The regional nature of nitrate-dominant haze pollution during autumn over the Pearl River Delta area

China has experienced severe haze and visibility degradation problems in recent years because of rapid urbanization and industrialization. In this study, daily atmospheric fine-aerosol samples were concurrently collected at three sites over the Pearl River Delta (PRD) region during 1–29 October 2014. PM_2.5 samples were analyzed for organic carbon and elemental carbon using a thermal/optical carbon analyzer. Major water-soluble inorganic ions including F⁻, Cl⁻, NO₃⁻, SO₄²⁻, Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺ were analyzed by ion chromatography. The results show that the temporal variations of PM_2.5 concentration at the three sites were highly parallel, with particle pollution events simultaneously observed during 13–19 October (EP1) and 23–27 October (EP2), suggesting that pollution events in the PRD region are usually regional. This can most likely be attributed to the significant influences of synoptic conditions, which regionally facilitate or block diffusion of air pollutants. The chemical compositions of PM_2.5 at the three sites were also found to be similar, dominated by organics and sulfates. The results indicate that accumulation of traffic emissions seems to play important roles in particle pollution for the PRD region, leading in this case to elevated nitrate contributions during both EP1 and EP2 at the three sites. Moreover, the authors found that locally emitted aerosols from biomass burning only markedly influenced the air condition at one site, Nanhai, during EP1, while regionally transported biomass burning aerosols from eastern and northern Guangdong Province influenced all three sampling sites when the PRD was dominated by polluted air from these directions during EP2.