Collective impacts of biomass burning and synoptic weather on surface PM2.5 and CO in Northeast China

Biomass burning has become one of the prominent sources of air pollution in Northeast China, where the land is largely covered by forest, grasslands, and agricultural crops. Nevertheless, the impact of biomass burning on air quality and the sensitivity of such impact to weather are rarely documented. In this study, we addressed these issues using satellite fire radiative power (FRP) and surface PM2.5 and CO data at 24 stations during fire seasons (March, April, October and November) in Northeast China from 2015 to 2017. Fire-polluted days were identified by tracing air parcels at the stations back to fire regions using a trajectory model, the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. The results show that 60-80% the polluted days in Northeast China can be attributed to biomass burning in the fire seasons, varying among subregions due to the differences in landscapes, fire activities, and weather conditions. On fire-polluted days, the mean PM2.5 and CO concentrations from all observation stations in Northeast China reached 111 mu g m(-3) and 1.3 mg m(-3), respectively. The hourly PM2.5 and CO concentrations on fire-polluted days could reach as high as 1000 mu g m(-3) and 10 mg m(-3), respectively. By subregions, the mean PM(2.5 )concentrations were 128, 112, 102, 106 mu g m(-3), respectively, in the central, eastern, northern, and southern subregions, while mean CO concentrations were 1.2, 1.3, 0.8, 1.5 mg m(-3), respectively, in the four corresponding subregions. On average, PM2.5 and CO concentrations on fire-polluted days elevated, respectively, by 22-54% and 4-11% from those on no-fire-polluted days, with the largest enhancement in the central subregion and the least in the southern subregion. We classified six predominant weather patterns during the fire seasons. When Northeast China was ahead of a strong Siberian High, fire-polluted days were least and FRP was weakest among the six patterns in most subregions, leading to lowest enhancement of PM2.5 and CO in the subregions. In contrast, two weather patterns exacerbated PM2.5 and CO pollution the most during fire episodes. Under one of the weather patterns, Northeast China was under a stagnant high-pressure system, resulting in poor dispersion conditions and thus high PM2.5 and CO pollution. The other pattern was characterized with high humidity and southwesterlies ahead of a trough, which brought about strong hygroscopic growth and regional transport of fire emissions. The degree to which the weather patterns impacted air pollution during fires varied largely among subregions. This study highlights the combined impacts of biomass burning and weather on air pollution, The findings and the developed methodology are helpful for forecasting air quality and implementing mitigation strategies during biomass burning.