A review of aerosol chemistry in Asia: insights from aerosol mass spectrometer measurements

Understanding secondary particles in a regional site of Yangtze River Delta: Insights from mass spectrometric measurement

Submicron particulate matter (PM1) poses significant risks to health risks and global climate. In this study, secondary organic aerosols (SOA) and inorganic compositions were examined for their physicochemical characteristics and evolution using high-resolution aerosol instruments in Changzhou over one-month period. The results showed that transport accompanied by regional static conditions leaded to the occurrence of heavy pollution. In addition, regional generation and local emissions also leaded to the occurrence of light and moderate pollution during the observation period in Changzhou. Organic aerosols (OA) and nitrate (NO3-) accounted for 45 % and 23 % of PM1, respectively. The increase in PM1 was dominated by the contribution of NO3- and OA. SOA was dominance in OA (63 % with 40 % MO-OOA), which was higher than primary organic aerosols (POA). Besides, photochemical reactions and the high oxidizing nature of the urban atmosphere promoted the production of OA, especially MO-OOA in Changzhou. Our results highlight that secondary particles contribute significantly to PM pollution in Changzhou, underlining the importance of controlling emissions of gaseous precursors, especially under high oxidation conditions.

Seasonal characterization of chemical and optical properties of water-soluble organic aerosol in Beijing

Water-soluble organic aerosol (WSOA) plays a crucial role in altering radiative forcing and impacting human health. However, our understanding of the seasonal variations of WSOA in Chinese megacities after the three-year clean air action plan is limited. In this study, we analyzed PM2.5 filter samples collected over one year (2020-2021) in Beijing to characterize the seasonal changes in the chemical and optical properties of WSOA using an offline aerosol mass spectrometer along with spectroscopy techniques. The mean mass concentration of WSOA during the observation period was 8.84 ± 7.12 μg m-3, constituting approximately 64-67 % of OA. Our results indicate the contribution of secondary OA (SOA) increased by 13-28 % due to a substantial reduction in primary emissions after the clean air action plan. The composition of WSOA exhibited pronounced seasonal variations, with a predominant contribution from less oxidized SOA in summer (61 %) and primary OA originating from coal combustion and biomass burning during the heating season (34 %). The mass absorption efficiency of WSOA at 365 nm in winter was nearly twice that in summer, suggesting that WSOA from primary emissions possesses a stronger light-absorbing capability than SOA. On average, water-soluble brown carbon accounted for 33-48 % of total brown carbon absorption. Fluorescence analysis revealed humic-like substances as the most significant fluorescence component of WSOA, constituting 82 %. Furthermore, both absorption and fluorescence chromophores were associated with nitrogen-containing compounds, highlighting the role of nitrogen-containing species in influencing the optical properties of WSOA. The results are important for chemical transport models to accurately simulate the WSOA and its climate effects.

Influence of ozone pollution on the mixing state and formation of oxygenated organics containing single particles

The formation and aging processes of oxygenated organic molecules (OOMs) are important for understanding the formation mechanisms of secondary organic aerosols (SOAs) in the field. In this study, we investigated the mixing states of OOM particles by identifying several oxygenated species along with the distributions of secondary organic carbon (SOC) during both clean and ozone (O3)-polluted periods in July and September of 2022 in Guangzhou, China. OOM-containing particles accounted for 57 % and 49 % of the total detected single particles in July and September, respectively. Most of the OOM particles were internally mixed with sulfate and nitrate, while elemental carbon and hydrocarbon species were absent. Despite the higher SOC/OC ratio in September (81 %) than it in July (72 %), comparative investigations of the mass spectra, diurnal patterns, and distributions of OOM particles revealed the same composition and aging states of OOMs in two O3 pollution periods. As the O3 concentration increased from the clean to the polluted periods, the ratio of SOC to OC increased along with the relative abundance of secondary OOM particles among total OOM particles. In contrast, the relative abundance of OC-type OOM particles gradually decreased, indicating the conversion of hydrocarbon species into OOMs as the SOC/OC ratio increased. Both the bulk analysis of SOC from filter measurement and the mixing states of OOM particles suggested that OOM production and degree of oxidation were higher in the O3-polluted periods than in the clean periods. These results elucidate the effects of O3 pollution on the OOM formation process and offer new perspectives for the joint investigation of SOA production based on filter sampling and single-particle measurements.

