Exploration of PM _2.5 sources on the regional scale in the Pearl River Delta based on ME-2 modeling

The Pearl River Delta (PRD) of China, which has a population of more than 58 million people, is one of the largest agglomerations of cities in the world and had severe PMinline-formula_2.5 pollution at the beginning of this century. Due to the implementation of strong pollution control in recent decades, PMinline-formula_2.5 in the PRD has continuously decreased to relatively lower levels in China. To comprehensively understand the current PMinline-formula_2.5 sources in the PRD to support future air pollution control strategies in similar regions, we performed regional-scale PMinline-formula_2.5 field observations coupled with a state-of-the-art source apportionment model at six sites in four seasons in 2015. The regional annual average PMinline-formula_2.5 concentration based on the 4-month sampling was determined to be 37 inline-formulaµg m⁻³, which is still more than 3 times the WHO standard, with organic matter (36.9 %) and inline-formula $M9inlinescrollmathmlchem{\mathrm{normal\; SO}}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 29pt17ptsvg-formulamathimg06a1e144313624090049b6627390d3e8 acp-18-11563-2018-ie00001.svg29pt17ptacp-18-11563-2018-ie00001.png (23.6 %) as the most abundant species. A novel multilinear engine (ME-2) model was first applied to a comprehensive PMinline-formula_2.5 chemical dataset to perform source apportionment with predetermined constraints, producing more environmentally meaningful results compared to those obtained using traditional positive matrix factorization (PMF) modeling. The regional annual average PMinline-formula_2.5 source structure in the PRD was retrieved to be secondary sulfate (21 %), vehicle emissions (14 %), industrial emissions (13 %), secondary nitrate (11 %), biomass burning (11 %), secondary organic aerosol (SOA, 7 %), coal burning (6 %), fugitive dust (5 %), ship emissions (3 %) and aged sea salt (2 %). Analyzing the spatial distribution of PMinline-formula_2.5 sources under different weather conditions clearly identified the central PRD area as the key emission area for inline-formulaSO₂, inline-formulaNO_x, coal burning, biomass burning, industrial emissions and vehicle emissions. It was further estimated that under the polluted northerly air flow in winter, local emissions in the central PRD area accounted for approximately 45 % of the total PMinline-formula_2.5, with secondary nitrate and biomass burning being most abundant; in contrast, the regional transport from outside the PRD accounted for more than half of PMinline-formula_2.5, with secondary sulfate representing the most abundant transported species.