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

Atmospheric brown carbon in China haze is dominated by secondary formation

Brown carbon (BrC) is a class of light-absorbing organic aerosols (OA) and has significant influence on atmospheric radiative forcing. However, the current limited understanding of the physicochemical properties of BrC restricts the accurate evaluation of its environmental effects. Here the optical characteristics and chemical composition of BrC during wintertime in the Yangtze River Delta (YRD) region, China were measured by using high-resolution aerosol mass spectrometry (HR-AMS) and UV-vis spectrometry. Our results showed that BrC in PM2.5 during the campaign was dominated by water-soluble organics, which consist of less oxidized oxygenated OA (LO-OOA), more oxidized oxygenated OA (MO-OOA), fossil fuel OA (FFOA) and biomass burning OA (BBOA). MO-OOA and BBOA were the strongest light absorbing BrC at 365 nm (Abs365), followed by LO-OOA and FFOA with a mass absorption coefficient (MAC) being 0.74 ± 0.04, 0.73 ± 0.03, 0.48 ± 0.04 and 0.39 ± 0.06 m2 g-1 during the campaign, respectively. In the low relative humidity (RH < 80 %) haze periods Abs365 of LO-OOA contributed to 44 % of the total light absorption at 365 nm, followed by MO-OOA (31 %), FFOA (21 %) and BBOA (4 %). In contrast, in the high-RH (RH > 80 %) haze periods Abs365 was dominated by MO-OOA, which accounted for 62 % of the total Abs365, followed by LO-OOA (17 %), BBOA (13 %) and FFOA (8 %). Chemical composition analysis further showed that LO-OOA and MO-OOA are produced from gas-phase photooxidation of VOCs and aerosol aqueous reactions, respectively, in which ammonia significantly enhanced the formation and light absorption of BrC in the high RH haze period. On average, >75 % of the total Abs365nm in the YRD region during the haze events was contributed by LO-OOA and MO-OOA, suggesting that atmospheric BrC in China haze periods is predominantly formed by secondary reactions.

Research progress on the characteristics, sources, and environmental and potential health effects of water-soluble organic compounds in atmospheric particulate matter

Water-soluble organic compounds (WSOCs) have received extensive attention due to their indistinct chemical components, complex sources, negative environmental impact, and potential health effects. To the best of our knowledge, until now, there has been no comprehensive review focused on the research progress of WSOCs. This paper reviewed the studies on chemical constituent and characterization, distribution condition, sources, environmental impact, as well as the potential health effects of WSOCs in the past 13 years. Moreover, the main existing challenges and directions for the future research on WSOCs were discussed from several aspects. Because of the complex composition of WSOCs and many unknown individual components that have not been detected, there is still a need for the identification and quantification of WSOCs. As modern people spend more time in indoor environments, it is meaningful to fill the gaps in the component characteristics and sources of indoor WSOCs. In addition, although in vitro cell experiments have shown that WSOCs could induce cellular oxidative stress and trigger the inflammatory response, the corresponding mechanisms of action need to be further explored. The current population epidemiology research of WSOCs is missing. Prospectively, we propose to conduct a comprehensive and simultaneous analysis strategy for concentration screening, source apportionment, potential health effects, and action mechanisms of WSOCs based on high throughput omics coupled with machine learning simulation and prediction.

Size-resolved light-absorbing organic carbon and organic molecular markers in Nanjing, east China: Seasonal variations and sources