Chemical characterization, source identification and potential health effects of PM2.5-bound non-polar organic compounds over a COALESCE network site - Bhopal, India

Year-long (2019) measurements of carbonaceous aerosols were performed at Bhopal, a regionally representative site as a part of the COALESCE (Carbonaceous Aerosol Emissions, Source apportionment and Climate Impacts) campaign. Aerosol-associated non-polar organic compounds (NPOCs) were analysed using thermal desorption (TD) Gas chromatography/Mass spectrometry (TD-GC/MS). The annual average of the total organic carbon (OC), elemental carbon (EC), and analysed PAHs (Polycyclic Aromatic Hydrocarbons), and n-alkanes were, 9.74 ± 9.47 μg m-3, 2.13 ± 3.12 μg m-3, 10.43 ± 5.49 ng m-3, and 114.93 ± 49.24 ng m-3, respectively. PAHs diagnostic ratios suggested emissions from petroleum, grass, wood, and coal combustion. Combustion derived PAHs (CombPAHs) accounted for 72.5 % of the total measured PAHs. During wintertime, based on Pyr/BaP ratio (∼0.6), gasoline exhaust emissions were higher compared to diesel exhaust emissions. The weak correlations between PAHs and meteorological parameters suggested that variations in PAH levels are primarily driven by alterations in emission sources. Total PAHs were correlated moderately with BrC (r2 = 0.60). The estimated lifetime lung cancer risk (LLCR) values on exposure to 16 USEPA priority PAHs (5 × 10-5) demonstrated that PAH levels in this region pose moderate health risks. Given observations from only campaign mode short-term measurements of NPOCs over India, this work provides a more comprehensive understanding of the concentrations, seasonal variations, and sources of n-alkanes and health risk associated with particle bound PAHs over the data-poor central Indian region.

Differentiated emissions and secondary organic aerosol formation potential of organic vapor from industrial coatings in China

Organic vapors emitted during solvent use are important precursors of secondary organic aerosols (SOAs). Industrial coatings are a major class of solvents that emit volatile and intermediate volatile organic compounds (VOCs and IVOCs, respectively). However, the emission factors and source profiles of VOCs and IVOCs from industrial coatings remain unclear. In this study, representative solvent- and water-based industrial paints were evaporated, sampled and tested using online and offline instruments. The VOC and IVOC emission factors for solvent-based paints are 129-254 and 25-80 g/kg, while for water-based paint are 13 and 32 g/kg, respectively. In solvent-based paints, the VOCs are mainly aromatics, while the IVOCs are composed of long-chain alkanes, alkenes, carbonyls and halocarbons. The VOCs and IVOCs in water-based paint are mostly oxygenates, such as ethanol, acetone, ethylene glycol, and Texanol. During the evaporation of solvent-based paints, the fraction of IVOCs increases along with those of alkenes and aldehydes, while the proportion of aromatics decreases. For water-based paint, the fraction of IVOCs slightly decreases with evaporation. The SOA formation potentials of solvent-based paints are 8.6-28.0 g/kg, much higher than that of water-based paint (0.65 g/kg); thus, substituting solvent-based paints with water-based paints may significantly decrease SOA formation.

Molecular tracers, mass spectral tracers and oxidation of organic aerosols emitted from cooking and fossil fuel burning sources

Secondary organic aerosol (SOA) composes a substantial fraction of atmospheric particles, yet the formation and aging mechanism of SOA remains unclear. Here we investigate the initial oxidation of primary organic aerosol (POA) and further aging of SOA in winter Beijing by using aerosol mass spectrometer (AMS) measurements along with offline molecular tracer analysis. Multilinear engine (ME-2) source apportionment was conducted to capture the characteristic of source-related SOA, and connect them with specific POA. Our results show that urban cooking and fossil fuel burning sources contribute significantly (17 % and 20 %) to total organic aerosol (OA) in winter Beijing. Molecular tracer analysis by two-dimensional gas chromatography-time-of-flight mass spectrometer (GC × GC-ToF-MS) reveals that cooking SOA (CSOA) is produced through both photooxidation and aqueous-phase processing, while less oxidized SOA (LO-SOA) is the photooxidation product of fossil fuel burning OA (FFOA) and may experience aqueous-phase aging to form more-oxidized oxygenated OA (MO-OOA). Furthermore, CHO_{m/z 69} and CHO_{m/z 85} are mass spectral tracers indicating the initial photooxidation, while CHO₂⁺ and C₂H₂O₂⁺ imply further aqueous-phase aging of OA. Tracer analysis indicates that the formation of diketones is involved in the initial photooxidation of POA, while the formation of glyoxal and diacids is involved in the further aqueous-phase aging of SOA.

