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The particulate matter (PM) pollution has been significantly improved by carrying out various valid emission control strategies since 2013 in China. Meanwhile the variation trend of nitrous acid (HONO) is worthy to investigate due to its vital role in the atmospheric oxidation process. In this study, field observation in the winter is conducted to investigate the concentration of HONO in an urban area of Beijing. In the winter of 2019, the mean HONO concentration is 1.38 ppbV during the whole winter. Photo-enhanced NO 2 heterogeneous reactions on the ground and aerosol surfaces were found as the possible daytime sources of HONO. Compared to O 3 , photolysis of HONO dominates the primary OH sources during the winter time. To understand the HONO pollution patterns by years variation, multi-year data is summarized and finds that primary pollutants including CO and NO decreased, but secondary pollutants i.e., HONO (mostly generated via secondary process) increased. Our study highlights the requirement to mitigate secondary pollution by control HONO concentration. [Display omitted] [ABSTRACT FROM AUTHOR]

Air pollution and the urban heat island (UHI) effect are two serious problems in many cities and have become increasingly serious with accelerated urbanization. The relationship between fine particulate matter (PM2.5) and the UHI effect has become a research topic of major interest to help avoid the vicious cycle of the UHI effect and air pollution, but recent studies still focus on correlation analysis, lacking attention to the spatiotemporal heterogeneity of their relationship. Combining remote sensing and in situ data, this study analyzed the spatiotemporal heterogeneity of the interaction between the UHI effect and PM2.5 in Beijing over four seasons by using a coupling coordination degree model, and the characteristics of spatiotemporal heterogeneity were analyzed from both biophysical and socioeconomic aspects. The modified normalized difference water index (MNDWI) and normalized difference vegetation index (NDVI) were selected as the indices from a biophysical perspective. Urban functional areas derived from points of interest (POIs) were used to represent the area's socioeconomic background. The results showed that the relationship between PM2.5 and the UHI effect has spatiotemporal heterogeneity. The areas exhibiting high degrees of coupling coordination were most widely distributed in spring, and the highest degree of coupling coordination appeared in summer. The four seasons of high coupling coordination areas occurred over four seasons, covering more than 58.3% of Beijing's area, mainly concentrated in business-related functional areas. Areas where the positive interaction between the UHI effect and air pollution was weak tended to have a higher proportion of vegetation. The results of this work are of theoretical and management value for mitigating the threat of rapid urbanization to human health. [ABSTRACT FROM AUTHOR]