Contribution of fossil and biomass-derived secondary organic carbon to winter water-soluble organic aerosols in Delhi, India

Delhi, among the world's most polluted megacities, is a hotspot of particulate matter emissions, with high contribution from organic aerosol (OA), affecting health and climate in the entire northern India. While the primary organic aerosol (POA) sources can be effectively identified, an incomplete source apportionment of secondary organic aerosol (SOA) causes significant ambiguity in the management of air quality and the assessment of climate change. Present study uses positive matrix factorization analysis on the water-soluble organic aerosol (WSOA) data from the offline-aerosol mass spectrometry (AMS). It revealed POA as the dominant source of WSOA, with biomass-burning OA (31-34 %) and solid fuel combustion OA (∼21 %) being two major contributors. Here we use water-solubility fingerprints to track the SOA precursors, such as oxalates or organic nitrates, instead of identifying them based on their O:C ratio. Non-fossil precursors dominate in more oxidized oxygenated organic carbon (MO-OOC) (∼90 %), a proxy for aged secondary organic carbon (SOC), by coupling offline-AMS with 14C measurements. On the contrary, the oxidation of fossil fuel emissions produces a large quantity of fresh fossil SOC, which accounts for ∼75 % of less oxidized oxygenated organic carbon (LO-OOC). Our study reveals that apart from major POA contributions, large fractions of fossil (10-14 %) and biomass-derived SOA (23-30 %) contribute significantly to the total WSOA load, having impact on climate and air quality of the Delhi megacity. Our study reveals that large-scale unregulated biomass burning was not only found to dominate in POA but was also observed to be a significant contributor to SOA with implications on human health, highlighting the need for effective control strategies.

Development and performance evaluation of online monitors for near real-time measurement of total and water-soluble organic carbon in fine and coarse ambient PM

In this study, we developed two online monitors for total organic carbon (TOC) and water-soluble organic carbon (WSOC) measurements in fine (dp < 2.5μm) and coarse (2.5μm < dp < 10μm) particulate matter (PM), respectively. Their performance has been evaluated in laboratory and field tests to demonstrate the feasibility of using these monitors to measure near real-time concentrations, with consideration of their potential for being employed in long-term measurements. The fine PM collection setup was equipped with a versatile aerosol concentration enrichment system (VACES) connected to an aerosol-into-liquid-sampler (AILS), whereas two virtual impactors (VIs) in tandem with a modified BioSampler were used to collect coarse PM. These particle collection setups were in tandem with a Sievers M9 TOC analyzer to read TOC and WSOC concentrations in aqueous samples hourly. The average hourly TOC concentration measured by our developed monitors in fine and coarse PM were 5.17 ± 2.41 and 0.92 ± 0.29 μg/m3, respectively. In addition, our TOC readings showed good agreement and were comparable with those quantified using Sunset Lab EC/OC analyzer operating in parallel as a reference. Furthermore, we conducted field tests to produce diurnal profiles of fine PM-bound WSOC, which can show the effects of ambient temperature on maximum values in the nighttime chemistry of the winter, as well as on increased photochemical activities in afternoon peaks during the summer. According to our experimental campaign, WSOC mean values during the study period (3.07 μg/m3 for the winter and 2.7 μg/m3 for the summer) were in a comparable range with those of earlier studies in Los Angeles. Overall, our results corroborate the performance of our developed monitors in near real-time measurements of TOC and WSOC, which can be employed for future source apportionment studies in Los Angeles and other areas, aiding in understanding the health impacts of different pollution sources.

Predicting Atmospheric Water-Soluble Organic Mass Reversibly Partitioned to Aerosol Liquid Water in the Eastern United States

Water-soluble organic matter (WSOM) formed through aqueous processes contributes substantially to total atmospheric aerosol, however, the impact of water evaporation on particle concentrations is highly uncertain. Herein, we present a novel approach to predict the amount of evaporated organic mass induced by sample drying using multivariate polynomial regression and random forest (RF) machine learning models. The impact of particle drying on fine WSOM was monitored during three consecutive summers in Baltimore, MD (2015, 2016, and 2017). The amount of evaporated organic mass was dependent on relative humidity (RH), WSOM concentrations, isoprene concentrations, and NOx/isoprene ratios. Different models corresponding to each class were fitted (trained and tested) to data from the summers of 2015 and 2016 while model validation was performed using summer 2017 data. Using the coefficient of determination (R2) and the root-mean-square error (RMSE), it was concluded that an RF model with 100 decision trees had the best performance (R2 of 0.81) and the lowest normalized mean error (NME < 1%) leading to low model uncertainties. The relative feature importance for the RF model was calculated to be 0.55, 0.2, 0.15, and 0.1 for WSOM concentrations, RH levels, isoprene concentrations, and NOx/isoprene ratios, respectively. The machine learning model was thus used to predict summertime concentrations of evaporated organics in Yorkville, Georgia, and Centerville, Alabama in 2016 and 2013, respectively. Results presented herein have implications for measurements that rely on sample drying using a machine learning approach for the analysis and interpretation of atmospheric data sets to elucidate their complex behavior.

Water-soluble organic aerosols over South Asia - Seasonal changes and source characteristics

Water-soluble organic carbon (WSOC) has been identified as a key component in atmospheric aerosols due to its ability to act as cloud condensation nuclei (CCN) owing to their highly hygroscopic nature. This paper discusses about the spatio-temporal variability in WSOC mass concentration, sources (primary and secondary contributions), the role of long-range air-mass transport in modulating their abundance, at distinct sectors over South Asia. We found from our observations that, photochemical ageing of primary organic aerosols that are derived from biomass emissions, significantly contribute to the total WSOC budget over South Asia. The wide range of water-soluble compounds released by biomass burning can contribute directly to the WSOC fraction or undergo further atmospheric processing, such as oxidation or ageing, leading to the formation of additional WSOC. WSOC/OC (organic carbon) ratio and the correlation between the WSOC and secondary organic carbon (SOC) are used for assessing the contribution from secondary sources. The three different ratios are used to delineate different source processes; OC/EC (elemental carbon) for source identification, WSOC/OC for long-range atmospheric transport (ageing) and WSOC/SOC to understand the primary and secondary contribution of WSOC. The present investigation revealed that, the primary OC that have undergone significant chemical processing as a result of long-range transport have a substantial influence on WSOC formation over South Asia, especially in Indo Gangetic Plain outflow regions such as southern peninsular and adjacent marine regions. Overall, oxidation and ageing of primary organic aerosols emitted from biomass burning was found to serve as an important source of WSOC over South Asia.

Oxidized and Unsaturated: Key Organic Aerosol Traits Associated with Cellular Reactive Oxygen Species Production in the Southeastern United States

Exposure to ambient fine particulate matter (PM2.5) is associated with millions of premature deaths annually. Oxidative stress through overproduction of reactive oxygen species (ROS) is a possible mechanism for PM2.5-induced health effects. Organic aerosol (OA) is a dominant component of PM2.5 worldwide, yet its role in PM2.5 toxicity is poorly understood due to its chemical complexity. Here, through integrated cellular ROS measurements and detailed multi-instrument chemical characterization of PM in urban southeastern United States, we show that oxygenated OA (OOA), especially more-oxidized OOA, is the main OA type associated with cellular ROS production. We further reveal that highly unsaturated species containing carbon-oxygen double bonds and aromatic rings in OOA are major contributors to cellular ROS production. These results highlight the key chemical features of ambient OA driving its toxicity. As more-oxidized OOA is ubiquitous and abundant in the atmosphere, this emphasizes the need to understand its sources and chemical processing when formulating effective strategies to mitigate PM2.5 health impacts.

Size distributions of molecular markers for biogenic secondary organic aerosol in urban Beijing

To understand the source, formation, and seasonality of biogenic secondary organic aerosol (BSOA), a nine-stage cascade impactor was utilized to collect size-segregated particulate samples from April 2017 to January 2018 in Beijing, China. BSOA tracers derived from isoprene, monoterpene, and sesquiterpene were measured with gas chromatography-mass spectrometry. Isoprene and monoterpene SOA tracers exhibited significant seasonal variations, with a summer maximum and a winter minimum. Dominance of 2-methyltetrols (isoprene SOA tracers) with a good correlation with levoglucosan (a biomass burning tracer), which was combined with the detection of methyltartaric acids (possible indicators for aged isoprene) in summer, implies possible biomass burning and long-range transport. In contrast, sesquiterpene SOA tracer (β-caryophyllinic acid) was dominant in winter and was probably associated with the local burning of biomass. Bimodal size distributions were observed for most isoprene SOA tracers, consistent with previous laboratory experiments and field studies showing that they can be formed not only in the aerosol phase but also in the gas phase. Monoterpene SOA tracers cis-pinonic acid and pinic acid showed a coarse-mode peak (5.8-9.0 μm) in four seasons due to their volatile nature. Sesquiterpene SOA tracer β-caryophyllinic acid showed a unimodal pattern with a major fine-mode peak (1.1-2.1 μm), which is linked to local biomass burning. The tracer-yield method was used to quantify the contributions of isoprene, monoterpene, and sesquiterpene to secondary organic carbon (SOC) and SOA. The highest isoprene SOC and SOA concentrations occurred in summer (2.00 μgC m^-3 and 4.93 μg m^-3, respectively), contributing to 1.61% of OC and 5.22% of PM_2.5, respectively. These results suggest that BSOA tracers are promising tracers for understanding the source, formation, and seasonality of BSOA.

