Evidence of Surface-Tension Lowering of Atmospheric Aerosols by Organics from Field Observations in an Urban Atmosphere: Relation to Particle Size and Chemical Composition.

Surface-active organics lower the aerosol surface tension (σs/a), leading to enhanced cloud condensation nuclei (CCN) activity and potentially exerting impacts on the climate. Quantification of σs/a is mainly limited to laboratory or modeling work for particles with selected sizes and known chemical compositions. Inferred values from ambient aerosol populations are deficient. In this study, we propose a new method to derive σs/a by combining field measurements made at an urban site in northern China with the κ-Köhler theory. The results present new evidence that organics remarkably lower the surface tension of aerosols in a polluted atmosphere. Particles sized around 40 nm have an averaged σs/a of 53.8 mN m-1, while particles sized up to 100 nm show σs/a values approaching that of pure water. The dependence curve of σs/a with the organic mass resembles the behavior of dicarboxylic acids, suggesting their critical role in reducing the surface tension. The study further reveals that neglecting the σs/a lowering effect would result in lowered ultrafine CCN (diameter <100 nm) concentrations by 6.8-42.1% at a typical range of supersaturations in clouds, demonstrating the significant impact of surface tension on the CCN concentrations of urban aerosols.

Wintertime fine aerosol particles composition and its evolution in two megacities of southern and northern China.

Study on fine aerosols composition can help understand the particles formation and is crucial for improving the accuracy of model simulations. Based on field data measured by a Q-ACSM (Quadrupole-Aerosol Chemical Speciation Monitor), we have comprehensively compared the characteristics, evolution, and potential formation mechanisms of the components in NR-PM2.5 during wintertime at two megacities (Beijing and Guangzhou) of southern and northern China. We show that as PM pollution intensifies, the mass fraction of the primary aerosols (e.g., COA, HOA) in PM2.5 in Guangzhou increased, along with a slight decline in proportion of both the secondary organic (SOA) and inorganic (SIA) aerosols; In contrast, in Beijing, the proportion of the SIA ramped up from 28 % to 53 % with the pollution evolution; and the fraction of SOA in total OA also increased due to a substantial increment in the proportion of MO-OOA (from 29 % to 48 %), suggesting a significance of the secondary processes in worsening aerosols pollution in Beijing. Our further analysis demonstrates a leading role of aqueous pathway in the secondary formation of aerosols at the Beijing site, presenting an exponential rising of SIA and SOA with the relative humidity (RH) increase. Compared to Beijing, however, we find that the photochemical oxidation other than aqueous process in Guangzhou plays a more critical role in those secondary aerosols formation. Combined with the Hysplit trajectory model, we identify the high humid conditions in Guangzhou are typically affected by clean marine air masses, explaining the slower response of secondary components to the RH changes. Moreover, the particles in Guangzhou were observed less hygroscopic that is adverse to the aerosol aqueous chemistry. The results provide basis for the precise control of PM pollution in different regions across China and would be helpful in improving model simulations.

Changes in wintertime visibility across China over 2013-2019 and the drivers: A comprehensive assessment using machine learning method.

Effective emission reduction measures have largely lowered the particulate concentration in China, but low-visibility events still occur frequently, greatly affecting people's daily life, travel, and health. In the context of carbon neutrality strategy and climate change, the mechanisms governing visibility changes may be undergoing a transformation. To address this critical issue, we have undertaken a comprehensive assessment by employing a novel approach that combines site observations, model-derived datasets, and machine learning techniques. Our analysis of the dataset shows varying degrees of improvement in wintertime visibility in regions such as North China, South China, and the Fenwei Plain over 2013-2019, but an unexpected deterioration (approximately 1 km yr-1) in central and southern China (CSC). We further elucidate key roles of PM2.5 reduction in these regions with visibility improvement; whereas the unsatisfactory visibility trend in CSC was caused by combined effect of relative humidity (RH) increase (47 %), aerosol hygroscopicity (κ) enhancement (9 %), and boundary layer (BLH) reduction (8 %), which greatly overwhelms the effect of PM2.5 reduction recently. Moreover, the study reveals a growing influence of RH on the wintertime visibility, reaching 40 % ± 24 % to the total contribution in 2019, while that of PM2.5 declined to 18 % ± 19 % and is expected to further diminish with emission reduction. Note those often-neglected factors-temperature, wind speed, BLH, and κ, account for over 40 % of the total contribution. Though the importance of aerosol hygroscopic growth to visibility was found decreasing recently, it retains non-negligible impacts on driving inter-annual visibility trends. This study yields innovative insights for air pollution control, underscoring the imperative of region-specific strategies to mitigate low-visibility events.

Contrasting the characteristics, sources, and evolution of organic aerosols between summer and winter in a megacity of China.

Organic aerosol (OA) accounts for the largest fraction of fine particles in the ambient atmosphere, however, its formation process remains highly uncertain. Here, with an aim of obtaining insights to OA formation mechanism, we have focused on contrasting the characteristics, sources and evolution of OA of PM2.5 (particulate matter with an aerodynamic diameter < 2.5 µm) between summer and winter based on two field campaigns of urban Beijing. The results show that secondary OA (SOA) dominates OA mass in PM2.5 both in summer and winter, accounting for 87 % and 74 %, respectively. This is much higher compared to the mass fraction of ~56 % that observed in PM1, indicating an important role of SOA in larger size particles. We also show that, the SOA is becoming more dominant in total OA and our observed high SOA proportion is reconciled with its overall upward trends in recent years. The observed mass concentrations of SOA (less oxidized oxygenated OA, LO-OOA; more oxidized OOA, MO-OOA) in winter are about twice higher than those in summer. The Van-Krevelen diagram implies that the addition of carboxylic acid may be a primary oxidation process of OA in summer, while the addition of alcohol/peroxide may play a dominant role in OA evolution in winter. Our observation and analysis illustrate a more efficient conversion from LO-OOA to MO-OOA through aqueous-phase processes in winter of Beijing. While, in summer, cooking OA may be easier to convert to MO-OOA through photochemical process than other OA factors. We further show that the POA is mainly locally emitted, while the origin of SOA is from both regional transport and locally formed. The results of this study may provide policy basis for the precise control of OA pollution and would also help to improve the accuracy of assessing the environmental and climate effects of OA.

Rapid narrowing of the urban-suburban gap in air pollutant concentrations in Beijing from 2014 to 2019.

Understanding the spatial patterns of atmospheric pollutants in urban and suburban areas is important for evaluating their effects on regional air quality, climate, and human health. The analyses of pollutant monitoring data of the China National Environmental Monitoring Center revealed that the differences in the concentrations of ambient O3, PM2.5, NO2, SO2, and CO between urban and suburban areas rapidly decreased from 2014 to 2019 in Beijing. Considering the negligible urbanization and interannual meteorological changes during the study period, the results reveal a quick response of the urban-to-suburban difference (ΔUrban-Suburban) in the ambient pollutants concentrations to emission reduction measures implemented in China in 2013. However, owing to the efficient O3 formation in summer in urban areas in recent years, we observed a more rapid decrease in the ΔUrban-Suburban in O3 concentration in summer (64.8%) than in winter (16.1%). In addition, the ΔUrban-Suburban in daytime summer O3 changed from negative in 2014-2018 to positive in 2019, indicating that the daytime O3 concentration in urban areas exceeded that in suburban areas. Furthermore, instantaneous changes in ΔUrban-Suburban in air pollutants were more sensitive to meteorological variations in 2014 than in 2019. The results indicate a less significant role of regional air mass transport in the spatial variability of pollutants under a future scenario of strong emission reduction in China.