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.

Characterization of organic vapors by a Vocus proton-transfer-reaction mass spectrometry at a mountain site in southeastern China.

Biogenic and anthropogenic organic vapors are crucial precursors of ozone and secondary organic aerosol (SOA) in the atmosphere. Here we conducted real-time measurements of gaseous organic compounds using a Vocus proton-transfer-reaction mass spectrometer (Vocus PTR-MS) at the Shanghuang mountain site (1128 m a.s.l.) in southeastern China during November 2022. Our results revealed a substantial impact of mixed biogenic and anthropogenic compounds at the mountain site, with oxygenated volatile organic compounds (OVOCs) comprising 74 % of the organic vapors. Two distinct periods, characterized by sunny days (P1) and persistent cloud events (P2), were observed. P1 exhibited higher concentrations of biogenic-related emissions compared to P2. For instance, isoprene, monoterpenes, and sesquiterpenes during P1 were 2.4-2.9 times higher than those during P2. OVOCs such as acetaldehyde, MVK + MACR, acetone, and MEK also showed higher concentrations during P1, indicating a dominant source from the photochemical oxidation of biogenic VOCs. Anthropogenic-related VOCs like benzene and toluene had higher concentrations during P2, displaying different diurnal cycles compared to P1. Our analysis identified four biogenic-related factors dominated by isoprene and sesquiterpene oxidation products, and two anthropogenic-related factors. During P1, biogenic sources contributed approximately 80 % to total organic compounds, while during P2, anthropogenic sources, particularly the aromatic-related factor, increased from 16 % to 35 %. Furthermore, a unique factor characterized by C2 amines and C3 amides and periodic plumes indicated the influence of industrial emissions from regional transport. The study highlights the significant variations in sources and compositions of gaseous organic compounds at regional mountain sites due to changes in meteorology and photochemical processing, potentially impacting regional ozone and SOA formation.

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.

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.

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 NOx 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.

Vertically Resolved Aerosol Chemistry in the Low Boundary Layer of Beijing in Summer.

Air quality in Beijing has been improved significantly in recent years; however, our knowledge of the vertically resolved aerosol chemistry in summer remains poor. Here, we carried out comprehensive measurements of aerosol composition, gaseous species, and aerosol optical properties on a meteorological tower in Beijing in summer and compared with those measured in winter. Our results showed that aerosol liquid water (ALW) contributing approximately 50% of the total mass with higher values aloft played a crucial role in aerosol formation. Particularly, the higher nitrate concentration in city aloft than at the ground level during daytime was mainly due to the enhanced gas-particle partitioning driven by ALW and particle acidity. The vertical profiles of organic aerosol (OA) factors varied more differently in the urban boundary layer. Although the ubiquitous decreases in primary OA with the increase in height were mainly due to the influences of local emissions and vertical convection, the vertical differences in oxygenated OA between summer and winter may be related to the photochemical processing of different biogenic and anthropogenic volatile organic compounds. The single-scattering albedo, brown carbon, and absorption Ångstrom exponent of aerosol particles also presented different vertical profiles between day and night due to the vertical changes in aerosol chemistry.

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 ÅngstrÓ§m 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.

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.