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

Özet

Rainwater samples were collected to investigate the water-soluble species levels and their sources at a semi-aridurban site of Xi'an, China, during 2015. The precipitation samples were analyzed for pH, electric conductivity(EC), ionic species ($Na^+4, $K^+$, $NH_4^+$, $Ca^{2+}$, $Mg^{2+}$, $F^-$, $Cl^-$, $No_3^-$ and $SO_4^{2-}$), and water-soluble organiccarbon and nitrogen (WSOC and WSTN). The annual volume-weighted mean (VWM) of pH and EC values were6.50 and 46.9 μS $cm^{-1}4, respectively. The total ionic concentrations in rainwater varied widely, ranging from121.9 to 2448.2 μeq $L^{-1}$ with an annual VWM of 686.4 μeq $L^{-1}$, $SO_4^{2-}$ was the largest contributor to anionswhile $Ca^{2+}$ was the most important cation. $SO_4^{2-}$ and $No_3^-$ levels decreased in a large scale by comparing withprior 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. Theaverage concentrations of water-soluble organic carbon and total nitrogen (WSOC and WSTN) were 2.9 and3.9 mg $L^{-1}$. The water-soluble organic nitrogen was the dominant component in WSTN. The neutralizationfactor showed that $NH_4^+$ and Ca2+ were the dominant neutralization substances in the rainwater, whereas theeffects 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. Sourceapportionment by a positive matrix factorization receptor model showed that the anthropogenic emissions werethe 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 wasthe dominant contributor (75.6%), followed by biomass burning (14.6%).