Characterization and Influence Factors of PM2.5 Emitted from Crop Straw Burning

Large quantities of particulate pollutants are emitted into the atmosphere during biomass burning processes. In China, large amounts of agricultural residues are burned in the field during harvest seasons, which influence regional air quality and human health. Corn and wheat are two major crops grown in China, whose burning was simulated in this study. The controlled laboratory simulation of straw burning was performed in the Laboratory of Biomass Burning Simulation at Peking University Shenzhen Graduate School. The burning simulation system was improved and verified. Straw burning aerosols (PM2.5) samples were collected and measured by gravimetric method. Organic carbon (OC) and elemental carbon (EC) were measured by thermal/optical method. Water-soluble inorganic ions and organic matter were measured by ion chromatography. Emission level, characterization and influence factors of crop straw burning aerosols are discussed. PM2.5 emission factors of corn and wheat straw burning are 1082.8 and 835.7 similar to 897.3 mg/kg, respectively. Organic matter (OM), which is calculated by multiplying organic carbon (OC) by 1.3, is the major component of PM2.5, accounting for 42%similar to 66% of the total mass. Nearly half (37%similar to 50%) of OM are water soluble. and K+ are two major components among water-soluble inorganic ions, accounting for 4%similar to 15% and 2%similar to 14% of total particle mass, respectively. The K+/EC ratio is 0.5 similar to 3.8. The proportions of these species in PM2.5 are comparable to previous studies. Straw moisture content and burning temperature influence the emission level and characterization of straw burning aerosols. Emission factors of PM2.5 and OC increase with the increase of straw moisture content because of incomplete burning. With higher moisture content, more thermal energy is used for the evaporation of water, lowering the burning temperature. Then less proportion of K+ and Cl- are released from biomass into the smoke. Therefore, their contributions to the particle mass decrease with the increase of straw moisture content. The emissions of PM2.5 and OC/EC by crop straw burning in the field are estimated based on the emission factors obtained in this study. Corn and wheat burning in the field yield 92.7 Gg PM2.5 and 47.5 Gg OC every year in China, accounting for important fractions among the total mass.