Non-linear effects of secondary organic aerosol formation and properties in multi-precursor systems

Secondary organic aerosol (SOA) contributes significantly to ambient fine particulate matter that affects climate and human health. Monoterpenes represent an important class of biogenic volatile organic compounds (VOCs) and their oxidation by nitrate radicals poses a substantial source of SOA globally. Here, we investigate the formation and properties of SOA from nitrate radical oxidation of two common monoterpenes, α-pinene and limonene. When two monoterpenes are oxidized simultaneously, we observe a ~50% enhancement in the formation of SOA from α-pinene and a ~20% reduction in limonene SOA formation. The change in SOA yields is accompanied by pronounced changes in aerosol chemical composition and volatility. These non-linear effects are not observed in a sequential oxidation experiment. Our results highlight that unlike currently assumed in atmospheric models, the interaction of products formed from individual VOCs should be accounted for to accurately describe SOA formation and its climate and health impacts.

Light absorption potential of water-soluble organic aerosols in the two polluted urban locations in the central Indo-Gangetic Plain

PM_2.5 (particulate matter having aerodynamic diameter ≤2.5 μm) samples were collected during wintertime from two polluted urban sites (Allahabad and Kanpur) in the central Indo-Gangetic Plain (IGP) to comprehend the sources and atmospheric transformations of light-absorbing water-soluble organic aerosol (WSOA). The aqueous extract of each filter was atomized and analyzed in a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). Water-soluble organic carbon (WSOC) and WSOA concentrations at Kanpur were ∼1.2 and ∼1.5 times higher than that at Allahabad. The fractions of WSOC and secondary organic carbon (SOC) to total organic carbon (OC) were also significantly higher ∼53% and 38%, respectively at Kanpur compared to Allahabad. This indicates a higher abundance of oxidized WSOA at Kanpur. The absorption coefficient (b_abs-365) of light-absorbing WSOA measured at 365 nm was 46.5 ± 15.5 Mm^-1 and 73.2 ± 21.6 Mm^-1 in Allahabad and Kanpur, respectively, indicating the dominance of more light-absorbing fractions in WSOC at Kanpur. The absorption properties such as mass absorption efficiency (MAE₃₆₅) and imaginary component of refractive index (k_abs-365) at 365 nm at Kanpur were also comparatively higher than Allahabad. The absorption forcing efficiency (Abs SFE; indicates warming effect) of WSOA at Kanpur was ∼1.4 times higher than Allahabad. Enhancement in light absorption capacity was observed with the increase in f44/f43 (fraction of m/z 44 (f44) to 43 (f43) in organic mass spectra) and O/C (oxygen to carbon) ratio of WSOA at Kanpur while no such trend was observed for the Allahabad site. Moreover, the correlation between carbon fractions and light absorption properties suggested the influence of low-volatile organic compounds (OC3 + OC4 fraction obtained from thermal/optical carbon analyzer) in increasing the light absorption capacity of WSOA in Kanpur.

Large contribution of fossil-derived components to aqueous secondary organic aerosols in China

Incomplete understanding of the sources of secondary organic aerosol (SOA) leads to large uncertainty in both air quality management and in climate change assessment. Chemical reactions occurring in the atmospheric aqueous phase represent an important source of SOA mass, yet, the effects of anthropogenic emissions on the aqueous SOA (aqSOA) are not well constrained. Here we use compound-specific dual-carbon isotopic fingerprints (δ¹³C and Δ¹⁴C) of dominant aqSOA molecules, such as oxalic acid, to track the precursor sources and formation mechanisms of aqSOA. Substantial stable carbon isotope fractionation of aqSOA molecules provides robust evidence for extensive aqueous-phase processing. Contrary to the paradigm that these aqSOA compounds are largely biogenic, radiocarbon-based source apportionments show that fossil precursors produced over one-half of the aqSOA molecules. Large fractions of fossil-derived aqSOA contribute substantially to the total water-soluble organic aerosol load and hence impact projections of both air quality and anthropogenic radiative forcing. Our findings reveal the importance of fossil emissions for aqSOA with effects on climate and air quality.

Quantifying the relative contributions of aqueous phase and photochemical processes to water-soluble organic carbon formation in winter in a megacity of South China

To identify potential formation mechanisms of water-soluble organic carbon (WSOC) and quantify their contributions to WSOC in urban Guangzhou of south China, a comprehensive campaign was carried out in winter of 2019-2020. During the campaign, WSOC, total carbon (TC), black carbon (BC), water-soluble inorganic ions (WSIIs) and fourteen elements in PM_2.5 were collected using inline instruments. Bulk PM_2.5 and size-segregated particle samples were also synchronously collected using offline instruments for analyzing the dominant chemical components including WSOC, organic carbon (OC), elemental carbon (EC) and WSIIs. In addition, gaseous pollutants (e.g., NH₃, SO₂, HNO₃, NO₂, O₃) and meteorological parameters were also measured during the same period. PM_2.5 pollution episodes during the campaign period were mainly driven by increased nitrate concentrations. The mass concentration of WSOC increased from 3.9 ± 1.1 μg m^-3 on non-episode days to 6.8 ± 0.6 μg m^-3 on episode days, although the mass ratio of WSOC to OC in PM_2.5 changed little (<4%). Photochemical processes dominated WSOC formation in the afternoon and aqueous phase chemical processes played the dominant role in the night, from which newly formed WSOC distributed in the condensation mode and the droplet mode, respectively. Source apportionment analysis using positive matrix factorization (PMF) model suggested that on average 35% and 65% of WSOC mass in PM_2.5 were related with the photochemical processes and aqueous phase chemical processes, respectively. Aqueous phase chemical processes were highly affected by nitrate pollution, which was closely related with O₃ pollution.

Water-soluble matter in PM2.5 in a coastal city over China: Chemical components, optical properties, and source analysis

Although marine and terrestrial emissions simultaneously affect the formation of atmospheric fine particles in coastal areas, knowledge on the optical properties and sources of water-soluble matter in these areas is still scarce. In this work, taking Qingdao, China as a typical coastal location, the chemical composition of PM_2.5 during winter 2019 was analyzed. Excitation-emission matrix fluorescence spectroscopy was combined with parallel factor analysis model to explain the components of water-soluble atmospheric chromophores of PM_2.5. Our analysis indicated that NO₃^-, NH₄⁺ and SO₄^2- ions accounted for 86.80% of the total ion mass, dominated by NO₃^-. The ratio of [NO₃^-]/[SO₄^2-] was up to 2.42 ± 0.84, suggesting that mobile sources play an important role in local pollutants emission. The result of positive correlation between Abs₃₆₅ with K⁺ suggests that biomass burning is an important source of water-soluble organic compounds (WSOC). Six types of fluorophores (C1-C6), all humic-like substances, were identified in WSOC. Humification index, biological index and fluorescence index in winter were 1.66 ± 0.34, 0.51 ± 0.44 and 1.09 ± 0.78, respectively, indicating that WSOC in Qingdao were mainly terrestrial organic matters. Overall, although the study area is close to the ocean, the contribution of terrestrial sources to PM_2.5, especially vehicle exhaust and coal combustion, is still much higher than that of marine sources. Our study provides a more comprehensive understanding of chemical and optical properties of WSOC based on PM_2.5 in coastal areas, and may provide ground for improving local air quality.

Gridded distribution of total suspended particulate matter (TSP) and their chemical characterization over Delhi during winter

In the present study, total suspended particulate matter (TSP) samples were collected at 47 different sites (47 grids of 5 × 5 km² area) of Delhi during winter (January-February 2019) in campaign mode. To understand the spatial variation of sources, TSP samples were analyzed for chemical compositions including carbonaceous species [organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC)], water-soluble total nitrogen (WSTN), water-soluble inorganic nitrogen (WSIN), polycyclic aromatic hydrocarbons (16 PAHs), water-soluble inorganic species (WSIS) (F^-, Cl^-, SO₄^2-, NO₂^-, NO₃^-, PO₄^3-, NH₄⁺, Ca²⁺, Mg²⁺, Na⁺, and K⁺), and major and minor trace elements (B, Na, Mg, Al, P, S, Cl, K, Ca, Ti, Fe, Zn, Cr, Mn, Cu, As, Pd, F, and Ag). During the campaign, the maximum concentration of several components of TSP (996 μg/m³) was recorded at the Rana Pratap Bagh area, representing a pollution hotspot of Delhi. The maximum concentrations of PAHs were recorded at Udhyog Nagar, a region close to heavily loaded diesel vehicles, small rubber factories, and waste burning areas. Higher content of Cl^- and Cl^-/Na⁺ ratio (>1.7) suggests the presence of nonmarine anthropogenic sources of Cl^- over Delhi. Minimum concentrations of OC, EC, WSOC, PAHs, and WSIS in TSP were observed at Kalkaji, representing the least polluted area in Delhi. Enrichment factor <5.0 at several locations and a significant correlation of Al with Mg, Fe, Ti, and Ca and C/N ratio indicated the abundance of mineral/crustal dust in TSP over Delhi. Principal component analysis (PCA) was also performed for the source apportionment of TSP, and extracted soil dust was found to be the major contributor to TSP, followed by biomass burning, open waste burning, secondary aerosol, and vehicular emissions.

