Predominant effects of emission reduction by recording 8-year water-soluble ions in precipitation in Taiyuan, North China

To investigate the trends of chemical characteristics of precipitation in Taiyuan, and factors influencing their variations, a total of 350 precipitation samples were collected from April 2011 to November 2018. The principal water-soluble ions were measured by ion chromatography. PH value (4.34-8.72) was recorded and 14% of all samples were acidic, and was negatively correlated with rainfall amount. The frequency of acid rain decreased year by year, and the acidity was still dominated by SO4 center dot 2- SO42-, Ca2+, NH4+ and NO3- were the most abundant ions, accounting for 85% of the total concentration, with a significantly decrease rate of 61% during 8 years due to a series of emission reduction measures. The average ratio of neutralizing potential to acidifying potential was 1.37 and experienced an evident increasing trend. This was primarily ascribed to reduced input of SO42- in precipitation. The precipitation acidity was predominantly neutralized by crustal ions Ca2+ and Mg2+. The ionic wet deposition flux was positively correlated to the rainfall amount. Based on Correlation analysis and Positive Matrix Factorization results, four pollution sources of coal combustion and vehicle mixing source (56%), dust (23%), agriculture and industry mixing source (16%) and biomass burning (5%) were identified. Back trajectory showed the air quality of Taiyuan was most significantly affected by the short-distance air mass from the southwest (40%). In order to achieve the SO2 (47%) and NOx (57%) emission reduction targets, we provided a specific energy reform plan in major cities affecting the air quality of Taiyuan before 2030.