Dramatic changes in aerosol composition during the 2016-2020 heating seasons in Beijing-Tianjin-Hebei region and its surrounding areas: The role of primary pollutants and secondary aerosol formation

With the implementation of a series of air pollution mitigation strategies during the past decade, great air quality improvements have been observed in the BTH region. Despite of significant decreases in gaseous pollutants, such as SO2 and NO2, the enhancement of secondary aerosol formation was observed. NO3- has surpassed SOq- and OM to become the dominant PM2.5 component. We find that the reduction of POC mainly dominated the decreasing trend of OC. As for secondary inorganic components, the key processes or factors controlling the spatial-temporal variation characteristics were different. The areas with large SOq- concentrations corresponded well to those with high SO2 concentrations, while the synchronized NO3- better followed spatial patterns in O3 than NO2. From 2016 to 2020, the response of SOq- to SO2 was close to a linear function, while the reaction of NO3- to the decrease of NO2 displayed nonlinear behavior. Such different relationships indicated that SOq - was predominantly controlled by SO2, while NO3- was not only related to NO2 but also determined by the secondary conversion process. The ratios of SOq -, NO3-, NH4+, and OC to EC between 2016 and 2020 were generally higher than 1 in typical BTH cities, and the ratio of NO3- to EC was exceptionally high, with a range reaching up to 200 %. Besides, this ratio coincided well with the enhancement of Ox, indicating the potential role of Ox to secondary NO3- formation. The diurnal cycle of NO3-, O3, and NO2 concentration change rate indicated that the relative increase of O3 during nighttime may offset the effective-ness of NOX emission reduction. This study provided observational evidence of enhanced secondary NO3 - formation with the rising trend of atmospheric oxidation and emphasized the importance of nighttime chemistry for NO3 - forma-tion in the BTH region.