Water-soluble components in rainwater over Xi'an in northwest China: Source apportionment and pollution controls effectiveness evaluation

Rainwater samples were collected to investigate the water-soluble species levels and their sources at a semi-arid urban site of Xi'an, China, during 2015. The precipitation samples were analyzed for pH, electric conductivity (EC), ionic species (Na ⁺ , K ⁺ , NH ₄ ⁺ , Ca ²⁺ , Mg ²⁺ , F ⁻ , Cl ⁻ , NO ₃ ⁻ , and SO ₄ ²⁻ ), and water-soluble organic carbon and nitrogen (WSOC and WSTN). The annual volume-weighted mean (VWM) of pH and EC values were 6.50 and 46.9 μS cm ⁻¹ , respectively. The total ionic concentrations in rainwater varied widely, ranging from 121.9 to 2448.2 μeq L ⁻¹ with an annual VWM of 686.4 μeq L ⁻¹ . SO ₄ ²⁻ was the largest contributor to anions while Ca ²⁺ was the most important cation. SO ₄ ²⁻ and NO ₃ ⁻ levels decreased in a large scale by comparing with prior study in six years ago. Moreover, the total wet depositions flux of water-soluble inorganic nitrogen (sum of NH ₄ ⁺ -N and NO ₃ ⁻ -N) and SO ₄ ^2- -S were reduced by 54.5 and 81.1% in 2015 compared with 2009, respectively. The results indicated that the pollution controls during the past years were more effective over Xi'an. The average concentrations of water-soluble organic carbon and total nitrogen (WSOC and WSTN) were 2.9 and 3.9 mg L ⁻¹ . The water-soluble organic nitrogen was the dominant component in WSTN. The neutralization factor showed that NH ₄ ⁺ and Ca ²⁺ were the dominant neutralization substances in the rainwater, whereas the effects of Mg ²⁺ , K ⁺ and Na ⁺ were negligible. The enrichment factor analysis indicated that SO ₄ ²⁻ and NO ₃ ⁻ were mainly originated from anthropogenic sources, while Ca ²⁺ was associated with terrestrial sources. Source apportionment by a positive matrix factorization receptor model showed that the anthropogenic emissions were the largest contributor to SO ₄ ²⁻ (61.9%), NO ₃ ⁻ (65.3%), and NH ₄ ⁺ (70.5%), followed by crustal sources (14.9, 21.3, and 18.4%) and biomass burning (13.8, 12.1, and 6.8%). For the major cations of Ca ²⁺ , crustal source was the dominant contributor (75.6%), followed by biomass burning (14.6%).