Chemistry of Precipitation Acidity at Irkutsk, Russia
2025
Hiroshi Hara | Izumi Noguchi
Precipitation pH at a Russin EANET site, Irkutsk, was discussed and explained from the viewpoint of acid&ndash: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&minus:, NO3&minus:, NH4+, and nss-Ca2+, divulged that nss-Ca2+ and nss-SO42&minus: were predominant species and that nss-Ca2+ dominated nss-SO42&minus: during winter months and vice versa in summer. Individual pHs were explored with the concentration difference between two ions, ([nss-SO42&minus:] &minus: [nss-Ca2+]), defined as D2, as well as two other differences in selected ion groups, D4 = ([nss-SO42&minus:] + [NO3&minus:]) &minus: ([NH4+] + [nss-Ca2+]) and D6 = ([nss-SO42&minus:] + [NO3&minus:]) &minus: ([NH4+] + [nss-Ca2+] + [nss-Mg2+] + [nss-K+]), to show some clear relationships with pH. The observed results were compared with theoretical calculations, Dn = [H+] &minus: HPCO2Ka1/[H+], where n is 2, 4, or 6. The distribution of &Delta:pH (=pHcalc &minus: 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.
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