Secondary organic aerosol formation in China from urban-lifestyle sources: Vehicle exhaust and cooking emission

An increasing number of people tend to live in cities, where they suffer from serious air pollution from anthropogenic sources. Vehicle exhaust and cooking emission are closely related to daily life of urban residents, and could be defined as "urban-lifestyle sources". The primary emissions of urban-lifestyle sources tend to form abundant secondary organic aerosols (SOA) through complicated atmospheric chemistry processes. The newly formed SOA is a kind of complex mixture and causes considerable health effects with high uncertainty. Most studies focus on formation pathway, mass growth potential and chemical feature of urban-lifestyle SOA under simple laboratory conditions. Few studies have measured the urban-lifestyle SOA in ambient air, let alone verified laboratory findings under complicated atmospheric conditions. In this work, we established a new method that combined laboratory simulation and field observation, which quantified the urban-lifestyle SOA with high time resolution under the real atmospheric condition. The complex SOA was measured and resolved by a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The multilinear engine model (ME-2) and multilinear correction methods were used to apply laboratory results into ambient SOA apportionment. It was found that the vehicle source dominated the SOA formation during the diurnal photochemical process, and the SOA:POA ratio of vehicle source was about 1.4 times larger than that of cooking source. The vehicle emission may undergo an alcohol/peroxide & carboxylic acid oxidation pathway and form higher oxidized SOA, while the cooking emission may undergo an alcohol/peroxide oxidation pathway and form relatively lower oxidized SOA. The vehicle SOA and cooking SOA contributed 45.6 % and 24.8 % of OA during a local episode in 2021 winter of downtown Beijing. Our findings could not only provide a new way to quantify urban SOA but also demonstrate some laboratory hypotheses, conducing to understand its ambient contributions, chemical features, and environmental effects.

Single-particle volatility and implications for brown carbon absorption in Beijing, China

Aerosol volatility has a substantial impact on gas-particle partitioning, aging process and hence brown carbon (BrC) absorption. Here we analyzed single-particle volatility in winter in Beijing using a thermodenuder coupled with a single particle aerosol mass spectrometer along with a suite of collocated measurements. Our results showed that elemental carbon, metals, organic nitrogen (ON) were the dominant low-volatility components. The ON-containing particles accounting for 50 % of the total low-volatility particles comprised mainly ON-organic carbon (ON-OC) particles which were associated with biomass burning and significantly enhanced during polluted periods with high relative humidity and nitrogen oxides (NOx) levels. By analyzing the relationship between single-particle volatility and BrC, we found that semi-volatile particles related to fossil fuel combustion contributed dominantly to the light absorption of BrC (~50 %). Comparatively, the low-volatility and semi-volatile particles related to biomass burning contributed 21-35 % and 10-15 %, respectively to the BrC light absorption. Our results demonstrated that single particles from different sources with different volatility showed different impacts on BrC absorption. Although low-volatility organic aerosol accounted only for ~16 % of the total ambient organics, they can contribute as much as ~30-40 % to BrC light absorption in winter in Beijing.

Tracing the predominant sources of carbon in PM2.5 using δ13C values together with OC/EC and select inorganic ions over two COALESCE locations

