Abstract. 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 PM 2.5 pollution at the beginning of this century. Due to the implementation of strong pollution control in recent decades, PM 2.5 in the PRD has continuously decreased to relatively lower levels in China. To comprehensively understand the current PM 2.5 sources in the PRD to support future air pollution control strategies in similar regions, we performed regional-scale PM 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 PM 2.5 concentration based on the 4-month sampling was determined to be 37 µg m −3 , which is still more than 3 times the WHO standard, with organic matter (36.9 %) and SO 2− 4 (23.6 %) as the most abundant species. A novel multilinear engine (ME-2) model was first applied to a comprehensive PM 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 PM 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 PM 2.5 sources under different weather conditions clearly identified the central PRD area as the key emission area for SO 2 , NO 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 PM 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 PM 2.5 , with secondary sulfate representing the most abundant transported species.

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13The Pearl River Delta (PRD) of China, which has a population of more than 58 million people, is 14 one of the largest agglomerations of cities in the world and ever experienced severe PM2.5 15 pollution at the beginning of this century. Due to the implementation of strong pollution control in 16 recent decades, PM2.5 in the PRD has continuously decreased to relatively lower levels in China. 17To comprehensively understand the current PM2.5 sources in the PRD to support future air 18 pollution control strategy in similar regions, we performed regional-scale PM2.5 field observations 19 coupled with a state-of-the-art source apportionment model at six sites in four seasons in 2015. 20The regional annual average PM2.5 concentration was determined to be 37 μg/m³, which is still 21 more than three times the WHO standard, with organic matter (36.9%) and SO₄²ˉ (23.6%) as the 22 most abundant species. A novel multilinear engine (ME-2) model was then applied to the PM2.5 23 dataset in the PRD to perform source apportionment with predetermined constraints, which 24 produced more environmentally meaningful results compared to those obtained using traditional 25 positive matrix factorization (PMF) modeling. The regional annual average PM2.5 source structure 26 was retrieved to be secondary sulfate (21%), vehicle emissions (14%), industrial emissions (13%), 27 secondary nitrate (11%), biomass burning(11%), secondary organic aerosol (SOA, 7%), coal 28 burning (6%), fugitive dust (5%), ship emissions (3%) and aged sea salt (2%). Analyzing the 29 spatial distribution of PM2.5 sources under different weather conditions clearly identified the 30 central PRD area as the key emission area for SO2, NOx, coal burning, biomass burning, industrial 31 emissions and vehicle emissions. It was further estimated that under the polluted northerly air flow 32 in winter, local emissions in the central PRD area accounted for approximately 45% of the total 33 PM2.5, with secondary nitrate and biomass burning being most abundant; in contrast, the regional 34 transport from outside the PRD accounted for more than half of PM2.5, with secondary sulfate 35 representing the most abundant transported species. 36 37