Spatio-temporal characteristics and source apportionment of water-soluble organic acids in PM 2.5 in the North China Plain

Water-soluble organic acids (WSOAs) are important chemical components of fine particulate matter (PM 2.5 ). WSOAs can aggravate PM 2.5 pollution and contribute to the acidity of aerosol and rain. PM 2.5 pollution in the North China Plain (NCP) has been extensively researched. However, with the decrease in PM 2.5 and enhancement of atmospheric oxidation in the NCP, it is unclear how the amount and source of WSOAs in ambient PM 2.5 change. In this study, we investigated WSOAs in PM 2.5 from 2018 to 2019 at urban and rural sites in the NCP. Our results showed that WSOAs accounted for 0.8% of the PM 2.5 mass with a dominance of oxalic acid (OA), followed by formic acid (FA) and methanesulfonic acid (MSA). High MSA concentrations at inland sites, such as Yucheng, indicated that MSA in PM 2.5 primarily originated from anthropogenic activities rather than marine sources, as previously inferred. WSOAs in ambient PM 2.5 were comparable at rural and urban sites with 0.73 ± 0.46 μg/m³ and 0.66 ± 0.40 μg/m³, respectively. The secondary transformation of pollutants from coal combustion and vehicle exhaust emissions was the major WSOAs source, with an average contribution of 70% at all sites. Photochemical oxidation played a stronger role in WSOAs transformation than aqueous oxidation. The potential source contribution function analysis revealed that southern Hebei and central Shandong provinces were potential source areas for WSOAs in the NCP. In the future, the integrated regional control of precursor and oxidant emissions will be important in the NCP.