As part of the COALESCE (Carbonaceous Aerosol Emissions, Source apportionment and Climate Impacts) campaign, ambient PM_2.5 was collected at two regional sites (Bhopal and Mysuru) in India during 2019. We utilized organic carbon (OC), elemental carbon (EC) and water-soluble inorganic ions together with δ¹³C values, to better understand total carbon (TC) sources at these locations. The annual average δ¹³C values (-26.2 ± 0.6‰) at Mysuru and Bhopal (-26.6 ± 0.6‰) were comparable. However, at Mysuru, except during winter, day-to-day variability was much lower (narrow range of -26.8 to -26.0‰) than that at Bhopal (range: -28.1 to -24.7‰), suggesting that TC was contributed by few sources, likely dominated by vehicular emissions. Seasonal average δ¹³C values at Bhopal increased slightly (-25.8 ± 0.5‰) during the winter (Jan-Feb) and decreased (-27.0 ± 0.3‰) during the monsoon (Jun-Sep) season compared to the annual average. The decrease in δ¹³C values during the monsoon season was likely driven by enhanced secondary organic aerosol formation. Further, based on MODIS derived fire spots and back trajectories, we infered that the δ¹³C values (-27.5 to -26.0‰) in Bhopal during the post-monsoon season (Oct-Dec) were indicative of dominant biomass burning contributions. The inorganic ions/TC ratio during this season suggested that biomass burning aerosol was aged and may have been transported from crop residue burning in the Indo-Gangetic plains. At Mysuru, like the trend at Bhopal, the δ¹³C values during the monsoon season were lower than those during the winter season. Finally, δ¹³C values were input to a Bayesian model-MixSIAR to demonstrate the usefulness of such models in apportioning TC. In its simplest implementation, the model separated TC sources into fossil fuel emissions and non-fossil fuel sources . Fossil fuel combustion emissions accounted for 47 ± 19% and 62 ± 22% of the TC at Bhopal and Mysuru, respectively.

Parameterization of the ambient aerosol refractive index with source appointed chemical compositions

The refractive index of ambient aerosols is widely used in the climate model and remote sensing. Traditionally, the real part of the refractive index (RRI) of the ambient aerosol is calculated from the measured mass fraction of the main inorganic components with known refractive index, without full resolving the effects of variation in the RRI of organic components, which always contribute more than 50 % of the total aerosol mass. For the first time, the ambient aerosol RRI and the aerosol chemical components were measured concurrently at a suburban site Changping, in Beijing, China. Measurements results show that the ambient aerosol ranges between 1.57 and 1.71 with a mean value of 1.66. The mean mass fractions of organic aerosol (OA), nitrate, sulfate, ammonium, and chloride to total non-refractory aerosol loading are 43.1 %, 21.9 %, 21.6 %, 13.1 %, and 0.3 % respectively. Source appointment analysis of the organic aerosol show that the fossil fuel-related OA, cooking OA, biomass burning OA, less oxidized oxygenated OA and more oxidized OOA contributes 18.0 %, 11.2 %, 4.1 %, 39.9 %, 26.7 % to the total aerosol. A new parameterization scheme of the ambient aerosol RRI, which considers the source appointed OA, is proposed based on the concurrent measurements of RRI and chemical composition. The measured and parameterized RRI shows good consistency with a correlation coefficient of 0.79 and slope of 0.98. Our measurement results reveal that a significant deviation of the calculated RRI exists without considering the variation of the RRI of the aerosol organic component. The parametrization scheme is adopted and applicable in aerosol model for bettering estimating the corresponding optical and radiative effects.

Secondary organic aerosol formation and aging from ambient air in an oxidation flow reactor during wintertime in Beijing, China

Secondary organic aerosols (SOA) constitute a large fraction of atmospheric aerosols, yet our knowledge of the formation and aging processes of SOA in megacities of China is still limited. In this work, the formation and aging processes of SOA in winter in Beijing was investigated using a high-resolution aerosol mass spectrometer (AMS) and an oxidation flow reactor (OFR). Our results showed that the OA enhancement from OH aging peaked at ∼3.9 equivalent days with an average enhancement of 0.9 (±0.3) μg m^-3. Positive matrix factorization analysis of AMS-OFR data identified three primary OA (POA) and two SOA factors. While the concentrations of POA factors decreased as a function of photochemical age, the two SOA factors showed clear enhancements by 2.5 and 4.3 μg m^-3 at ∼3.9 and ∼2.6 days of equivalent photochemical age, respectively. The average contribution of SOA to the total OA was 47% in ambient air and 87% in OFR-oxidized ambient air. The elevated oxygen-to-carbon (O/C) ratio from 0.49 to 0.77-0.82 and the decreased hydrogen-to-carbon (H/C) from 1.37 to ∼1.1 highlighted the formation of more oxidized SOA during photochemical aging in winter in Beijing. The ubiquitous SOA enhancement as a function of OA levels indicated the significant formation potential of SOA in winter, and it varied differently among different episodes. In particular, we observed a maximum SOA enhancement of 38.6 μg m^-3 during a biomass burning event. This result demonstrates that photochemical oxidation of ubiquitous biomass burning emissions can be a large source of SOA in winter in North China Plain.

