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The evolution of haze, involving multiple processes such as nucleation, coagulation, and condensation, may exert complex effects on aerosols' cloud condensation nuclei (CCN) activity and number concentration (NCCN). Based on field campaigns carried out in the winters of 2014 and 2016 in Beijing, we show that NCCNwas significantly enhanced by the evolution of haze, substantially driven by the nucleation process (or new particle formation). The enhancement factor of NCCNby such nucleation‐initiated haze episodes, E_NCCN, defined as the ratio of NCCNafter haze events to NCCNprior to haze events, ranged from 2.2 to 6.5 at a supersaturation (S) = 0.76% and from 4.2 to 17.3 at S= 0.23%, the magnitude of which partially depends on the severity of the haze event. The enhancements are much greater than those previously observed and those from model simulations of contribution from new particle formation. This suggests that CCN sources from new particle formation may be underestimated, needing reevaluation in polluted environments where the subsequent growth of newly formed particles can last 2–3 days, yielding more CCN‐sized particles. We further quantified that the changes in particle size and composition during the nucleation‐initiated evolution of haze are responsible for > 80% and 12–20%, respectively, of the enhancement in CCN activity. The changes in particle composition had a limited impact because most of the ambient particles were already hydrophilic, with hygroscopic parameters of 0.2–0.65. Nucleation‐initiated haze events lead to more CCN‐sized particlesThe effects of changes in particle size and composition on CCN activity during the evolution of haze events are quantifiedThe source of CCN from new particle formation needs reevaluation in polluted regions where the subsequent growth of nucleated particles may last 2–3 days

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/ais 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/aby 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/aof 53.8 mN m–1, while particles sized up to 100 nm show σs/avalues approaching that of pure water. The dependence curve of σs/awith 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/alowering 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.

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. 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. 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

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 βpunder ambient RH conditions are consistent with those in ALWC at the three sites. The PM2.5in 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 βpwith related factors at the three sites are demonstrated. βpis more sensitive to particle hygroscopicity and mass in NJ and ambient RH in BJ. The relative contributions of these factors to βpat the three sites under different pollution conditions are differentiated and quantified. The factor with the largest impact on the variability in βpshifts 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 βpunder different pollution conditions are similar, both dominated by PM2.5. 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. 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 This study differentially quantifies the impacts of aerosol loading and properties and meteorology on scattering at three sites in China Scattering of light by aerosol is most sensitive to particle hygroscopicity and mass in Nanjing and humidity in Beijing Humidity substantially affects scattering during heavy haze in Beijing, whereas particle mass is more impactful in Nanjing and Guangzhou

A new mechanism of new particle formation (NPF) is investigated using comprehensive measurements of aerosol physicochemical quantities and meteorological variables made in three continents, including Beijing, China; the Southern Great Plains site in the USA; and SMEAR II Station in Hyytiälä, Finland. Despite the considerably different emissions of chemical species among the sites, a common relationship was found between the characteristics of NPF and the stability intensity. The stability parameter (ζ = Z/L, where Zis the height above ground and Lis the Monin–Obukhov length) is found to play an important role; it drops significantly before NPF as the atmosphere becomes more unstable, which may serve as an indicator of nucleation bursts. As the atmosphere becomes unstable, the NPF duration is closely related to the tendency for turbulence development, which influences the evolution of the condensation sink. Presumably, the unstable atmosphere may dilute pre-existing particles, effectively reducing the condensation sink, especially at coarse mode to foster nucleation. This new mechanism is confirmed by model simulations using a molecular dynamic model that mimics the impact of turbulence development on nucleation by inducing and intensifying homogeneous nucleation events.