Strong biomass burning contribution to ambient aerosol during heating season in a megacity in Northeast China: Effectiveness of agricultural fire bans?

Abstract Sustainable use of crop residues remains a challenge in main agricultural regions of China such as the Northeast Plain. Here we investigated the impacts of biomass burning on fine particle (PM2.5) during a six-month long heating season in the Harbin-Changchun (HC) metropolitan area, China's only national-level city cluster located in the severe cold climate region. Temporal variation of PM2.5 was found to coincide with that of levoglucosan. This was attributed to the strong contribution of biomass burning to organic aerosol (the dominant component in PM2.5), as supported by the source apportionment results and high levoglucosan-to-organic carbon (OC) ratios. Furthermore, the variation of biomass burning contribution was inferred to be driven mainly by agricultural fires with relatively low combustion efficiencies, based on a synthesis of the relationship between OC and elemental carbon (EC), the dependence of EC on carbon monoxide, and the relative abundances of different tracers for biomass burning. Nitrate formation was enhanced during biomass burning episodes whereas no evidence was observed to indicate enhanced sulfate formation or net increase of OC mass due to secondary formation. This study demonstrates the importance of open burning as a source of haze pollution in the HC region.