Molecular composition and sources of water-soluble organic aerosol in summer in Beijing

Selective molecular characterization of organic aerosols using in situ laser desorption ionization mass spectrometry

RATIONALE

The sources and chemical compositions of organic aerosol (OA) exert a significant influence on both regional and global atmospheric conditions, thereby having far-reaching implications on environmental chemistry. However, existing mass spectrometry (MS) methods have limitations in characterizing the detailed composition of OA due to selective ionization as well as fractionation during cold-water extraction and solid-phase extraction (SPE).

METHODS

A comprehensive MS study was conducted using aerosol samples collected on dusty, clean, and polluted days. To supplement the data obtained from electrospray ionization (ESI), a strategy for analyzing OAs collected using the quartz fiber filter directly utilizing laser desorption ionization (LDI) was employed. Additionally, the ESI method was conducted to explore suitable approaches for determining various OA compositions from samples collected on dusty, clean, and polluted days.

RESULTS

In situ LDI has the advantages of significantly reducing the sample volume, simplifying sample preparation, and overcoming the problem of overestimating sulfur-containing compounds usually encountered in ESI. It is suitable for the characterization of highly unsaturated and hydrophobic aerosols, such as brown carbon-type compounds with low volatility and high stability, which is supplementary to ESI.

CONCLUSIONS

Compared with other ionization methods, in situ LDI helps provide a complementary description of the molecular compositions of OAs, especially for analyzing OAs in polluted day samples. This method may contribute to a more comprehensive MS analysis of the elusive compositions and sources of OA in the atmosphere.

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.

Size-Resolved Molecular Characterization of Water-Soluble Organic Matter in Atmospheric Particulate Matter from Northern China

Atmospheric particulate matter (PM) affects visibility, climate, biogeochemical cycles and human health. Water-soluble organic matter (WSOM) is an important component of PM. In this study, PM samples with size-resolved measurements at aerodynamic cut-point diameters (Dp) of 0.01-18μm were collected in the rural area of Baoding and the urban area of Dalian, Northern China. Non-targeted analysis was adopted for the characterization of the molecule constitutes of WSOM in different sized particles using Fourier transform-ion cyclotron resonance mass spectrometry. Regardless of the location, the composition of WSOM in Aitken mode particles (aerodynamic diameter < 0.05 μm) was similar. The WSOM in accumulation mode particles (0.05-2 μm) in Baoding was predominantly composed of CHO compounds (84.9%), which were mainly recognized as lignins and lipids species. However, S-containing compounds (64.2%), especially protein and carbohydrates species, accounted for most of the WSOM in the accumulation mode particles in Dalian. The CHO compounds (67.6%-79.7%) contributed the most to the WSOM in coarse mode particles (> 2 μm) from both sites. Potential sources analysis indicated the WSOM in Baoding were mainly derived from biomass burning and oxidation reactions, while the WSOM in Dalian arose from coal combustion, oxidation reactions, and regional transport.

Significant Reductions in Secondary Aerosols after the Three-Year Action Plan in Beijing Summer

Air quality in China has continuously improved during the Three-Year Action Plan (2018-2020); however, the changes in aerosol composition, properties, and sources in Beijing summer remain poorly understood. Here, we conducted real-time measurements of aerosol composition in five summers from 2018 to 2022 along with WRF-Community Multiscale Air Quality simulations to characterize the changes in aerosol chemistry and the roles of meteorology and emission reductions. Largely different from winter, secondary inorganic aerosol and photochemical-related secondary organic aerosol (SOA) showed significant decreases by 55-67% in summer, and the most decreases occurred in 2021. Comparatively, the decreases in the primary aerosol species and gaseous precursors were comparably small. While decreased atmospheric oxidation capacity as indicated by ozone changes played an important role in changing SOA composition, the large decrease in aerosol liquid water and small increase in particle acidity were critical for nitrate changes by decreasing gas-particle partitioning substantially (∼28%). Analysis of meteorological influences demonstrated clear and similar transitions in aerosol composition and formation mechanisms at a relative humidity of 50-60% in five summers. Model simulations revealed that emission controls played the decisive role in reducing sulfate, primary OA, and anthropogenic SOA during the Three-Year Action Plan, while meteorology affected more nitrate and biogenic SOA.

Environmental factors driving the formation of water-soluble organic aerosols: A comparative study under contrasting atmospheric conditions

Water-soluble organic carbon (WSOC), as major fractions of atmospheric aerosols, have gained attention due to their light-absorption properties. To illustrate the sources and key environmental factors driving WSOC formation under different atmospheric conditions, a comparative study was conducted by summarizing the results obtained from five field campaigns at inland (urban, suburban or regional) sites and a coastal site during different seasons. Organic carbon concentrations varied from 8.5 μg/m³ at the summer regional site to 17.5 μg/m³ at the winter urban site, with 46 %- 89 % of the mass as WSOC. Based on correlation analysis, primary combustion emissions were more important in winter than in summer, and secondary formation was an important source of WSOC during winter, summer and autumn. Atmospheric oxidants (NO₂, O₃), aerosol liquid water (ALW) and ambient RH were important factors influencing the WSOC formation, while their roles varied in different atmospheres. We observed a seasonal transition of atmospheric oxidants dominating the WSOC formation from O₃ and NO₂-driven conditions in summer to NO₂-driven conditions in winter. Elevated ALW or ambient RH generally favor the WSOC formation, while the WSOC dependence of ALW varied among different ALW ranges. As the increasing of ALW or ambient RH, a transition of WSOC formation from "RH/ALW-limited regime" under low-ALW conditions, to "RH/ALW and precursor-driven regime" under medium-ALW/RH, and to "precursor-limited (RH/ALW-excess) regime" were observed for the inland atmospheric conditions. Under the high-RH and ALW conditions in coastal areas, ALW or ambient RH was generally not a limiting factor for WSOC formation.