Seasonal changes in stable carbon isotopic composition in the bulk aerosol and gas phases at a suburban site in Prague

Isotope fractionation between the gas and aerosol phases is an important phenomenon for studying atmospheric processes. Here, for the first time, seasonally resolved stable carbon isotope ratio (δ¹³C) values are systematically used to study phase interactions in bulk aerosol and gaseous carbonaceous samples. Seasonal variations in the δ¹³C of total carbon (TC; δ¹³C_TC) and water-soluble organic carbon (WSOC; δ¹³C_WSOC) in fine aerosol particles (PM_2.5) as well as in the total carbon of part of the gas phase (TCgas; δ¹³C_TCgas) were studied at a suburban site in Prague, Czech Republic, Central Europe. Year-round samples were collected for the main and backup filters from 14 April 2016 to 1 May 2017 every 6 days with a 48 h sampling period (n = 66). During all seasons, the highest ¹³C enrichment was found in WSOC, followed by particulate TC, whereas the highest ¹³C depletion was found in gaseous TC. We observed a clear seasonal pattern for all δ¹³C, with the highest values in winter (avg. δ¹³C_TC = -25.5 ± 0.8‰, δ¹³C_WSOC = -25.0 ± 0.7‰, δ¹³C_TCgas = -27.7 ± 0.5‰) and the lowest values in summer (avg. δ¹³C_TC = -27.2 ± 0.5‰, δ¹³C_WSOC = -26.4 ± 0.3‰, δ¹³C_TCgas = -28.9 ± 0.3‰). This study supports the existence of different aerosol sources at the site during the year. Despite the different seasonal compositions of carbonaceous aerosols, the isotope difference (Δδ¹³C) between δ¹³C_TC (aerosol) and δ¹³C_TCgas (gas phase) was similar during the seasons (year avg. 1.97 ± 0.50‰). Moreover, Δδ¹³C between WSOC and TC in PM_2.5 showed a difference between spring and winter, but in general, these values were also similar year-round (year avg. 0.71 ± 0.37‰). During the entire period, TCgas and WSOC were the most ¹³C-depleted and most ¹³C-enriched fractions, respectively, and although the resulting difference Δ(δ¹³C_WSOC - δ¹³C_TCgas) was significant, it was almost invariant throughout the year (2.67 ± 0.44‰). The present study suggests that the stable carbon isotopic fractionation between the bulk aerosol and gas phases is probably not entirely dependent on the chemical composition of individual carbonaceous compounds from different sources.

Organic aerosol formation and aging processes in Beijing constrained by size-resolved measurements of radiocarbon and stable isotopic 13C

This study investigates the sources and atmospheric processes of size-resolved carbonaceous aerosols in winter 2018 in urban Beijing, based on analysis of dual-carbon isotopes (i.e., radiocarbon and the stable isotope ¹³C). We found a size dependence of fossil source contributions to elemental carbon (EC), but no clear size dependence for organic carbon (OC). Comparable fossil source contributions to water-insoluble OC (WIOC; 55 ± 3%) and to water-soluble OC (WSOC; 54 ± 4%) highlight the importance of secondary aerosol formation, considering that fossil sources emit only small amounts of primary WSOC. OC concentrations increased during high PM_2.5 pollution events, with increased fossil and non-fossil WSOC concentrating at larger particles (0.44-2.5 µm) than WIOC (0.25-2.5 µm), highlighting the aqueous-phase chemistry as an important pathway for OC production. The ratio of ¹³C/¹²C (expressed as δ¹³C) of total carbon (-27.0‰ to -23.3‰) fell in the range of anthropogenic aerosol, reflecting small biogenic influence. δ¹³C of OC increased with desorption temperature steps (200 °C, 350 °C and 650 °C). The strongly enriched δ¹³C_OC,650 (-26.9‰ to -20.3‰) and large mass fraction of OC_650°C in total desorbed OC, both increasing with the increase of particle sizes, were caused by photochemical aging, especially during low and moderate PM_2.5 pollution events, when regional, aged aerosol played an important role. During low pollution events, higher δ¹³C_OC,650 and WSOC/OC ratios reflect a larger contribution and more extensive chemical processing of aged aerosol. In contrast, relatively low δ¹³C_OC,200 (-27.2‰ to -25.7‰) suggests the influence of secondary OC formation on the more volatile OC desorbed at 200 °C. δ¹³C_OC,200 was similar for all particle sizes and for different pollution events, pointing to an internal mixture of local and aged regional OC. Our results show that the organic aerosol in Beijing arises from a mixture of various sources and complex formation processes, spanning local to regional scales. Particle sizes < 250 nm show strong contribution from local secondary OC formation, whereas refractory OC in particles around 1 µm shows strong evidence for regional aging processes. In summary, primary emission, secondary and aqueous-phase formation, and (photo-)chemical aging all need to be considered to understand organic aerosol in this region and their importance varies with particle size.

Diurnal variability in the spectral characteristics and sources of water-soluble brown carbon aerosols over Delhi

It is well established that light-absorbing organic aerosols (commonly known as brown carbon, BrC) impact climate. However, uncertainties remain as their contributions to absorption at different wavelengths are often ignored in climate models. Further, BrC exhibits differences in absorption at different wavelengths due to the variable composition including varying sources and meteorological conditions. However, diurnal variability in the spectral characteristics of water-soluble BrC (hereafter BrC) is not yet reported. This study presents unique measurement hitherto lacking in the literature. Online measurements of BrC were performed using an assembled system including a particle-into-liquid sampler, portable UV-Visible spectrophotometer with liquid waveguid capillary cell, and total carbon analyzer (PILS-LWCC-TOC). This system measured the absorption of ambient aerosol extracts at the wavelengths ranging from 300 to 600 nm with 2 min integration time and water-soluble organic carbon (WSOC) with 4 min integration time over a polluted megacity, New Delhi. Black carbon, carbon monoxide (CO), nitrogen oxides (NOx), and the chemical composition of non-refractory submicron aerosols were also measured in parallel. Diurnal variability in absorption coefficient (0.05 to 65 Mm^-1), mass absorption efficiency (0.01 to 3.4 m^-2 gC^-1) at 365 nm, and absorption angstrom exponent (AAE) of BrC for different wavelength range (AAE_300-400: 4.2-5.8; AAE_400-600: 5.5-8.0; and AAE_300-600: 5.3-7.3) is discussed. BrC chromophores absorbing at any wavelength showed minimum absorption during afternoon hours, suggesting the effects of boundary layer expansion and their photo-sensitive/volatile nature. On certain days, a considerable presence of BrC absorbing at 490 nm was observed during nighttime that disappears during the daytime. It appeared to be associated with secondary BrC. Observations also infer that BrC species emitted from the biomass and coal burning are more absorbing among all sources. A fraction of BrC is likely associated with trash burning, as inferred from the spectral characteristics of Factor-3 from the PMF analysis of BrC spectra. Such studies are essential in understanding the BrC characteristics and their further utilization in climate models.

Chemical Composition and Source Apportionment of Total Suspended Particulate in the Central Himalayan Region

The present study analyzes data from total suspended particulate (TSP) samples collected during 3 years (2005–2008) at Nainital, central Himalayas, India and analyzed for carbonaceous aerosols (organic carbon (OC) and elemental carbon (EC)) and inorganic species, focusing on the assessment of primary and secondary organic carbon contributions (POC, SOC, respectively) and on source apportionment by positive matrix factorization (PMF). An average TSP concentration of 69.6 ± 51.8 µg m−3 was found, exhibiting a pre-monsoon (March–May) maximum (92.9 ± 48.5 µg m−3) due to dust transport and forest fires and a monsoon (June–August) minimum due to atmospheric washout, while carbonaceous aerosols and inorganic species expressed a similar seasonality. The mean OC/EC ratio (8.0 ± 3.3) and the good correlations between OC, EC, and nss-K+ suggested that biomass burning (BB) was one of the major contributing factors to aerosols in Nainital. Using the EC tracer method, along with several approaches for the determination of the (OC/EC)pri ratio, the estimated SOC component accounted for ~25% (19.3–29.7%). Furthermore, TSP source apportionment via PMF allowed for a better understanding of the aerosol sources in the Central Himalayan region. The key aerosol sources over Nainital were BB (27%), secondary sulfate (20%), secondary nitrate (9%), mineral dust (34%), and long-range transported mixed marine aerosol (10%). The potential source contribution function (PSCF) and concentration weighted trajectory (CWT) analyses were also used to identify the probable regional source areas of resolved aerosol sources. The main source regions for aerosols in Nainital were the plains in northwest India and Pakistan, polluted cities like Delhi, the Thar Desert, and the Arabian Sea area. The outcomes of the present study are expected to elucidate the atmospheric chemistry, emission source origins, and transport pathways of aerosols over the central Himalayan region.

Chemical Synthesis of Multifunctional Air Pollutants: Terpene-Derived Nitrooxy Organosulfates

Nitrooxy organosulfates derived from terpenes (NOS_TP) represent an important class of products formed between anthropogenic pollution (e.g., SO₂ and NO_x) and natural emissions. NOS_TP compounds have been consistently detected in atmospheric environments under varying urban influences. Their chemical linkages to both anthroposphere and biosphere make them valuable markers for tracking anthroposphere-biosphere interactions. However, their quantification, formation, and transformation kinetics in atmospheric aerosols are hindered due to the lack of NOS_TP standards. In this work, we developed two routes for the first concise chemical synthesis of eight NOS_TP from terpenes including α-pinene, β-pinene, limonene, limonaketone, and β-caryophyllene. Subsequently, six of the synthesized NOS_TP were for the first time positively identified in ambient aerosol samples, clarifying certain misidentifications in previous studies. More significantly, the availability of authentic standards allows irrefutable observation of three carbon skeleton-rearranged NOS_TP, two derived from α-pinene, and one derived from β-caryophyllene, revealing the occurrence of previously unrecognized transformation pathways in the formation of NOS_TP. Two synthesized NOS_TP from β-pinene and limonene could not be detected, likely due to rapid hydrolysis of their immediate hydroxynitrate precursors outcompeting sulfation. Such mechanistic evidence is valuable in understanding the atmospheric chemistry of NOS_TP and related compounds. This work demonstrates the usefulness of authentic standards in probing the NOS_TP formation mechanisms in the atmosphere. Comparison of NOS_TP ambient samples collected from four Chinese cities in two winter months indicates that anthropogenic chemical factors could outcompete terpene emissions in the formation of NOS_TP.

