Chemical characteristics of PM_2.5: Impact of biomass burning at an agricultural site of the North China Plain during a season of transition

Abstract. Biomass burning (BB) activities are ubiquitous in China, especially in North China, where there is an enormous rural population and winter heating custom. In order to better understand their impacts on aerosol chemical characteristics in rural and agricultural areas of the North China Plain, BB tracers (i.e., levoglucosan (LG), mannosan (MN) and potassium (K⁺)), as well as other chemical components were quantified at a rural site (Gucheng, GC) from 15 October to 30 November, during a transition heating season, when the field burning of agricultural residues was becoming intense. The measured daily average PM_2.5 concentrations of LG, MN and K⁺ during this study were 0.79 ± 0.75 μg m⁻³, 0.03 ± 0.03 μg m⁻³ and 1.52 ± 0.62 μg m⁻³. Due to the planetary boundary layer development, carbonaceous components and BB tracers showed higher levels at nighttime than daytime, while OM and secondary inorganic ions were enhanced during daytime, likely due to enhanced photochemical activity. An episode with high levels of BB tracers was encountered at the end of October, 2016, with high LG at 4.37 μg m⁻³. Based on the comparison of chemical components during different BB periods, it appeared that biomass combustion can obviously elevate carbonaceous components levels, whereas there seems to be essentially no effect on secondary inorganic ions in the ambient air. Moreover, the LG / MN ratios in different BB periods were consistent, while the LG / K⁺ ratio during intensive BB periods was significantly elevated at times, with K⁺ not increasing as much as LG during intensive BB episodes. This indicated that there were other sources of K⁺ in the study region, such as fireworks, fertilizer use, or soil resuspension, which don't have variable contributions of K⁺ during the intensive BB periods; however, local soft wood and vegetation combustion can't be excluded, which have efficient formation of levoglucosan during flaming fires.