Chemical characterization and sources of submicron aerosols in Lhasa on the Qinghai-Tibet Plateau: Insights from high-resolution mass spectrometry

In recent years, a great number of studies has been carried out in urban cities regarding urban particulate matter (PM) pollution in China, especially in eastern China. Lhasa, the capital of the Tibet Autonomous Region in western China, is the highest (3650 m a.s.l.) city in China and has notably different lifestyles and PM sources comparing with those in eastern China. However, there is currently a lack of studies on PM pollution in this city. In this study, an Aerodyne high-resolution time-of-flight aerosol mass spectrometer was deployed along with other co-located instruments to explore the chemical characterization of ambient submicron PM (PM₁) in Lhasa from 31 August 2019 to 26 September 2019. The mean ambient PM₁ mass loading through this study was 4.72 μg m^-3. Organic aerosols (OAs) played a dominant role with an average contribution of 82.6% to PM₁, followed by 5.4% nitrate, 4.7% ammonium, 3.4% sulfate, 3.1% BC, and 0.7% chloride. The relatively lower contribution from secondary inorganic aerosols (nitrate and sulfate) in this study was distinctly different from that in eastern China, indicating lower fossil fuel usage in this city. Via positive matrix factorization (PMF), organic aerosols were decomposed into four components containing a traffic-related hydrocarbon-like OA (HOA), a cooking-related OA (COA), a biomass burning-related OA (BBOA), as well as an oxygenated OA (OOA). The OOA and COA had higher contributions (34% and 35%, respectively) to total OAs, while the rest accounted for 17% for HOA and 14% for BBOA. However, an increased mass fraction of BBOA (up to 36%) was found during the Sho Dun Festival, suggesting the importance of biomass burning emissions during the religious activities in this city. Frequent new particle formation events were observed during this study and the contribution of chemical species for the particle growth was also explored.

Source apportionment of carbonaceous aerosols in diverse atmospheric environments of China by dual-carbon isotope method

Carbonaceous aerosols are major components in PM_2.5 of both polluted and clean atmosphere. Accurate source apportionment of carbonaceous aerosols may support effective PM_2.5 control. Dual-carbon isotope method (¹⁴C and ¹³C) was adopted to identify the contribution of three main air pollution sources biogenic and biomass (f_bb), liquid fossil (f_liq.fossil) and coal (f_coal). The aerosol samples were collected at three types of sites with distinctly different degree of air pollution: urban, rural and regional background. The seasonal variation of source apportionment of the carbonaceous aerosols in urban Beijing was discussed. Modern biogenic and biomass made an absolute dominance of 92.9 ± 0.5% contribution to the carbonaceous aerosols at the background site Mt. Yulong due to long-range transport from Southeast Asia. The three main sources contributed jointly to the atmospheric carbonaceous aerosols at the rural site Wangdu and the urban site Beijing. The biogenic and biomass source was the major contribution in summer (47.0 ± 0.3%) and autumn (49.3 ± 0.3%) of Beijing, while coal source increased from summer (26.8 ± 13.8%) to autumn (34.7 ± 11.5%). Heating significantly increased the coal source to the dominant contribution (47.0 ± 16.9%) in winter of Beijing. Separate day and night time coal contributions were used to evaluate the two origins of coal combustion: industrial use vs. residential use. The results of source apportionment for carbonaceous aerosols provide scientific support for the prevention and control of air pollution.

Facial Fabrication of Large-Scale SERS-Active Substrate Based on Self-Assembled Monolayer of Silver Nanoparticles on CTAB-Modified Silicon for Analytical Applications

Surface-enhanced Raman spectroscopy (SERS) has been proven to be a promising analytical technique with sensitivity at the single-molecule level. However, one of the key problems preventing its real-world application lies in the great challenges that are encountered in the preparation of large-scale, reproducible, and highly sensitive SERS-active substrates. In this work, a new strategy is developed to fabricate an Ag collide SERS substrate by using cetyltrimethylammonium bromide (CTAB) as a connection agent. The developed SERS substrate can be developed on a large scale and is highly efficient, and it has high-density “hot spots” that enhance the yield enormously. We employed 4-methylbenzenethiol(4-MBT) as the SERS probe due to the strong Ag–S linkage. The SERS enhancement factor (EF) was calculated to be ~2.6 × 10⁶. The efficacy of the proposed substrate is demonstrated for the detection of malachite green (MG) as an example. The limit of detection (LOD) for the MG assay is brought down to 1.0 × 10⁻¹¹ M, and the relative standard deviation (RSD) for the intensity of the main Raman vibration modes (1620, 1038 cm⁻¹) is less than 20%.