Chemical Characteristics and Sources of Water-Soluble Organic Nitrogen Species in PM2.5 in Nanjing, China

Water-soluble organic nitrogen (WSON) is an important component of PM2.5 which may affect air quality, climate and human health. Herein, one-year field samples of atmospheric PM2.5 (June 2017–May 2018) were collected in northern Nanjing. Chemical characterization of PM2.5 major components as well as WSON were conducted, and WSON composition and sources were further investigated via measurements by a Aerodyne soot particle aerosol mass spectrometer (SP-AMS) as well as positive matrix factorization (PMF). Inorganic ions, mainly consisting of ammonium, sulfate, and nitrate, were found to dominate PM2.5 mass (58.7%), followed by organic matter (OM) (22.6%), and elemental carbon (EC) (2.1%). Water-soluble OM dominated OM (65.1%), and its temporal variation was closely correlated with that of secondary organic matter, while time series of water-insoluble OM concentrations correlated tightly with that of primary organic matter. Average WSON concentration was 2.15 μg/m3, which was highest in winter and lowest in summer. Correlation analysis of WSON with PM2.5 components also indicated that WSON was mainly from secondary sources. SP-AMS revealed that WSON mass spectrum was composed of CxHyNp+ (91.2%) and CxHyOzNp+ (8.8%), indicating dominance of amines and other oxygenated ON compounds. PMF analysis resolved two primary sources (traffic, biomass burning) and two secondary sources (less-oxidized and more-oxidized factors) of WSOM and WSON, and the secondary source dominated both WSOM and WSON. Contribution of the more-oxidized ON factor was very high in winter, and the less-oxidized factor was significant in summer, indicating a likely important role of aqueous-phase processing in winter as well as photochemical oxidation in summer to WSON.

The impact of stay-home policies during Coronavirus-19 pandemic on the chemical and toxicological characteristics of ambient PM2.5 in the metropolitan area of Milan, Italy

The goal of this study was to characterize changes in components and toxicological properties of PM2.5 during the nationwide 2019-Coronavirus (COVID-19) lockdown restrictions in Milan, Italy. Time-integrated PM2.5 filters were collected at a residential site in Milan metropolitan area from April 11th to June 3rd at 2020, encompassing full-lockdown (FL), the followed partial-lockdown (PL2), and full-relaxation (FR) periods of COVID-19 restrictions. The collected filters were analyzed for elemental and organic carbon (EC/OC), water-soluble organic carbon (WSOC), individual organic species (e.g., polycyclic aromatic hydrocarbons (PAHs), and levoglucosan), and metals. According to online data, nitrogen dioxide (NO2) and benzene (C6H6) levels significantly decreased during the entire COVID-19 period compared to the same time span in 2019, mainly due to the government-backed shutdowns and curtailed road traffic. Similarly, with a few exceptions, surrogates of tailpipe emissions (e.g., traffic-associated PAHs) as well as re-suspended road dust (e.g., Fe, Mn, Cu, Cr, and Ti) were relatively lower during FL and PL2 periods in comparison with year 2019, whereas an increasing trend in mass concentration of mentioned species was observed from FL to PL2 and FR phases due to the gradual lifting of lockdown restrictions. In contrast, comparable concentrations of ambient PM2.5 and black carbon (BC) between lockdown period and the same time span in 2019 were attributed to the interplay between decreased road traffic and elevated domestic biomass burning as a result of adopted stay-home strategies. Finally, the curtailed road traffic during FL and PL2 periods led to ~25% drop in the PM2.5 oxidative potential (measured via 2',7'-dichlorodihydrofluorescein (DCFH) and dithiothreitol (DTT) assays) with respect to the FR period as well as the same time span in 2019. The results of this study provide insights into the changes in components and oxidative potential of PM2.5 in the absence of road traffic during COVID-19 restrictions.

Absorption properties and forcing efficiency of light-absorbing water-soluble organic aerosols: Seasonal and spatial variability

Light-absorbing organic aerosols, also known as brown carbon (BrC), enhance the warming effect of the Earth's atmosphere. The seasonal and spatial variability of BrC absorption properties is poorly constrained and accounted for in the climate models resulting in a substantial underestimation of their radiative forcing estimates. This study reports seasonal and spatial variability of absorption properties and simple forcing efficiency of light-absorbing water-soluble organic carbon (WSOC, SFE_WSOC) by utilizing current and previous field-based measurements reported mostly from Asia along with a few observations from Europe, the USA, and the Amazon rainforest. The absorption coefficient of WSOC at 365 nm (b_abs-365) and the concentrations of carbonaceous species at Kanpur were about an order of magnitude higher during winter than in the monsoon season owing to differences in the boundary layer height, active sources and their strengths, and amount of seasonal wet precipitation. The WSOC aerosols during winter exhibited ∼1.6 times higher light absorption capacity than in the monsoon season at Kanpur site. The assessment of spatial variability of the imaginary component of the refractive index spectrum (k_λ) across South Asia has revealed that it varies from ∼1 to 2 orders of magnitude and light absorption capacity of WSOC ranges from 3 to 21 W/g. The light absorption capacity of WSOC aerosols exhibited less spatial variability across East Asia (5-13 W/g) when compared to that in the South Asia. The photochemical aging of WSOC aerosols, indicated by the enhancement in WSOC/OC ratio, was linked to degradation in their light absorption capacity, whereas the absorption Ångström exponent (AAE) remained unaffected. This study recommends the adoption of refined climate models where sampling regime specific absorption properties are calculated separately, such that these inputs can better constrain the model estimates of the global effects of BrC.

Oxidative potential of atmospheric PM10 at five different sites of Ahmedabad, a big city in Western India

The current study presents the oxidative potential (OP) along with a wide range of chemical speciation of particulate matter with an aerodynamic diameter less than 10 μm (PM₁₀) at five sites representing different environments in Ahmedabad, a big city in western India. On an average, PM₁₀ concentrations were 116 ± 36, 228 ± 43, 133 ± 29, 101 ± 21, and 70 ± 20 μg m^-3; volume-normalized OP (OP_V) were 2.51 ± 0.71, 5.62 ± 0.68, 2.69 ± 0.76, 2.14 ± 0.41, and 1.55 ± 0.51 nmol DTT min^-1 m^-3; and mass-normalized OP (OP_M) were 22 ± 3, 25 ± 5, 21 ± 6, 21 ± 2, and 22 ± 3 pmol DTT min^-1 μg^-1 over Bapunagar (backward residential area), Narol (industrial), Paldi (bus transport hub), Income Tax (huge running traffic) and Science City (posh residential area), respectively. Overall, OP_V showed a significant linear correlation with PM₁₀, whereas OP_M showed near uniformity with increasing PM₁₀. Although the OP_M values were similar, the site-to-site variability in PM₁₀ concentration reflects the corresponding health risks associated with PM₁₀ exposure for the people living in these areas. Further, a noticeable temporal variation in OP_M at Narol and Paldi suggests that species with diverse OP_M contributed to PM₁₀ on different days. A strong linear relationship between the ratio of OP_V to the mass concentration of organic carbon (OP_OC) and the ratio of m/z 43 signal to total water-soluble organic aerosols (WSOA) signals (f43) suggests that the fossil-fuel combustion derived WSOA have higher OP. Furthermore, the relationships of OP with water-soluble trace metals and brown carbon are also investigated and discussed in this paper. Nitrogenous organic compounds particularly emitted from the traffic-related sources in Paldi and Income Tax have higher OP_OC than those emitted from other sources over Bapunagar, Narol, and Science City.

Apportioned primary and secondary organic aerosol during pollution events of DISCOVER-AQ Houston

Understanding the drivers for high ozone (O3) and atmospheric particulate matter (PM) concentrations is a pressing issue in urban air quality, as this understanding informs decisions for control and mitigation of these key pollutants. The Houston, TX metropolitan area is an ideal location for studying the intersection between O3 and atmospheric secondary organic carbon (SOC) production due to the diversity of source types (urban, industrial, and biogenic) and the on- and off-shore cycling of air masses over Galveston Bay, TX. Detailed characterization of filter-based samples collected during Deriving Information on Surface Conditions from Column and VERtically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) Houston field experiment in September 2013 were used to investigate sources and composition of organic carbon (OC) and potential relationships between daily maximum 8 h average O3 and PM. The current study employed a novel combination of chemical mass balance modeling defining primary (i.e. POC) versus secondary (i.e. SOC) organic carbon and radiocarbon (14C) for apportionment of contemporary and fossil carbon. The apportioned sources include contemporary POC (biomass burning [BB], vegetative detritus), fossil POC (motor vehicle exhaust), biogenic SOC and fossil SOC. The filter-based results were then compared with real-time measurements by aerosol mass spectrometry. With these methods, a consistent urban background of contemporary carbon and motor vehicle exhaust was observed in the Houston metropolitan area. Real-time and filter-based characterization both showed that carbonaceous aerosols in Houston was highly impacted by SOC or oxidized OC, with much higher contributions from biogenic than fossil sources. However, fossil SOC concentration and fractional contribution had a stronger correlation with daily maximum 8 h average O3, peaking during high PM and O3 events. The results indicate that point source emissions processed by on- and off-shore wind cycles likely contribute to peak events for both PM and O3 in the greater Houston metropolitan area.

