Impact of Crop Residue Burning on PM_2.5 Inorganic Components in Beijing-Tianjin-Hebei and Surrounding Areas

Tremendous advances in atmospheric aerosol particle research have taken place in the last decade in the context of climate and global change science. Biomass burning, one of the largest sources of accumulation mode particles globally, is closely studied for its radiative, geochemical, and dynamic impacts. In addition to primary aerosols such as organic carbon (OC) and black carbon (BC), straw burning emits a large amount of gaseous precursor of aerosols. The transformation of these gaseous precursors to secondary aerosols is one of the important ways to the formation of heavy pollution episodes. Due to rapid economic growth and urbanization in China, crop residues are often burnt in a couple of days post harvests to prepare for planting the next season's crops. The North China Plain is a major agricultural base in China with straw burning widely in the field. Remote sensing data, ground monitoring data, meteorological data are used for analyzing effects on the haze pollution from autumn crop residue burning over Beijing-Tianjin-Hebei area. Results indicate that lots of heavy pollution processes are related to the pollutant transmission from the crop residue burning in surrounding regions. Emission characteristics of straw burning during the autumn harvest season (October 2014) are analyzed, using the regional chemical transport model WRF-Chem. Effects of straw burning on gaseous precursors of inorganic aerosols and their oxidation products as well as resulting changes of sulfate, nitrate and ammonium in PM_2.5 are studied. It's found that during the autumn harvest season of 2014, the straw burning emissions in Henan and Shandong provinces tend to affect Beijing-Tianjin-Hebei urban areas under the influence of the southeast wind. A large number of VOCs emitted by straw burning lead to an increase in the concentration of major oxidants in the atmosphere which enhances the regional atmospheric oxidation capacity. When the straw burning plume carrying a large number of VOCs is mixed with the urban air mass mainly composed of fossil fuel emissions, the increase of atmospheric oxidation accelerates the oxidation process of gaseous precursors such as NO₂ and SO₂ emitted by anthropogenic sources and increases the conversion rate of sulfate and nitrate. Ammonia-rich state in Beijing-Tianjin-Hebei area are favorable for the formation of secondary inorganic aerosols, and straw burning intensifies the development of this process, and then results in a significant increase in the concentration of nitrate, sulfate and ammonium.