Owing to the potential influence of light-absorbing organic carbon (OC), also termed "brown carbon" (BrC), on the planetary radiation budget, many studies have focused on its absorption in single-sized ranges of particulate matter (PM). However, the size distribution and organic tracer-based source apportionment of BrC absorption have not been extensively investigated. In this study, size-resolved PM samples were collected using multi-stage impactors from eastern Nanjing during each season in 2017. The light absorption of methanol-extractable OC at 365 nm (Abs₃₆₅, Mm^-1) was determined using spectrophotometry, and a series of organic molecular markers (OMMs) was measured using a gas chromatography-mass spectrometer. Fine PM with an aerodynamic diameter <2.1 μm (PM_2.1) dominated Abs₃₆₅ (79.8 ± 10.4%) of the total size ranges with maxima and minima in winter and summer, respectively. The distributions of Abs₃₆₅ shifted to larger PM sizes from winter to spring and summer due to lower primary emissions and increased BrC chromophores in dust. Except for low-volatility (p^o,*_L < 10^-10 atm) polycyclic aromatic hydrocarbons (PAHs), the non-polar OMMs, including n-alkanes, PAHs, oxygenated PAHs, and steranes, showed a bimodal distribution pattern. Secondary products of biogenic precursors and biomass burning tracers presented a unimodal distribution peaking at 0.4-0.7 μm, while sugar alcohols and saccharides were enriched in coarse PM. Their seasonal variations in average concentrations reflected intense photochemical reactions in summer, more biomass burning emissions in winter, and stronger microbial activity in spring and summer. Positive matrix factorization was used for the source apportionment of Abs₃₆₅ in fine and coarse PM samples. Biomass burning contributed an average of 53.9% to the Abs₃₆₅ of PM_2.1 extracts. The Abs₃₆₅ of coarse PM extracts was associated with various dust-related sources where the aging processes of aerosol organics could occur.

Effects of NO2 and RH on secondary organic aerosol formation and light absorption from OH oxidation of ο-xylene

Ο-xylene is an important aromatic volatile organic compound (VOC) in the atmosphere over urban areas. In this work, the effect of nitrogen dioxide (NO₂) concentration and relative humidity (RH) on the mass concentration of secondary organic aerosols (SOA) formed from ο-xylene OH oxidization was investigated in a photooxidation chamber. The ο-xylene SOA mass concentration increased from 54.2 μg m^-3 to 127.2 μg m^-3 during dry conditions, but decreased from 177.7 μg m^-3 to 146.5 μg m^-3 during high RH conditions when the initial NO₂ concentration increased form 0 ppbv to about 900 ppbv. An increase in the ratio of [NO₃^-]/[Org] and a decrease in the oxidation state of carbon (OS_C) of SOA suggested that acid-catalyzed heterogeneous reaction was responsible for enhancing SOA formation with increasing NO₂ concentrations in dry conditions. In contrast, in humid conditions, the high molecular diffusion capacity of SOA could promote the reactivity of OH towards the interior of SOA, and the enhancement of nitrous acid (HONO) formation under high NO₂ conditions could promote the SOA aging processes and be responsible for the decreasing trend of SOA formation with NO₂. Light absorption by SOA was also measured, and both NO₂ and RH enhanced the mass absorption coefficient (MAC_{λ = 365 nm}) value for the optical properties of ο-xylene SOA. The highest MAC_{λ = 365 nm} value of ο-xylene SOA was 0.89 m² g^-1, observed during humid conditions with an initial NO₂ concentration of 862 ppbv, which was 3.9 times higher than in the experiment conducted in the absence of NO₂ under dry conditions. The formation of nitrogen-containing organic compounds (NOCs) and humic-like substances (HULIS) were responsible for the increased MAC_{λ = 365 nm} values of ο-xylene derived SOA. This study provides new insight into the effect of NO₂ on SOA formation through the change in ο-xylene photooxidation under different RH conditions, and the complex effect of multiple environmental factors on SOA formation was also important and should not be ignored.