Chemical characteristics and sources of water-soluble organic aerosol in southwest suburb of Beijing

PM_2.5 filter sampling and components measurement were conducted in autumn and winter from 2014 to 2015 at a suburban site (referred herein as "LLH site") located in the southwest of Beijing. The offline aerosol mass spectrometry (offline-AMS) analysis and positive matrix factorization (PMF) were applied for measurement and source apportionment of water-soluble organic aerosol (WSOA). Organic aerosol (OA) always dominated PM_2.5 during the sampling period, especially in winter. WSOA pollution was serious during the polluted period both in autumn (31.1 µg/m³) and winter (31.9 µg/m³), while WSOA accounted for 54.4% of OA during the polluted period in autumn, much more than that (21.3%) in winter. The oxidation degree of WSOA at LLH site was at a high level (oxygen-to-carbon ratio, O/C=0.91) and secondary organic aerosol (SOA) contributed more mass ratio of WSOA than primary organic aerosol (POA) during the whole observation period. In winter, coal combustion OA (CCOA) was a stable source of OA and on average accounted for 25.1% of WSOA. In autumn, biomass burning OA (BBOA) from household combustion contributed 38.3% of WSOA during polluted period. In addition to oxygenated OA (OOA), aqueous-oxygenated OA (aq-OOA) was identified as an important factor of SOA. During heavy pollution period, the mass proportion of aq-OOA to WSOA increased significantly, implying the significant SOA formation through aqueous-phase process. The result of this study highlights the concentration on controlling the residential coal and biomass burning, as well as the research needs on aqueous chemistry in OA formation.

Chemical Composition and Light Absorption of PM2.5 Observed at Two Sites near a Busy Road during Summer and Winter

To examine the difference in the major chemical composition of fine particulate matter (PM2.5) between two roadway sites, 24 h integrated PM2.5 samples were simultaneously collected both 15 m (Buk-Ku District Office (BKO) site) and 150 m (Chonnam National University campus (CNU) site) away from busy roads during the summer and winter periods; these samples were taken to determine the concentrations of organic and elemental carbon (OC and EC), water-soluble organic carbon (WSOC), and water-soluble inorganic species. In addition, the real-time aerosol light absorption coefficients (Abs) were measured using a dual-spot seven-wavelength aethalometer at the CNU site to evaluate the influence of traffic and biomass burning (BB) emissions on the concentrations of organic aerosol particles. The hourly NO2 concentration was also observed at an air pollution monitoring network that is about 2 km away from the CNU site. During summer, 24 h PM2.5 concentrations (PM2.5 episode) which exceeded the Korean PM2.5 standard (35 μg/m3) were linked to increases in organic matter (OM) and SO42− concentrations that accounted for on average 35–41% and 26–30%, respectively, of the PM2.5 at the two sites. The increased SO42− concentration was most likely attributable to the inflow of long-range transported aerosols, rather than local production, as demonstrated by both the MODIS (Moderate Resolution Imaging Spectroradiometer) images and transport pathways of air masses reaching the sites. On the other hand, the OM, WSOC, and EC concentrations were directly attributable to traffic emissions at the sampling sites, as supported by the tight correlation between the OC and EC. A small difference between the absorption Ångström exponent (AAE) values calculated at wavelengths of 370–950 nm (AAE370–950nm) and 370–520 nm (AAE370–520nm), and the poor correlation of absorption coefficient by brown carbon (BrC) at 370 nm (AbsBrC370nm) with K+ (R2 = 0.00) also suggest a significant contribution of traffic emissions to OM. However, the wintertime PM2.5 episode was strongly related to the enhanced OM and NO3− concentrations, which contributed 26–28% and 22–23% of the PM2.5 concentration, respectively. It is interesting to note that there were two distinct OC/EC ratios in winter: a lower OC/EC (~3.0), which indicates a significant contribution of traffic emissions to the OC and EC, and a higher OC/EC (~6.5), which suggests an additional influence of BB emissions as well as traffic emissions at the sites. Strong correlations between the OC and EC (R2 = 0.72–0.83) and the enhanced AAE370–520nm values compared to the AAE370–950nm support that BB emissions were also an important contributor to the wintertime OM concentrations as well as traffic emissions at the two sites. A good correlation between the gaseous NO2 and NO3− and meteorological conditions (e.g., low wind speed and high relative humidity) suggest that the heterogeneous oxidation of NO2 on moist particles could be an important contributor to wintertime particulate NO3− formation at the sites. The OC concentrations during summer and winter were higher at the BKO site, with a higher traffic flow and a shorter distance from the roadway than at the CNU site. However, there were slight differences in the concentrations of secondary inorganic species (NO3−, SO42−, and NH4+) between the sites during summer and winter.

Chemical and isotopic characteristics of PM10 over the Bay of Bengal: Effects of continental outflow on a marine environment

Pollutants transport from South and Southeast Asia can profoundly affect the marine atmospheric boundary layer (MABL) over the Bay of Bengal (BoB). This study presents chemical and stable isotopic composition of PM₁₀ collected at Port Blair Island (11.6°N, 92.7°E) located in the middle of the BoB during the late northeast monsoon (February-April), a period when the BoB receives considerable continental outflow. These samples (n = 50) were analysed for major ions, carbonaceous species, trace metals, and isotopic composition of total C, N, and S components. Mass concentration of PM₁₀ ranged from 24 to 65 μg m^-3 during the study period. The dominance of continental inputs over a marine realm was evident by a significant amount of non-sea-salt (nss)-SO₄^2- (range: 1.8 to 16.9 μg m^-3), which accounts for ~65% of the total water-soluble inorganic constituents. The impact of anthropogenic emissions was further evident from the widespread depletion of chloride (range: 57-100%, avg.: 98 ± 7%) from sea-salt aerosols. Carbonaceous species (elemental carbon and organic matter) contributed nearly 35% to PM₁₀. Further, average δ¹³C (-25.6‰ ± 0.5) and δ³⁴S (4.5‰ ± 1.3) values observed over the marine study region were similar to those found in typical urban environments. δ¹⁵N values (13.7‰ ± 5.1) show the significant presence of combustion sources along with the effect of atmospheric processing. Aerosol δ¹³C values correlate positively with the ratio of water-soluble organic carbon to total organic carbon, indicating the aging of organic aerosols during the transport. Chemical and isotopic data suggest that both biomass burning (BB) and fossil fuel burning (FFB) contributed to ambient PM₁₀ with relatively more contribution of BB during February to early March and that of FFB during late March to middle of April. In aggregate, this study provides newer insights into sources of carbonaceous species and their chemical processing in MABL of BoB.

Source Apportionment of PM2.5 in Guangzhou Based on an Approach of Combining Positive Matrix Factorization with the Bayesian Mixing Model and Radiocarbon

To accurately apportion the sources of aerosols, a combined method of positive matrix factorization (PMF) and the Bayesian mixing model was applied in this study. The PMF model was conducted to identify the sources of PM2.5 in Guangzhou. The secondary inorganic aerosol source was one of the seven main sources in Guangzhou. Based on stable isotopes of oxygen and nitrogen (δ15N-NO3− and δ18O-NO3−), the Bayesian mixing model was performed to apportion the source of NO3− to coal combustion, traffic emission and biogenic source. Then the secondary aerosol source was subdivided into three sources according to the discrepancy in source apportionment of NO3− between PMF and Bayesian mixing model results. After secondary aerosol assignment, the six main sources of PM2.5 were traffic emission (30.6%), biomass burning (23.1%), coal combustion (17.7%), ship emission (14.0%), biomass boiler (9.9%) and industrial emission (4.7%). To assess the source apportionment results, fossil/non-fossil source contributions to organic carbon (OC) and element carbon (EC) inferred from 14C measurements were compared with the corresponding results in the PMF model. The results showed that source distributions of EC matched well between those two methods, indicating that the PMF model captured the primary sources well. Probably because of the lack of organic molecular markers to identify the biogenic source of OC, the non-fossil source contribution to OC in PMF results was obviously lower than 14C results. Thus, an indicative organic molecular tracer should be used to identify the biogenic source when accurately apportioning the sources of aerosols, especially in the region with high plant coverage or intense biomass burning.

A critical review on VOCs adsorption by different porous materials: Species, mechanisms and modification methods

Volatile organic compounds (VOCs) have attracted world-wide attention regarding their serious hazards on ecological environment and human health. Industrial processes such as fossil fuel combustion, petrochemicals, painting, coatings, pesticides, plastics, contributed to the large proportion of anthropogenic VOCs emission. Destructive methods (catalysis oxidation and biofiltration) and recovery methods (absorption, adsorption, condensation and membrane separation) have been developed for VOCs removal. Adsorption is established as one of the most promising strategies for VOCs abatement thanks to its characteristics of cost-effectiveness, simplicity and low energy consumption. The prominent progress in VOCs adsorption by different kinds of porous materials (such as carbon-based materials, oxygen-contained materials, organic polymers and composites is carefully summarized in this work, concerning the mechanism of adsorbate-adsorbent interactions, modification methods for the mentioned porous materials, and enhancement of VOCs adsorption capacity. This overview is to provide a comprehensive understanding of VOCs adsorption mechanisms and up-to-date progress of modification technologies for different porous materials.

An annual time series of weekly size-resolved aerosol properties in the megacity of Metro Manila, Philippines

Size-resolved aerosol samples were collected in Metro Manila between July 2018 and October 2019. Two Micro-Orifice Uniform Deposit Impactors (MOUDI) were deployed at Manila Observatory in Quezon City, Metro Manila with samples collected on a weekly basis for water-soluble speciation and mass quantification. Additional sets were collected for gravimetric and black carbon analysis, including during special events such as holidays. The unique aspect of the presented data is a year-long record with weekly frequency of size-resolved aerosol composition in a highly populated megacity where there is a lack of measurements. The data are suitable for research to understand the sources, evolution, and fate of atmospheric aerosols, as well as studies focusing on phenomena such as aerosol-cloud-precipitation-meteorology interactions, regional climate, boundary layer processes, and health effects. The dataset can be used to initialize, validate, and/or improve models and remote sensing algorithms.

