Transboundary biomass-burning influence on springtime PM2.5 population exposure across southern China during 2015-2019.
Abstract
Background
Every spring (March-April), biomass-burning fires in the Indo-China Peninsula (ICP) send smoke downwind into southern China, coinciding with elevated springtime PM2.5 in Yunnan, Guangxi, and Guangdong. The radiative and ecological effects of this transport are established, but its springtime population-exposure burden remains poorly characterised.
Objective
To quantify springtime PM2.5 population exposure under WHO and Chinese benchmarks, and attribute the ICP fire contribution with a controlled model experiment.
Methods
Daily PM2.5 from 51 cities was combined with WorldPop population data to compute exposure above the WHO 2021 guideline (24-h, 15 μg/m3) and China's GB 3095-2012 Grade II standard (75 μg/m3) for March-April 2015-2019, and paired GEOS-Chem simulations at 2° × 2.5° with and without ICP fire attributed the fire contribution.
Results
At observed PM2.5 concentrations, exposure above the WHO 24-h guideline was near-universal (the population-weighted share of exceedance days reached 78 to 96% every year). Exposure above the higher China Grade II standard was rare in all three provinces, with the population-weighted share of exceedance days spanning 1 to 6% in Yunnan, 1 to 6% in Guangxi, and 0 to 5% in Guangdong; in absolute terms this corresponds to an average daily exposed population of about 6 million across the corridor, under 3% of its residents, with Guangdong's larger population offsetting its lower exceedance fraction. A controlled GEOS-Chem experiment with and without ICP fire attributed 25% (9.6 μg/m3) of Yunnan's springtime PM2.5 to ICP burning, 6% (3.3 μg/m3) in Guangxi and 4% (1.6 μg/m3) in Guangdong, a monotonic west-to-east gradient paralleled by ozone (10%, 5%, 2%). Translating this attribution into exposure, ICP fire carried an estimated average daily population of 2.2-2.6 million above the China PM2.5 standard (36-44% of all above-standard exposure), and about 11 million above the WHO ozone guideline.
Conclusions
The model-attributed ICP-fire contribution is largest in Yunnan and attenuates eastward, while national-standard exceedance remains episodic across the corridor; this pattern supports province-specific, transport-aware air-quality assessment.