Real-time characterization of aerosol particle composition, sources and influences of increased ventilation and humidity in an office

Most of human exposure to atmospheric pollutants occurs indoors, and the components of outdoor aerosols may have been changed in the way before reaching indoor spaces. Here we conducted real-time online measurements of mass concentrations and chemical composition of black carbon and the non-refractory species in PM_2.5 in an occupied office for approximately one month. The open-close windows and controlled dampness experiments were also performed. Our results show that indoor aerosol species primarily originate from outdoors with indoor/outdoor ratio of these species typically less than unity except for certain organic aerosol (OA) factors. All aerosol species went through filtration upon transport indoors. Ammonium nitrate and fossil fuel OA underwent evaporation or particle-to-gas partitioning, while less oxidized secondary OA (SOA) underwent secondary formation and cooking OA might have indoor sources. With higher particulate matter (PM) mass concentration outdoors than in the office, elevated natural ventilation increased PM exposure indoors and this increased exposure was prolonged when outdoor PM was scavenged. We found that increasing humidity in the office led to higher indoor PM mass concentration particularly more oxidized SOA. Overall, our results highlight that indoor exposure of occupants is substantially different from outdoor in terms of mass concentrations and chemical species.

Using Micro-Raman Spectroscopy to Investigate Chemical Composition, Mixing States, and Heterogeneous Reactions of Individual Atmospheric Particles

Measuring the chemical composition of individual atmospheric aerosol particles can provide direct evidence of their heterogeneous reactions and mixing states in the atmosphere. In this study, micro-Raman spectroscopy was used to measure the chemical composition of 1200 individual atmospheric particles in 11 samples collected in Beijing air. (NH₄)₂SO₄, NH₄NO₃, various minerals, carbonaceous species (soot and organics), and NaNO₃ were identified in the measured particles according to their characteristic Raman peaks. These species represented the main components of aerosol particles. In individual particles, NH₄NO₃ and (NH₄)₂SO₄ either existed separately or were internally mixed. Possible reaction pathways of CaCO₃ particles in the atmosphere were proposed based on the results of this study and laboratory simulations on heterogeneous reactions in the literature. CaCO₃ reacted with N- and S-containing (nitrogen- and sulfur-containing) acidic gases to produce Ca(NO₃)₂ and CaSO₄. Ca(NO₃)₂ further reacted with S-containing acidic gases and oxidants to produce CaSO₄. Of the soot-containing particles, 23% were internal mixtures of soot and inorganic material. Of the organics-containing particles, 57% were internal mixtures of organic and inorganic materials. Micro-Raman spectroscopy directly identified functional groups and molecules in individual atmospheric particles under normal ambient conditions, rendering it a powerful tool for measuring the chemical composition of individual atmospheric particles with a diameter of ≥1.0 μm.

Larger than expected variation range in the real part of the refractive index for ambient aerosols in China

The real part of the refractive index (RRI) of ambient aerosol, which is widely used in remote sensing and atmospheric models, is one of the key factors determining its particles' optical properties. The characteristics of ambient aerosol RRI in China have not yet been well studied owing to a lack of observations. For the first time, the properties of aerosol RRI were studied based on field measurements in China at four sites with different atmospheres. The results revealed that the measured ambient aerosol RRI varied significantly between 1.36 and 1.78, increasing with the mass ratio of organic components. The scattering coefficient and direct radiative effects of the aerosols were estimated to increase by factors of 2 and 3, respectively, when RRI increased from 1.36 to 1.78. Our results indicate that variation in ambient aerosol RRI should be considered in aerosol and climate models to achieve an accurate estimation of aerosol's radiative impacts.

