Chemistry of Precipitation Acidity at Irkutsk, Russia

Precipitation pH at a Russin EANET site, Irkutsk, was discussed and explained from the viewpoint of acid–base chemical equilibrium theory. The datasets employed were the wet-only daily basis observations at a subarctic climate urban site from 1999 to 2020. A distinct seasonal variation in pH was discovered for the monthly means over the entire period, from pH5.24 in July to pH6.66 in February, bracketing pH5.6, a criterion for acid rain. Monthly concentration variations in major ions, nss-SO42−, NO3−, NH4+, and nss-Ca2+, divulged that nss-Ca2+ and nss-SO42− were predominant species and that nss-Ca2+ dominated nss-SO42− during winter months and vice versa in summer. Individual pHs were explored with the concentration difference between two ions, ([nss-SO42−] − [nss-Ca2+]), defined as D2, as well as two other differences in selected ion groups, D4 = ([nss-SO42−] + [NO3−]) − ([NH4+] + [nss-Ca2+]) and D6 = ([nss-SO42−] + [NO3−]) − ([NH4+] + [nss-Ca2+] + [nss-Mg2+] + [nss-K+]), to show some clear relationships with pH. The observed results were compared with theoretical calculations, Dn = [H+] − HPCO2Ka1/[H+], where n is 2, 4, or 6. The distribution of ΔpH (=pHcalc − pHobs) for D2 demonstrated the most reasonable pattern with a median of zero among the three Dns. These quantitative results conclude that the Irkutsk pH was controlled dominantly by sulfuric acid and calcium carbonate.