Evaluating the Effects of Biomass Burning on Severe Haze Formation in a Megacity of Sichuan Basin, Southwestern China

Severe haze pollution has long been an environmental problem, which is complicated and poorly understood in the Sichuan Basin (SCB). In this study, a field observation was carried out to investigate the factors driving haze formation in urban Chengdu, a typical megacity in the SCB. It was found that the accumulation of biomass burning organic aerosol (BBOA) played an important role in haze formation in urban Chengdu. The average mass fraction of BBOA in PM_2.5 increased from ∼1% during clear days to ∼10% during severe haze episodes. A method combining backward trajectory analysis with fire spot distribution was used to evaluate the effects of regional transport of biomass burning (BB) emissions. The results showed that BBOA concentration increased by ∼3 times and PM_2.5 concentration increased by ∼54% when BB emissions were transported from adjacent areas to urban Chengdu. Moreover, the parameter f₆₀ (the ratio of the integrated signal at m/z 60 to the total signal in the organic component mass spectrum), which indicated the impacts of BB emissions, was reassessed to be 0.54% instead of the widely used value 0.3% previously. Our results uncovered the importance of BB emissions on haze formation in urban areas in the SCB and provided new insights into pollutant mitigation strategies in the region.