Changes in primary and secondary aerosols during a controlled Chinese New Year

Large reductions in anthropogenic emissions during the Chinese New Year (CNY) holiday in Beijing have been well reported. However, the changes during the CNY of 2021 are different because most people stayed in Beijing to control the spread of coronavirus disease (COVID-19). Here a high-resolution aerosol mass spectrometer (HR-AMS) was deployed for characterization of the changes in size-resolved aerosol composition and sources during the CNY. We found that the reductions in traffic-related NO_x and fossil fuel-related organic aerosol (OA), and cooking OA (1.3-12.7%) during the CNY of 2021 were much smaller than those in previous CNY holidays of 2013, 2015, and 2020. In contrast, the mass concentrations of secondary aerosol species except nitrate showed ubiquitous increases (17.6-30.4%) during the CNY of 2021 mainly due to a 4-day severe haze episode. OA composition also changed substantially during the CNY of 2021. In particular, we observed a large increase by nearly a factor of 2 in oxidized primary OA likely from biomass burning, and a decrease of 50.1% in aqueous-phase secondary OA. A further analysis of the severe haze episode during the CNY illustrated a rapid transition of secondary formation from photochemical to aqueous-phase processing followed by a scavenging process, leading to significant changes in aerosol composition, size distributions, and oxidation degree of OA. A parameterization relationship between oxygen-to-carbon (O/C) and f₄₄ (fraction of m/z 44 in OA) from a collocated capture vaporizer aerosol chemical speciation monitor (CV-ACSM) was developed, which has a significant implication for characterization of OA evolution and the impacts on hygroscopicity due to the rapidly increased deployments of CV-ACSM worldwide.

Rapid transition of aerosol optical properties and water-soluble organic aerosols in cold season in Fenwei Plain

The Fenwei Plain (FWP) continues to be one of the most polluted regions in China despite the improvement of air quality in recent years. However, our understanding of aerosol optical properties (AOP) and its relationship with aerosol composition particularly in cold season is far from complete. Here we conducted three-month measurements of AOP from November 2020 to January 2021 in the FWP along with fine particle composition and water-soluble organic aerosol (WSOA) measurements. Our results showed rapid transitions in AOP from November to January due to the enhanced primary emissions and the decreased aqueous-phase processing. The single scattering albedo (SSA) decreased from 0.85 to 0.78, while the absorption Ångstrӧm exponent (AAE) increased from 1.41 to 1.60, demonstrating the increasing role of absorbing aerosol and brown carbon in cold season. Further analysis showed that SSA increased significantly with the fraction of secondary inorganic aerosol, while AAE was highly correlated with the fraction of primary OA (POA), highlighting the different impacts of primary and secondary aerosol on AOP. Chemical apportionment showed the dominant contributions of ammonium nitrate (29%) and ammonium sulfate (27%) to particle extinction before heating season, while that of POA increased to 27% during heating season. Although the pollution level showed a clear increase during the heating season, the changes in visibility were small due to the decreased mass extinction efficiency from 3.48 to 2.91 m² g^-1. Positive matrix factorization illustrated a clear transition in WSOA composition from the dominance of secondary OA (SOA) in November to POA in heating season. Compared with the large decrease in water-soluble aqueous-phase SOA, the consistently high concentration of photochemical-related SOA elucidated the presence of strong photochemical processing in cold season. Overall, our results demonstrate the significant transition in primary emissions and secondary formation in cold season, and such changes have affected AOP substantially.

A chemical cocktail during the COVID-19 outbreak in Beijing, China: Insights from six-year aerosol particle composition measurements during the Chinese New Year holiday

The rapidly spread coronavirus disease (COVID-19) has limited people's outdoor activities and hence caused substantial reductions in anthropogenic emissions around the world. However, the air quality in some megacities has not been improved as expected due to the complex responses of aerosol chemistry to the changes in precursors and meteorology. Here we demonstrate the responses of primary and secondary aerosol species to the changes in anthropogenic emissions during the COVID-19 outbreak in Beijing, China along with the Chinese New Year (CNY) holiday effects on air pollution by using six-year aerosol particle composition measurements. Our results showed large reductions in primary aerosol species associated with traffic, cooking and coal combustion emissions by 30-50% on average during the CNY, while the decreases in secondary aerosol species were much small (5-12%). These results point towards a future challenge in mitigating secondary air pollution because the reduced gaseous precursors may not suppress secondary aerosol formation efficiently under stagnant meteorological conditions. By analyzing the long-term measurements from 2012 to 2020, we found considerable increases in the ratios of nitrate to sulfate, secondary to primary OA, and sulfur and nitrogen oxidation capacity despite the overall decreasing trends in mass concentrations of most aerosol species, suggesting that the decreases in anthropogenic emissions have facilitated secondary formation processes during the last decade. Therefore, a better understanding of the mechanisms driving the chemical responses of secondary aerosol to the changes in anthropogenic emissions under complex meteorological environment is essential for future mitigation of air pollution in China.