Components, acidification characteristics, and sources of atmospheric precipitation in Beijing from 1997 to 2020

Policies such as the Air Pollution Prevention and Control Action Plan have been implemented in Beijing to address acid rain pollution. In this study, we evaluate the effects of these policies by analyzing rainfall monitoring data from 1997 to 2020, acidification characteristics, and chemical components of precipitation in Beijing. We found that precipitation pH values had significantly changed in 2008, showing an overall declining trend during 1997-2008 before gradually recovering. The frequency of acid rain was first increased and then decreased, reaching zero levels after 2017. The multi-year average pH in urban (1997-2020), suburban (1997-2020), and background (2004-2020) sites was 5.74 +/- 0.67 (A) over cap, 6.53 +/- 0.54 (A) over cap, and 6.49 +/- 0.64 (A) over cap, respectively. The predominant cations in precipitation were NH4 + and Ca2+, where Ca2+ concentrations dropped from 332.1 mu eq.L-1 in 1997 to 179.3 mu eq.L-1 by 2015 and 81.39 mu eq.L 1 by 2020. In contrast, NH4+ concentrations were first increased from 193.4 mu eq.L-1 in 1997 to 270.5 mu eq.L-1 in 2015 and then decreased to 206.78 mu eq.L-1 by 2020. The concentration of the predominant anions, SO42- and NO3, was generally decreased; SO42- concentration was decreased from 350.88 mu eq.(-1) in 1999 to 88.19 mu eq.L-1 in 2020. However, NO3 underwent a down-up-down trend, dropping slowly from 125.3 mu eq.L-1 in 1997 to 52.2 mu eq.L-1 in 2004, then significantly increasing to 173.06 mu eq.L-1 in 2014 before showing a downward trend in volatility. The [SO42-]:[NO3] ratio was gradually decreased from 3.3 in 1997 to 1.5 in 2015, then dropped to 0.9 in 2020, indicating that the type of precipitation gradually shifted from a predominantly sulfuric acid type to a more mixed type. Conversely, the [Ca2+]:[NH4+] ratio showed a decreasing trend, suggesting NH4+ as the principal neutralizing agent in recent years. The correlation coefficients among the major crustal source components (Ca2+, K+, and Mg2+) were relatively high in the precipitation, suggesting that the contribution from ground dust was significant. The major acidic anions were primarily attributed to human activities.