Characteristics and sources of water-soluble organic aerosol in a heavily polluted environment in Northern China

Water-soluble organic aerosol (WSOA) in fine particles (PM_2.5) collected during wintertime in a polluted city (Handan) in Northern China was characterized using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (AMS). Through comparing with real-time measurements from a collocated Aerosol Chemical Speciation Monitor (ACSM), we determined that WSOA on average accounts for 29% of total organic aerosol (OA) mass and correlates tightly with secondary organic aerosol (SOA; Pearson's r = 0.95). The mass spectra of WSOA closely resemble those of ambient SOA, but also show obvious influences from coal combustion and biomass burning. Positive matrix factorization (PMF) analysis of the WSOA mass spectra resolved a water-soluble coal combustion OA (WS-CCOA; O/C = 0.17), a water-soluble biomass burning OA (WS-BBOA; O/C = 0.32), and a water-soluble oxygenated OA (WS-OOA; O/C = 0.89), which account for 10.3%, 29.3% and 60.4% of the total WSOA mass, respectively. The water-solubility of the OA factors was estimated by comparing the offline AMS analysis results with the ambient ACSM measurements. OOA has the highest water-solubility of 49%, consistent with increased hygroscopicity of oxidized organics induced by atmospheric aging processes. In contrast, CCOA is the least water soluble, containing 17% WS-CCOA. The distinct characteristics of WSOA from different sources extend our knowledge of the complex aerosol chemistry in the polluted atmosphere of Northern China and the water-solubility analysis may help us to understand better aerosol hygroscopicity and its effects on radiative forcing in this region.

Long-term characterization of aerosol chemistry in cold season from 2013 to 2020 in Beijing, China

Severe haze episodes in cold season in Beijing have been mitigated greatly during the last decade. However, the changes in aerosol chemistry as responses to the large reductions in gaseous precursors during the two phases of clean air action, i.e., phase Ⅰ (2013-2017) and phase Ⅱ (2018-2020), are less understood. Here we characterized such changes in cold season (January-March) by using five-year real-time aerosol particle composition measurements. Our results showed consistently large reductions for all chemical species from 2013 to 2020 with the largest decreases being chloride (95%) and organics (74%) followed by sulfate (69%), while the decreases in nitrate were comparatively small (44%). However, the contributions of sulfate were fairly stable despite the increased nitrate contributions from 18% in 2013 to 30% in 2020. Organic aerosol (OA) composition also changed significantly since 2018 with large increases in the contributions of secondary OA and corresponding decreases in primary OA from fossil fuel combustion and cooking emissions. The changes in aerosol chemistry were closely related to the different reductions in gaseous precursors, e.g., SO₂ vs. NO₂, and the enhanced secondary processes, e.g., the increases in O₃, sulfur and nitrogen oxidation efficiency. Further, we found that the changes in aerosol chemistry in cold season during the phase Ⅱ of clean air action (2018-2020) started to slow down with relatively small changes in PM_2.5 and secondary inorganic species. Our results point towards a future challenge in mitigating air pollution in cold season, and the need of more stringent and scientific strategies to control secondary aerosol pollution in an environment with enhanced oxidation capacity and high precursors.

Dust-Dominated Coarse Particles as a Medium for Rapid Secondary Organic and Inorganic Aerosol Formation in Highly Polluted Air

Secondary aerosol (SA) frequently drives severe haze formation on the North China Plain. However, previous studies mostly focused on submicron SA formation, thus our understanding of SA formation on supermicron particles remains poor. In this study, PM_2.5 chemical composition and PM₁₀ number size distribution measurements revealed that the SA formation occurred in very distinct size ranges. In particular, SA formation on dust-dominated supermicron particles was surprisingly high and increased with relative humidity (RH). SA formed on supermicron aerosols reached comparable levels with that on submicron particles during evolutionary stages of haze episodes. These results suggested that dust particles served as a medium for rapid secondary organic and inorganic aerosol formation under favorable photochemical and RH conditions in a highly polluted environment. Further analysis indicated that SA formation pathways differed among distinct size ranges. Overall, our study highlights the importance of dust in SA formation during non-dust storm periods and the urgent need to perform size-resolved aerosol chemical and physical property measurements in future SA formation investigations that are extended to the coarse mode because the large amount of SA formed thereon might have significant impacts on ice nucleation, radiative forcing, and human health.