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 SO and NO, the enhancement of secondary aerosol formation was observed. NO has surpassed SO and OM to become the dominant PM 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 SO concentrations corresponded well to those with high SO concentrations, while the synchronized NO better followed spatial patterns in O than NO. From 2016 to 2020, the response of SO to SO was close to a linear function, while the reaction of NO to the decrease of NO displayed nonlinear behavior. Such different relationships indicated that SO was predominantly controlled by SO, while NO was not only related to NO but also determined by the secondary conversion process. The ratios of SO, NO, NH, and OC to EC between 2016 and 2020 were generally higher than 1 in typical BTH cities, and the ratio of NO to EC was exceptionally high, with a range reaching up to 200 %. Besides, this ratio coincided well with the enhancement of O, indicating the potential role of O to secondary NO formation. The diurnal cycle of NO, O, and NO concentration change rate indicated that the relative increase of O during nighttime may offset the effectiveness of NO emission reduction. This study provided observational evidence of enhanced secondary NO formation with the rising trend of atmospheric oxidation and emphasized the importance of nighttime chemistry for NO formation in the BTH region.