Seasonal variations, temperature dependence, and sources of size-resolved PM components in Nanjing, east China

Size-segregated ambient particulate matter (PM) samples were collected seasonally in suburban Nanjing of east China from 2016 to 2017 and chemically speciated. In both fine (< 2.1 µm, PM_2.1) and coarse (> 2.1 µm, PM_>2.1) PM, organic carbon (OC) accounted for the highest fractions (26.9% ± 10.9% and 23.1% ± 9.35%) of all measured species, and NO₃^- lead in average concentrations of water-soluble inorganic ions (WSIIs). The size distributions of measured components were parameterized using geometric mean diameter (GMD). GMD values of NO₃^-, Cl^-, OC, and PM for the whole size range varied from < 2.1 µm in winter to > 2.1 μm in warm seasons, which was due to the fact that the size distributions of semi-volatile components (e.g., NH₄NO₃, NH₄Cl, and OC) had a dependency on the ambient temperature. Unlike OC, elemental carbon (EC), and elements, NH₄⁺, NO₃^-, and SO₄^2- exhibited an increase trend in GMD values with relative humidity, indicating that the hygroscopic growth might also play a role in driving seasonal changes of PM size distributions. Positive matrix factorization was performed using compositional data of fine and coarse particles, respectively. The secondary formation of inorganic salts contributing to the majority (> 70%) of fine PM and 20.2% ± 19.9% of speciated coarse PM. The remaining coarse PM content was attributed to a variety of dust sources. Considering that coarse and fine PM had comparable mass concentrations, more attention should be paid to local dust emissions in future air quality plans.

Probing autoxidation of oleic acid at air-water interface: A neglected and significant pathway for secondary organic aerosols formation

Fatty acids have been proposed to be a potential source of precursors for SOAs, but the autoxidation process was neglected in the oxidation studies. Here, the autoxidation of oleic acid was explored using microdroplet mass spectrometry. Bulk solution, concentration and solvent composition experiments provided direct evidences for that the autoxidation occurred at or near the air-water interface. The kinetic data showed an acceleration at this interface and was comparable to ozonation, indicating that autoxidation is an important pathway for SOAs formation. In addition, intermediates/products were captured and identified using tandem mass spectrometry, spin-trapping and quenched agents. The autoxidation mechanism was divided into addition intermediates (AIs) and Criegee intermediates (CIs) pathways mediated by hydroxyl radicals (OH). The CI chemistry which is ubiquitous in gas phase was observed at the air-water interface, and this leaded to the mass/volume loss of aerosols. Inversely, the AI chemistry caused the increase of mass, density and hygroscopicity of aerosols. AI chemistry was dominated compared to CI chemistry, but varied by concerning aerosol sizes, ultraviolet light (UV) and charge. Moreover, the MS approach of selectively probing the interfacial substances at the scale of sub-seconds opens new opportunities to study heterogeneous chemistry in atmosphere.

Soil dust as a potential bridge from biogenic volatile organic compounds to secondary organic aerosol in a rural environment

The role of coarse particles has recently been proven to be underestimated in the atmosphere and can strongly influence clouds, ecosystems and climate. However, previous studies on atmospheric chemistry of volatile organic compounds (VOCs) have mostly focused on the products in fine particles, it remains less understood how coarse particles promote secondary organic aerosol (SOA) formation. In this study, we investigated water-soluble compounds of size-segregated aerosol samples (0.056 to >18 μm) collected at a coastal rural site in southern China during late summer and found that oxygenated organic matter was abundant in the coarse mode. Comprehensive source apportionment based on mass spectrum and ¹⁴C analysis indicated that different from fossil fuel SOA, biogenic SOA existed more in the coarse mode than in the fine mode. The SOA in the coarse mode showed a unique correlation with biogenic VOCs. ¹³C and elemental composition strongly suggested a pathway of heterogeneous reactions on coarse particles, which had an abundant low-acidic aqueous environment with soil dust to possibly initiate iron-catalytic oxidation reactions to form SOA. This potential pathway might complement understanding of both formation of biogenic SOA and sink of biogenic VOCs in global biogeochemical cycles, warrantying future relevant studies.

