Significant influence of the intensive agricultural activities on atmospheric PM2.5 during autumn harvest seasons in a rural area of the North China Plain

Abstract The levels and composition of atmospheric PM2.5 were comprehensively investigated over an agricultural field in the North China Plain (NCP) during the autumn harvesting season for the first time to reveal the influence of concentrated agricultural activities on atmospheric PM2.5. In comparison with the pre-harvesting period, PM2.5 concentration increased by a factor of 1.20–1.73 during the harvesting and post-harvesting periods despite of relatively higher wind speeds. Organic carbon (OC) in PM2.5 increased most significantly, with its average concentration (and its proportion in PM2.5) increased from 9.0 μg/m3 (15.6%) during pre-harvesting period to 18.8 μg/m3 (28.7%) during the harvesting period and to 28.8 μg/m3 (32.1%) during the postharvesting period, implying that OC emission relating to the harvest event made conspicuous contribution to atmospheric PM2.5. The sources of OC varied significantly during the process of the harvest event, e.g., the ratio of OC to element carbon (EC) increased from 3.33 during the pre-harvesting period to 4.54 during the harvesting period and to 5.36 during the post-harvesting period. Straw crushing and agricultural machineries during the harvesting period were suspected to the major sources for the elevation of OC because of sparse fire spots around the sampling site, whereas biomass burning during the post-harvesting period became the dominant contributor to atmospheric OC. Besides OC and EC, the concentrations of Cl- and K+ were also evidently increased from the preharvesting period to the post-harvesting period. The remarkable elevation of both Cl- and K+ also linked with the harvest event besides biomass burning, e.g., the release from straw crushing during the harvesting period and fertilization of the compound fertilizer with NH4Cl during the post-harvesting period. The maximal concentrations of the crustal components (Na2+, Mg2+ and Ca+) appeared during the harvesting period, revealing that straw crushing for returning field was their important source. The atmospheric SO42− concentration exhibited a steady decreasing trend from 15.2 μg/m3 to 8.3 μg/m3, whereas NO3− concentration significantly increased from 11.2 μg/m3 during the harvesting period to 16.2 μg/m3 during the post-harvesting period. The significant increase of NO3− concentration during the post-harvesting period was mainly attributed to the fertilization event which resulted in emissions of NH3, NOx and HONO to accelerate NO3− formation. Given the considerable contribution (more than 20%) on PM2.5, the intensive agricultural activities during the harvesting season in the NCP should be aroused greater attention, especially for the current disposal of crop straws and the surplus fertilization.