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The Clean Air Action implemented by the Chinese government in 2013 has greatly improved air quality in the North China Plain (NCP). In this work, we report changes in the chemical components of atmospheric fine particulate matter (PM 2.5) at four NCP sampling sites from 2012/2013 to 2017 to investigate the impacts and drivers of the Clean Air Action on aerosol chemistry, especially for secondary inorganic aerosols (SIA). During the observation period, the concentrations of PM 2.5 and its chemical components (especially SIA, organic carbon (OC), and elemental carbon (EC)) and the frequency of polluted days (daily PM 2.5 concentration ≥ 75 μg m−3) in the NCP, declined significantly at all four sites. Asynchronized reduction in SIA components (large decreases in SO 4 2− with stable or even increased NO 3 − and NH 4 +) was observed in urban Beijing, revealing a shift of the primary form of SIA, which suggested the fractions of NO 3 − increased more rapidly than SO 4 2− during PM 2.5 pollution episodes, especially in 2016 and 2017. In addition, unexpected increases in the sulfur oxidation ratio (SOR) and the nitrogen oxidation ratio (NOR) were observed among sites and across years in the substantially decreased PM 2.5 levels. They were largely determined by secondary aerosol precursors (i.e. decreased SO 2 and NO 2), photochemical oxidants (e.g. increased O 3), temperature, and relative humidity via gas-phase and heterogeneous reactions. Our results not only highlight the effectiveness of the Action Plan for improving air quality in the NCP, but also suggest an increasing importance of SIA in determining PM 2.5 concentration and composition. [Display omitted] • PM 2.5 and its chemical components were reduced in North China Plain during 2012–2017. • Reduced air pollutant emissions contributed more to PM 2.5 decline than meteorology. • NO 3 − became increasingly important in the formation of secondary inorganic aerosols. • Both sulfur oxidation ratio and nitrogen oxidation ratio increased unexpectedly. [ABSTRACT FROM AUTHOR]

Airborne bacteria are important components of fine particulate matter (PM 2.5), and have received increasing attention because of their impacts on public health and ecological systems. However, the relationships between the bacterial community and PM 2.5 pollution are poorly understood. The properties of bacterial communities in Beijing at low, medium and high PM 2.5 levels were analyzed during winter (December 2015–January 2016; January 2017) and spring (March 2016–May 2016; April 2017–May 2017). Variations in bacterial concentrations, Shannon and Simpson indices and relative abundance were significantly related to the seasons. In winter, there were no significant differences in bacterial communities among three PM 2.5 pollution levels. In contrast, significant correlations between bacterial abundance and PM 2.5 levels were observed in spring, and the bacterial concentrations, community richness and diversity indices were significantly higher on heavily polluted days compared to other pollution levels. Correlation results showed that relative humidity (RH), wind speed (WS), and O 3 were most closely associated with microbial community structure in winter (P < 0.05), but temperature (T), NO 2 , SO 2 , and CO in spring, while CO, NO 2 , O 3 , RH, and WS had significant relationships (|r| = 0.360–0.553, P < 0.05) with bacterial concentrations in winter, but PM 2.5 , O 3 , T, and RH (|r| = 0.281–0.527, P < 0.05) in spring. Chemical composition, especially that of secondary aerosol particles, which were mainly produced from anthropogenic sources (e.g. fossil fuel combustion, road traffic and industrial emissions), exerted the most control of bacterial community structure of PM 2.5. The effects of PM 2.5 levels on the bacterial community were modified by environmental conditions, heavy metal and nutrient contents of the PM 2.5. The relationships described provide a foundation for further research into the environmental and PM 2.5 chemical composition controls on the bacterial community and related health risk from air pollution. [Display omitted] • Conditional responses of bacterial communities to PM 2.5 pollution. • Stable bacterial communities across different PM 2.5 levels in winter. • Significant changes of bacterial communities to PM 2.5 levels in spring. • Secondary aerosol particles exerted the most control of bacterial community structure. [ABSTRACT FROM AUTHOR]