Distinctive Sources Govern Organic Aerosol Fractions with Different Degrees of Oxygenation in the Urban Atmosphere

Understanding how the sources of an atmospheric organic aerosol (OA) govern its burden is crucial for assessing its impact on the environment and adopting proper control strategies. In this study, the sources of OA over Beijing were assessed year-around based on the combination of two separation approaches for OA, one from chemical fractionation into the high-polarity fraction of water-soluble organic matter (HP-WSOM), humic-like substances (HULIS), and water-insoluble organic matter (WISOM), and the other from statistical grouping using positive matrix factorization (PMF) of high-resolution aerosol mass spectra. Among the three OA fractions, HP-WSOM has the highest O/C ratio (1.36), followed by HULIS (0.56) and WISOM (0.17). The major sources of different OA fractions were distinct: HP-WSOM was dominated by more oxidized oxygenated OA (96%); HULIS by cooking-like OA (40%), less oxidized oxygenated OA (27%), and biomass burning OA (21%); and WISOM by fossil fuel OA (77%). In addition, our results provide evidence that mass spectral-based PMF factors are associated with specific substructures in molecules. These structures are further discussed in the context of the FT-IR results. This study presents an overall relationship of OA groups monitored by chemical and statistical approaches for the first time, providing insights for future source apportionment studies.

Molecular Speciation of Size Fractionated Particulate Water-Soluble Organic Carbon by Two-Dimensional Nuclear Magnetic Resonance (NMR) Spectroscopy

Particulate matter is associated with increased morbidity and mortality; its effects depend on particle size and chemical content. It is important to understand the composition and resultant toxicological profile of particulate organic compounds, the largest and most complex fraction of particulate matter. The objective of the study was to delineate the nuclear magnetic resonance (NMR) spectral fingerprint of the biologically relevant water-soluble organic carbon (WSOC) fraction of size fractionated urban aerosol. A combination of one and two-dimensional NMR spectroscopy methods was used. The size distribution of particle mass, water-soluble extract, non-exchangeable organic hydrogen functional types and specific biomarkers such as levoglucosan, methane sulfonate, ammonium and saccharides indicated the contribution of fresh and aged wood burning emissions, anthropogenic and biogenic secondary aerosol for fine particles as well as primary traffic exhausts and pollen for large particles. Humic-like macromolecules in the fine particle size range included branched carbon structures containing aromatic, olefinic, keto and nitrile groups and terminal carboxylic and hydroxyl groups such as terpenoid-like polycarboxylic acids and polyols. Our study show that 2D-NMR spectroscopy can be applied to study the chemical composition of size fractionated aerosols.

Size distributions of nitrated phenols in winter at a coastal site in north China and the impacts from primary sources and secondary formation

Nitrated phenols in particulate matters are among the major components of brown carbon, harm plant growth and human health. To understand the size distributions of nitrated phenols in the polluted coastal region and the factors influencing these distributions, size-resolved particulate matters were collected from a rural site in the coastal city of Qingdao, China, in January 2019, and analyzed for the presence of 11 nitrated phenols. The average concentrations of total nitrated phenols in fine- and coarse-mode particles were 123.6 and 37.2 ng m^-3, respectively. 4-Nitrophenol was found to be the dominant nitrated phenol, followed by 3-methyl-6-nitrocatechol, 3-methyl-4-nitrophenol, and 4-nitrocatechol. On average, maximum concentrations of nitrated phenols were in condensation-mode particles, whereas a minor concentration peak of nitro-salicylic acids was present in droplet-mode particles. In addition, a minor concentration peak of 4-methyl-2,6-dinitrophenol was noticed in coarse-mode particles. Comparisons of the size distributions under different situations confirmed that both primary emissions and secondary formation had significant effects on the abundances and particle-sizes of nitrated phenols. Coal combustion in residential villages and firework burning during a festival led to a sharp increase of nitrated phenols in condensation-mode particles, whereas dust promoted their heterogeneous formation in coarse-mode particles, and high humidity in the coastal area facilitated their aqueous formation in droplet-mode particles.