Significant Contribution of Primary Sources to Water-Soluble Organic Carbon During Spring in Beijing, China

Despite the significant role water-soluble organic carbon (WSOC) plays in climate and human health, sources and formation mechanisms of atmospheric WSOC are still unclear; especially in some heavily polluted areas. In this study, near real-time WSOC measurement was conducted in Beijing for the first time with a particle-into-liquid-sampler coupled to a total organic carbon analyzer during the springtime, together with collocated online measurements of other chemical components in fine particulate matter with a 1 h time resolution, including elemental carbon (EC), organic carbon (OC), multiple metals, and water-soluble ions. Good correlations of WSOC with primary OC, as well as carbon monoxide, indicated that major sources of WSOC were primary instead of secondary during the study period. The positive matrix factorization model-based source apportionment results quantified that 68 ± 19% of WSOC could be attributed to primary sources, with predominant contributions by biomass burning during the study period. This finding was further confirmed by the estimate with the modified EC-tracer method, suggesting significant contribution of primary sources to WSOC. However, the relative contribution of secondary source to WSOC increased during haze episodes. The WSOC/OC ratio exhibited similar diurnal distributions with O3 and correlated well with secondary WSOC, suggesting that the WSOC/OC ratio might act as an indicator of secondary formation when WSOC was dominated by primary sources. This study provided evidence that primary sources could be major sources of WSOC in some polluted megacities, such as Beijing. From this study, it can be seen that WSOC cannot be simply used as a surrogate of secondary organic aerosol, and its major sources could vary by season and location.

Severe air pollution and characteristics of light-absorbing particles in a typical rural area of the Indo-Gangetic Plain

Total suspended particles (TSP) were collected in Lumbini from April 2013 to March 2016 to better understand the characteristics of carbonaceous aerosol (CA) concentrations, compositions and sources and their light absorption properties in rural region of severe polluted Indo-Gangetic Plain (IGP). Extremely high TSP (203.9 ± 109.6 μg m^-3), organic carbon (OC 32.1 ± 21.7 μg m^-3), elemental carbon (EC 6.44 ± 3.17 μg m^-3) concentrations were observed in Lumbini particularly during winter and post-monsoon seasons, reflecting the combined influences of emission sources and weather conditions. SO₄^2- (7.34 ± 4.39 μg m^-3) and Ca²⁺ (5.46 ± 5.20 μg m^-3) were the most dominant anion and cation in TSP. These components were comparable to those observed in urban areas in South and East Asia but significantly higher than those in remote regions over the Himalayas and Tibetan Plateau, suggesting severe air pollution in the study region. Various combustion activities including industry, vehicle emission, and biomass burning are the main reasons for high pollutant concentrations. The variation of OC/EC ratio further suggested that biomass such as agro-residue burning contributed a lot for CA, particularly during the non-monsoon season. The average mass absorption cross-section of EC (MAC_EC) and water-soluble organic carbon (MAC_WSOC) were 7.58 ± 3.39 and 1.52 ± 0.41 m² g^-1, respectively, indicating that CA in Lumbini was mainly affected by local emissions. Increased biomass burning decreased MAC_EC; whereas, it could result in high MAC_WSOC during the non-monsoon season. Furthermore, dust is one important factor causing higher MAC_WSOC during the pre-monsoon season.

Resolving ambient organic aerosol formation and aging pathways with simultaneous molecular composition and volatility observations

Organic aerosol (OA) constitutes a significant fraction of atmospheric fine particle mass. However, the precursors and chemical processes responsible for a majority of OA are rarely conclusively identified. We use online observations of hundreds of simultaneously measured molecular components obtained from 15 laboratory OA formation experiments with constraints on their effective saturation vapor concentrations to attribute the VOC precursors and subsequent chemical pathways giving rise to the vast majority of OA mass measured in two forested regions. We find that precursors and chemical pathways regulating OA composition and volatility are dynamic over hours to days, with their variations driven by coupled interactions between multiple oxidants. The extent of physical and photochemical aging, and its modulation by NOx, were key to a uniquely comprehensive combined composition-volatility description of OA. Our findings thus provide some of the most complete mechanistic-level guidance to the development of OA descriptions in air quality and Earth system models.

Aerosol Liquid Water Promotes the Formation of Water-Soluble Organic Nitrogen in Submicrometer Aerosols in a Suburban Forest

Water-soluble organic nitrogen (WSON) affects the formation, chemical transformations, hygroscopicity, and acidity of organic aerosols as well as biogeochemical cycles of nitrogen. However, large uncertainties exist in the origins and formation processes of WSON. Submicrometer aerosol particles were collected at a suburban forest site in Tokyo in summer 2015 to investigate the relative impacts of anthropogenic and biogenic sources on WSON formations and their linkages with aerosol liquid water (ALW). The concentrations of WSON (ave. 225 ± 100 ngN m^-3) and ALW exhibited peaks during nighttime, which showed a significant positive correlation, suggesting that ALW significantly contributed to WSON formation. Further, the thermodynamic predictions by ISORROPIA-II suggest that ALW was primarily driven by anthropogenic sulfate. Our analysis, including positive matrix factorization, suggests that aqueous-phase reactions of ammonium and reactive nitrogen with biogenic volatile organic compounds (VOCs) play a key role in WSON formation in submicrometer particles, which is particularly significant in nighttime, at the suburban forest site. The formation of WSON associated with biogenic VOCs and ALW was partly supported by the molecular characterization of WSON. The overall result suggests that ALW is an important driver for the formation of aerosol WSON through a combination of anthropogenic and biogenic sources.

Abundant Biogenic Oxygenated Organic Aerosol in Atmospheric Coarse Particles: Plausible Sources and Atmospheric Implications

Secondary organic aerosol (SOA) is a key component in atmospheric aerosols, strongly influencing air quality and climate. Most previous studies focused on SOA formation in the fine aerosol mode, and little is known about SOA formation across a broader size range, especially for the coarse aerosol mode. In this study, we coupled radiocarbon analysis and the offline aerosol mass spectrometric method to characterize water-soluble organic matter in size-segregated samples between 0.056 and 18 μm collected in urban Shenzhen, China. For the first time, detailed size distributions of different types of oxygenated organic aerosols (OOAs) are obtained. Fossil fuel OOA was mostly distributed in fine particles, and biogenic OOA occurred mostly in coarse particles. Organic composition and correlation analyses suggested that the major source of the coarse-mode OOA was more plausible to be heterogeneous reactions of biogenic volatile organic compounds (VOCs) on soil dust rather than primary biological materials. If so, this mechanism would complement the missing sinks of biogenic VOCs, significantly influence the regional and global organic aerosol budgets, and thus should be considered in air quality and climate models. This study highlights the urgent need for laboratory simulations of heterogeneous reactions of various VOCs on soil dust.

Insights into characteristics of light absorbing carbonaceous aerosols over an urban location in Southeast Asia

Light absorbing carbonaceous aerosols (LACA) consisting of black carbon (BC) and brown carbon (BrC) have received considerable attention because of their climate and health implications, but their sources, characteristics and fates remain unclear in Southeast Asia (SEA). In this study, we investigated spatio-temporal characteristics of LACA, their radiative properties and potential sources in Singapore under different weather conditions. Hourly BC concentrations, measured from May 2017 to March 2018, ranged from 0.31 μg/m³ to 14.37 μg/m³ with the mean value being 2.44 ± 1.51 μg/m³. High mass concentrations of BC were observed during the south-west monsoon (SWM, 2.60 ± 1.56 μg/m³) while relatively low mass concentrations were recorded during the north-east monsoon (NEM, 1.68 ± 0.96 μg/m³). There was a shift in the Absorption Ångström exponent (AAE) from 1.1 to 1.4 when the origin of LACA changed from fossil fuel (FF) to biomass burning (BB) combustion. This shift is attributed to the presence of secondary BrC in LACA, derived from transboundary BB emissions during the SWM. Lower AAE values were observed when local traffic emissions were dominant during the NEM. This explanation is supported by measurements of water-soluble organic carbon (WSOC) in LACA and the corresponding AAE values determined at 365 nm using a UV-vis spectrophotometer. The AAE values, indicative of the presence of brown carbon (BrC), showed that photochemically aged LACA contribute to an enhancement in the light absorption of aerosols. In addition, spatio-temporal characteristics of BC in the intra-urban environment of Singapore were investigated across diverse outdoor and indoor microenvironments. High variability of BC was evident across these microenvironments. Several air pollution hotspots with elevated BC concentrations were identified. Overall, the results stress a need to control anthropogenic emissions of BC and BrC in order to mitigate near-term climate change impacts and provide health benefits.

