The effect of black carbon aging from NO2 oxidation of SO2 on its morphology, optical and hygroscopic properties.

Atmospheric aging of black carbon (BC) leads to changes in its physiochemical properties, exerting complex effects on environment and climate. In this study, we have conducted laboratory chamber experiments to investigate the effects of BC aging on its morphology, hygroscopicity and optical properties by exposing monodisperse fresh BC particles to ambient ubiquitous species of nitrogen dioxide (NO2), sulfur dioxide (SO2) and ammonia (NH3) in absence of UV light. We show a rapid aging from highly fractal to compacted aggregates for the monodisperse BC particles with an initial diameter of 150 nm, with decline in the dynamic shape factor (χ) from about 1.8 to nearly 1. The effective density of the monodisperse BC particles increases from ∼0.54 to 1.50 g cm-3 accordingly. The aging process leads to that the light scattering, absorption, and single scattering albedo of the monodisperse BC particles are strongly enhanced by factors of 7.0, 1.8 and 3.0 respectively. By comparing with the BC aging from other mechanisms, we reveal a critical role of the composition of the coating materials on BC in determining its light absorption enhancement. Moreover, due to strong water uptake capacity of the aged BC particles, the light absorption enhancement (Eabs) could be 40-60% higher at humid atmosphere compared with dry conditions. This BC aging process from NO2 oxidation of SO2 may occur commonly in polluted regions and thus considerably alter its effects on regional air quality and climate.

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.