Differences in the chemical compositions and formation mechanisms of PM2.5 between urban and rural areas in Xinxiang, Central China around the Beijing Winter Olympics

Abstract

Xinxiang, as an important industrial city in Henan Province and one of the “2 + 26” cities, is encountering severe fine particulate matter (PM2.5) pollution. During the Beijing Winter Olympics Games, a series of pollution control measures were implemented in Xinxiang, which provides an excellent opportunity to investigate the local PM2.5 characteristics and formation mechanisms and assess the effectiveness of the control measures. From January 20 to February 20, 2022, samples of PM2.5 were collected in urban and rural areas of Xinxiang, and their chemical compositions were detected. Three research phases were divided according to control measures: P1, P2 and P3, representing before pollution control period, stricter pollution control period, and looser pollution control period, respectively. It was found that PM2.5 levels decreased significantly during P2 and P3, by more than 30%. There was a significant decrease in SO42−, NO3− and NH4+ (SNA) in PM2.5, but not in organic carbon (OC) and element carbon. Moreover, the secondary formations of SO42− and NO3− also decreased clearly during these two phases, especially for nocturnal NO3−. SO42− was mainly generated through the NO2 reaction pathway (SO2 oxidation by NO2 in aerosol water), and the reaction could also promote NO3− production. As NO2 is critical in this reaction, future pollution control should focus on reducing NO2. The PM2.5 in urban and rural areas had quite different compositions. Specifically, the rural area had better reaction conditions for sulfate (lower temperature). In addition, urban area had more OC that came from secondary reactions, while in rural area it comes more from biomass and coal combustion. Therefore, different pollution control measures should be taken to reduce their OC concentrations in the future. The study will provide a more scientific basis for formulating more scientific control measures of PM2.5 in Xinxiang and other cities in Central China.