Search

The scattering of light by aerosol particles dictates atmospheric visibility, which is a straightforward indicator of air quality. It is affected by numerous factors, such as particle number size distribution, particle mass concentration (PM2.5), ambient relative humidity (RH), and chemical composition. The latter two factors jointly influence the aerosol liquid water content (ALWC). Here, the particle backscattering coefficient (βp) under ambient RH conditions is investigated to differentiate and quantify the contributions of aerosol properties and meteorology using comprehensive observational datasets acquired at three megacities in China, that is, Beijing (BJ), Nanjing (NJ), and Guangzhou (GZ). Overall, the temporal variations in βp under ambient RH conditions are consistent with those in ALWC at the three sites. The PM2.5 in BJ is systematically higher than in NJ and GZ, while ambient RH and aerosol hygroscopicity in NJ are much higher than in BJ and GZ. Notable differences in the variations of βp with related factors at the three sites are demonstrated. βp is more sensitive to particle hygroscopicity and mass in NJ and ambient RH in BJ. The relative contributions of these factors to βp at the three sites under different pollution conditions are differentiated and quantified. The factor with the largest impact on the variability in βp shifts from particle mass to ambient RH as air quality deteriorated to heavy pollution in BJ. The opposite is true in NJ. In GZ, the contributions of these factors to changes in βp under different pollution conditions are similar, both dominated by PM2.5. Plain Language Summary: Scattering of light by aerosol is a straightforward measure of air quality because of its close link to visibility. It is dictated primarily by aerosol mass loading, size distribution, chemical composition, and humidity. We used comprehensive observational datasets acquired at three sites in China to differentiate and quantify the contributions to the scattering of light by aerosol loading measured by aerosol particulate mass and aerosol properties, as well as humidity. The findings differ among the three cities due to differences in aerosol properties and meteorology. Scattering of light by aerosol is more sensitive to particle hygroscopicity and mass concentration in Nanjing and more sensitive to ambient humidity in Beijing. Local emission control measures can effectively reduce aerosol mass concentrations in Nanjing and Guangzhou. Weather conditions are more influential in Beijing due to the dominant influence of humidity. Key Points: This study differentially quantifies the impacts of aerosol loading and properties and meteorology on scattering at three sites in ChinaScattering of light by aerosol is most sensitive to particle hygroscopicity and mass in Nanjing and humidity in BeijingHumidity substantially affects scattering during heavy haze in Beijing, whereas particle mass is more impactful in Nanjing and Guangzhou [ABSTRACT FROM AUTHOR]

In this study, we report the phenomenon of fast changes in aerosol hygroscopicity between clean and polluted periods observed frequently in urban Beijing during winter using a hygroscopicity tandem mobility analyzer. The cause of this phenomenon and the formation process of particles in different modes are discussed. During clean periods, ultrafine‐mode particles (i.e., nucleation and Aitken modes) stem mainly from nucleation events with subsequent growth. During heavily polluted periods, ultrafine‐mode particles originate chiefly from primary emissions. Larger‐mode particles like accumulation‐mode particles are mainly from primary emissions during clean periods and aqueous reactions during polluted periods. This finding based on hygroscopicity tandem mobility analyzer measurements can make up the deficiency of mass‐dependent instruments in analyzing sources and chemical processes of ultrafine‐mode particles. Plain Language Summary: The mass concentration of ultrafine‐mode particles in the atmosphere is extremely low, and their properties are rarely investigated using instruments specialized in measuring such things. The hygroscopicity tandem mobility analyzer makes it possible to study sources and chemical processes of ultrafine‐mode particles. They were measured in terms of aerosol number concentration, and aerosol hygroscopicity indirectly reflects aerosol chemical compositions. The properties of ultrafine‐ and accumulation‐mode particles differed considerably between clean and polluted urban environments. Investigated were their different formation processes. Key Points: Aerosol hygroscopicity changes quickly between clean and polluted periods in BeijingUltrafine‐mode particles stem chiefly from nucleation in clean environments and from primary emissions in polluted environmentsUltrafine‐ and accumulation‐mode particles have distinct properties in urban environments [ABSTRACT FROM AUTHOR]