Characterization of organic aerosols and their precursors in southern China during a severe haze episode in January 2017

The rapid industrialization and economic development in the Pearl River Delta (PRD) region of southern China have led to a substantial increase in anthropogenic emissions and hence frequent haze pollution over the past two decades. In early January 2017, a severe regional haze pollution episode was captured in the PRD region, with a peak PM_2.5 concentration of around 400μgm^-3, the highest value ever reported at this site. During the haze episode, elevated concentrations of oxygenated volatile organic compounds (OVOCs, 33±16 ppbv) and organic matter (41±15μg m^-3) were observed, indicating the enhanced roles of secondary organic aerosols (SOAs) in the formation of haze pollution. Water-soluble organic carbon (WSOC, 12.8±5.5μg C m^-3) dominated the organic aerosols, with a WSOC/OC ratio of 0.63±0.12 and high correlation (R=0.85) with estimated secondary organic carbon (SOC), suggesting the predominance of a secondary origin of the measured organic aerosols during the haze episode. Four carboxylic acids (oxalic, acetic, formic, and pyruvic acids) were characterized in the aerosols (1.30±0.38μgm^-3) and accounted for 3.6±1.2% of WSOC in carbon mass, with oxalic acid as the most abundant species. The simultaneous measurements of volatile organic compounds (VOCs), OVOCs, and organic acids in aerosols at this site provided an opportunity to investigate the relationship between the precursors and the products, as well as the potential formation pathways. Water-soluble aldehydes and ketones, predominantly produced via the oxidation of anthropogenic VOCs (mainly propane, toluene, n-butane, and m, p-xylene), were the main contributors of the organic acids. The formation of OVOCs is largely attributed to gas-phase photochemical oxidation, whereas the WSOC and dicarboxylic acids were produced from both photochemistry and nocturnal heterogeneous reactions. These findings provided further insights into the oxidation and evolution of organic compounds during the haze pollution episode.

Monoterpene and Sesquiterpene α-Hydroxy Organosulfates: Synthesis, MS/MS Characteristics, and Ambient Presence

Organosulfates (OSs) derived from biogenic volatile organic compounds are important compounds signifying interactions between anthropogenic sulfur pollution and natural emission. In this work, we substantially expand the OS standard library through the chemical synthesis of 26 α-hydroxy OS standards from eight monoterpenes (i.e., α- and β-pinene, limonene, sabinene, Δ³-carene, terpinolene, and α- and γ-terpinene) and two sesquiterpenes (i.e., α-humulene and β-caryophyllene). The sulfation of unsymmetrically substituted 1,2-diol intermediates produced a regioisomeric mixture of two OSs. The major regioisomeric OSs were isolated and purified for full NMR characterization, while the minor regioisomers could only be determined by liquid chromatograph-mass spectrometer (MS). The tandem mass spectra of the molecular ion formed through electrospray ionization confirmed the formation of abundant bisulfate ion fragments (m/z 97) and certain minor ion fragments characteristic of the carbon backbone. A knowledge of the MS/MS spectra and chromatographic retention times for authentic standards allows us to identify α-hydroxy OSs derived from six monoterpenes and β-caryophyllene in ambient samples. Notably, among two possible regioisomers of α-hydroxy OSs, we only detected the isomers with the sulfate group at the less substituted carbon position derived from α-pinene, limonene, sabinene, Δ³-carene, and terpinolene in the ambient samples. This observation sheds light on the atmospheric OS formation mechanisms.

Evaluation of factors influencing secondary organic carbon (SOC) estimation by CO and EC tracer methods

Secondary organic carbon (SOC) is known to account for a substantial fraction of fine-mode carbonaceous aerosol. Owing to a limited understanding of SOC formation processes and the absence of direct measurement methods, SOC concentrations are mostly estimated using a tracer-based method utilizing either elemental carbon (EC) or carbon monoxide (CO) as tracers. The performance of these tracer-based methods depends heavily on accurate determination of the (OC/Tracer)_pri value. The minimum R squared (MRS) method is currently recognized as a relatively reasonable tool to determine (OC/Tracer)_pri. This study estimated SOC based on the MRS method with EC and CO as tracers, followed by the Monte Carlo method to analyze quantitatively the effects of measurement uncertainty, emission scenario and sample size on SOC estimates. We report here four major findings: i) the concentration of O₃ was used as an indicator to atmospheric secondary reaction potential, and it was found that the mass proportion of SOC in total OC estimated by CO as the tracer is more consistent with the seasonality of actual secondary reaction potential; ii) the estimation results are highly sensitive to the measurement uncertainty in different emission scenarios, which leads us to conclude that the CO tracer method is more robust than the EC tracer method due to large inherent uncertainties in current EC measurements; iii) oversimplification of emission scenarios has substantial impacts on the estimated SOC value, and careful evaluation of the interdependence between sources should be performed to minimize this bias; and iv) the estimation bias of SOC can be reduced by increasing the sample size, and the tracer method can be expected to generate robust results for sample sizes over 1000. These findings are important in terms of providing a reference to choose appropriate tracers, emission scenarios and sample sizes for robust estimation of SOC in future studies.

Vapor-pressure pathways initiate but hydrolysis products dominate the aerosol estimated from organic nitrates

Organic nitrates contribute significantly to the total organic aerosol burden. However, the formation and loss mechanisms of particulate organic nitrates (PONs) remain poorly understood. In this study, with the CMAQ modeling system, we implement a detailed biogenic volatile organic carbon gas phase oxidation mechanism and an explicit representation of multiphase organic nitrate formation and loss, including both aqueous-phase uptake and vapor-pressure driven partitioning into organic aerosol as well as condensed-phase reactions. We find vapor-pressure dependent partitioning is the leading mechanism for formation of PONs and hydrolysis is a major loss mechanism for PON resulting in substantial amounts of organic aerosol that originate as an organic nitrate. Partitioning and hydrolysis together can produce high concentrations (up to 5 μg/m3) of PON-derived aerosols over the southeast United States. The main source of PON-derived aerosols is monoterpene nitrates that have been chemically processed to lose their nitrate functionality through aqueous chemistry. In contrast, the major portion of aqueous aerosol and in-cloud PON, which retains its nitrate moiety, are soluble isoprene nitrates. We evaluate the model using the observations from the Southern Oxidant and Aerosol Study (SOAS) campaign in the Southeast US in summer 2013 and show implementing aerosol-phase pathways for organic nitrates dramatically improves the magnitude of total alkyl nitrates (ANs) in CMAQ. The contribution of PONs to the total ANs at the SOAS site is estimated to be ~20%, a value in the range of the measurements. The predicted AN composition is shifted from monoterpene to isoprene and anthropogenic organic nitrates.

Size-Segregated Characteristics of Carbonaceous Aerosols during the Monsoon and Non-Monsoon Seasons in Lhasa in the Tibetan Plateau

In this paper, we intensively collected atmospheric particulate matter (PM) with different diameters (size ranges: <0.49, 0.49–0.95, 0.95–1.5, 1.5–3.0, 3.0–7.2, and >7.2 μm) in Lhasa during the monsoon and non-monsoon seasons. The results clearly showed that the concentrations of PM, organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) during the non-monsoon season were much higher than the concentrations during the monsoon season. During the monsoon season, a bimodal size distribution of the OC and WSOC, which were at <0.49 μm and >7.2 μm, respectively, and a unimodal size distribution at <0.49 μm for the EC were observed. However, during the non-monsoon season, there was a trimodal size distribution of the OC and WSOC (<0.49 μm, 1.5–3.0 μm, and >7.2 μm), and a unimodal size distribution of the EC (<0.49 μm). Possible sources of the carbonaceous components were revealed by combining the particle size distribution and the correlation analysis. OC, EC, and WSOC were likely from the photochemical transformation of biogenic and anthropogenic VOC, and the incomplete combustion of biomass burning and fossil fuels at <0.49 μm, whilst they were also likely to be from various types of dust and biogenic aerosols at >7.2 μm. OC and WSOC at 1.5–3.0 μm were likely to have been from the burning of yak dung and photochemical formation. The above results may draw attention in the public and scientific communities to the issues of air quality in the Tibetan Plateau.

Source signatures from combined isotopic analyses of PM2.5 carbonaceous and nitrogen aerosols at the peri-urban Taehwa Research Forest, South Korea in summer and fall

Isotopes are essential tools to apportion major sources of aerosols. We measured the radiocarbon, stable carbon, and stable nitrogen isotopic composition of PM_2.5 at Taehwa Research Forest (TRF) near Seoul Metropolitan Area (SMA) during August-October 2014. PM_2.5, TC, and TN concentrations were 19.4 ± 10.1 μg m^-3, 2.6 ± 0.8 μg C m^-3, and 1.4 ± 1.4 μg N m^-3, respectively. The δ¹³C of TC and the δ¹⁵N of TN were - 25.4 ± 0.7‰ and 14.6 ± 3.8‰, respectively. EC was dominated by fossil-fuel sources with F_ff (EC) of 78 ± 7%. In contrast, contemporary sources were dominant for TC with F_c (TC) of 76 ± 7%, revealing the significant contribution of contemporary sources to OC during the growing season. The isotopic signature carries more detailed information on sources depending on air mass trajectories. The urban influence was dominant under stagnant condition, which was in reasonable agreement with the estimated δ¹⁵N of NH₄⁺. The low δ¹⁵N (7.0 ± 0.2‰) with high TN concentration was apparent in air masses from Shandong province, indicating fossil fuel combustion as major emission source. In contrast, the high δ¹⁵N (16.1 ± 3.2‰) with enhanced TC/TN ratio reveals the impact of biomass burning in the air transported from the far eastern border region of China and Russia. Our findings highlight that the multi-isotopic composition is a useful tool to identify emission sources and to trace regional sources of carbonaceous and nitrogen aerosols.

Urban pollution greatly enhances formation of natural aerosols over the Amazon rainforest

One of the least understood aspects in atmospheric chemistry is how urban emissions influence the formation of natural organic aerosols, which affect Earth's energy budget. The Amazon rainforest, during its wet season, is one of the few remaining places on Earth where atmospheric chemistry transitions between preindustrial and urban-influenced conditions. Here, we integrate insights from several laboratory measurements and simulate the formation of secondary organic aerosols (SOA) in the Amazon using a high-resolution chemical transport model. Simulations show that emissions of nitrogen-oxides from Manaus, a city of ~2 million people, greatly enhance production of biogenic SOA by 60-200% on average with peak enhancements of 400%, through the increased oxidation of gas-phase organic carbon emitted by the forests. Simulated enhancements agree with aircraft measurements, and are much larger than those reported over other locations. The implication is that increasing anthropogenic emissions in the future might substantially enhance biogenic SOA in pristine locations like the Amazon.

Drought Impacts on Secondary Organic Aerosol: A Case Study in the Southeast United States

Secondary organic aerosol (SOA) is a significant component of fine particulate matter, and it has increased during past drought periods in the U.S. Here, we use the Community Multiscale Air Quality (CMAQ) model to characterize the complex effects of drought on SOA through a case study comparing a drought period (June 2011) and a wet period (June 2013) over the southeast U.S. The model simulates a 68% (1.7 μg/m³) higher SOA concentration at the surface during drought and attributes 98% of this increase to biogenic SOA. Through model sensitivity simulations, the SOA increase associated with drought is attributed to 54% from accelerated gas-phase reactions oxidizing volatile organic compounds (VOCs) to SOA, 45% from higher emissions of biogenic VOCs, 18% from enhanced acid-catalyzed production of isoprene SOA in aerosol water due to changing sulfate, 3% from enhanced in-cloud aqueous phase chemistry. Because the higher SOA levels overwhelm the reduced precipitation, there is an increase in wet deposition flux in the drought month which offsets 20% of the total SOA increase. If anthropogenic emissions are held constant, anthropogenic SOA is 51% higher during drought, highlighting the importance of meteorological impacts on chemistry.

Light absorption of organic carbon and its sources at a southeastern U.S. location in summer

Light-absorbing organic carbon (OC), also referred to as "brown carbon" (BrC), has been intensively investigated in atmospheres impacted by biomass burning. However, other BrC sources (e.g., secondary formation in the atmosphere) are rarely studied in ambient aerosols. In the current work, forty-five PM_2.5 filter samples were collected in Research Triangle Park (RTP), NC, USA from June 1st to July 15th, 2013. The bulk carbonaceous components, including OC, elemental carbon (EC), water soluble OC (WSOC), and an array of organic molecular markers were measured; an ultraviolet/visible spectrometer was used to measure the light absorption of methanol extractable OC and WSOC. The average light absorption per OC and WSOC mass of PM_2.5 samples in summer RTP are 0.36 ± 0.16 m² gC^-1 and 0.29 ± 0.13 m² gC^-1, respectively, lower than the ambient aerosol samples impacted by biomass burning and/or fossil fuel combustion (0.7-1.6 m² gC^-1) from other places. Less than 1% of the aqueous extracts absorption is attributed to the light-absorbing chromophores (nitroaromatic compounds) identified in this work. To identify the major sources of BrC absorption in RTP in the summer, Positive Matrix Factorization (PMF) was applied to a dataset containing optical properties and chemical compositions of carbonaceous components in PM_2.5. The results suggest that the formation of biogenic secondary organic aerosol (SOA) containing organosulfates is an important BrC source, contributing up to half of the BrC absorption in RTP during the summertime.

Characteristics of Carbonaceous PM2.5 in a Small Residential City in Korea

PM2.5 has been a serious issue in South Korea not only in urban and industrial areas but also in rural and background areas. In this study, PM2.5 and its carbonaceous compounds including organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and polycyclic aromatic hydrocarbons (PAHs) were collected and analyzed in a small residential city. The PM2.5 concentration frequently exceeded the national ambient air quality standard during the spring and the winter, which often occurred concurrently with fog and mist events. Over the whole sampling period, both OC and the OC/EC ratio were considerably higher than the ratios in other cities in Korea, which suggests that sources other than vehicular emissions were important. The top 10% of OC/EC ratio samples could be explained by regional and long-range transport because there was a strong correlation between primary and secondary organic carbon. However, biomass combustion was likely to account for the consistently high OC concentration due to a strong correlation between WSOC and primary OC as well as the diagnostic ratio results of PAHs.

Comparison of Measurement-Based Methodologies to Apportion Secondary Organic Carbon (SOC) in PM2.5: A Review of Recent Studies

Secondary organic aerosol (SOA) is known to account for a major fraction of airborne particulate matter, with significant impacts on air quality and climate at the global scale. Despite the substantial amount of research studies achieved during these last decades, the source apportionment of the SOA fraction remains difficult due to the complexity of the physicochemical processes involved. The selection and use of appropriate approaches are a major challenge for the atmospheric science community. Several methodologies are nowadays available to perform quantitative and/or predictive assessments of the SOA amount and composition. This review summarizes the current knowledge on the most commonly used approaches to evaluate secondary organic carbon (SOC) contents: elemental carbon (EC) tracer method, chemical mass balance (CMB), SOA tracer method, radiocarbon (14C) measurement and positive matrix factorization (PMF). The principles, limitations, challenges and good practices of each of these methodologies are discussed in the present article. Based on a comprehensive—although not exhaustive—review of research papers published during the last decade (2006–2016), SOC estimates obtained using these methodologies are also summarized for different regions across the world. Conclusions of some studies which are directly comparing the performances of different methodologies are then specifically discussed. An overall picture of SOC contributions and concentrations obtained worldwide for urban sites under similar conditions (i.e., geographical and seasonal ones) is also proposed here. Finally, further needs to improve SOC apportionment methodologies are also identified and discussed.

Seasonal variation and size distribution of biogenic secondary organic aerosols at urban and continental background sites of China

Size-resolved biogenic secondary organic aerosols (BSOA) derived from isoprene and monoterpene photooxidation in Qinghai Lake, Tibetan Plateau (a continental background site) and five cities of China were measured using gas chromatography/mass spectrometry (GC/MS). Concentrations of the determined BSOA are higher in the cities than in the background and are also higher in summer than in winter. Moreover, strong positive correlations (R²=0.44-0.90) between BSOA and sulfate were found at the six sites, suggesting that anthropogenic pollution (i.e., sulfate) could enhance SOA formation, because sulfate provides a surface favorable for acid-catalyzed formation of BSOA. Size distribution measurements showed that most of the determined SOA tracers are enriched in the fine mode (<3.3μm) except for cis-pinic and cis-pinonic acids, both presented a comparable mass in the fine and coarse (>3.3μm) modes, respectively. Mass ratio of oxidation products derived from isoprene to those from monoterpene in the five urban regions during summer are much less than those in Qinghai Lake region. In addition, in the five urban regions relative abundances of monoterpene oxidation products to SOA are much higher than those of isoprene. Such phenomena suggest that BSOA derived from monoterpenes are more abundant than those from isoprene in Chinese urban areas.

Experimental and model estimates of the contributions from biogenic monoterpenes and sesquiterpenes to secondary organic aerosol in the southeastern United States

Atmospheric organic aerosol (OA) has important impacts on climate and human health but its sources remain poorly understood. Biogenic monoterpenes and sesquiterpenes are important precursors of secondary organic aerosol (SOA), but the amounts and pathways of SOA generation from these precursors are not well constrained by observations. We propose that the less-oxidized oxygenated organic aerosol (LO-OOA) factor resolved from positive matrix factorization (PMF) analysis on aerosol mass spectrometry (AMS) data can be used as a surrogate for fresh SOA from monoterpenes and sesquiterpenes in the southeastern US. This hypothesis is supported by multiple lines of evidence, including lab-in-the-field perturbation experiments, extensive ambient ground-level measurements, and state-of-the-art modeling. We performed lab-in-the-field experiments in which the ambient air is perturbed by the injection of selected monoterpenes and sesquiterpenes, and the subsequent SOA formation is investigated. PMF analysis on the perturbation experiments provides an objective link between LO-OOA and fresh SOA from monoterpenes and sesquiterpenes as well as insights into the sources of other OA factors. Further, we use an upgraded atmospheric model and show that modeled SOA concentrations from monoterpenes and sesquiterpenes could reproduce both the magnitude and diurnal variation of LO-OOA at multiple sites in the southeastern US, building confidence in our hypothesis. We estimate the annual average concentration of SOA from monoterpenes and sesquiterpenes in the southeastern US to be roughly 2 μg m-3.

Additional Benefits of Federal Air-Quality Rules: Model Estimates of Controllable Biogenic Secondary Organic Aerosol

Atmospheric models that accurately describe the fate and transport of trace species for the right reasons aid in the development of effective air-quality management strategies that safeguard human health. Controllable emissions facilitate the formation of biogenic secondary organic aerosol (BSOA) to enhance the atmospheric fine particulate matter (PM2.5) burden. Previous modeling with the EPA's Community Multiscale Air Quality (CMAQ) model predicted that anthropogenic primary organic aerosol (POA) emissions had the greatest impact on BSOA. That experiment included formation processes involving semivolatile partitioning but not aerosol liquid water (ALW), a ubiquitous PM constituent. We conduct 17 summertime CMAQ simulations with updated chemistry and evaluate changes in BSOA due to the removal of individual pollutants and source sectors for the contiguous U.S. CMAQ predicts SO2 from electricity generating units, and mobile source NOX emissions have the largest impacts on BSOA. The removal of anthropogenic NOX, SO2, and POA emissions during the simulation reduces the nationally averaged BSOA by 23, 14, and 8% and PM2.5 by 9.2, 14, and 5.3%, respectively. ALW mass concentrations decrease by 10 and 35% in response to the removal of NOX and SO2 emissions. This work contributes chemical insight into ancillary benefits of Federal NOX and SO2 rules that concurrently reduce organic PM2.5 mass.