Calcium and ammonium now control the pH of wet and bulk deposition in Ohio, U.S.

As precipitation acidity in the United States has decreased dramatically over the past forty years, new questions have emerged about the role of base cations and nitrogen (N) species in regulating precipitation pH. To address these questions, we studied precipitation chemistry in Ohio, a state with relatively high N deposition and a mix of urban, industrial, and agricultural land use. We used long-term data from the National Atmospheric Deposition Program (NADP) to assess temporal changes in the drivers of wet deposition pH in Ohio. We supplemented our analysis with short-term data on spatial patterns of bulk deposition pH collected along an agricultural-to-urban transect in central Ohio. From 1979 to 2019, wet deposition pH at the Ohio NADP sites increased from 4.22 ± 0.05 to 5.33 ± 0.21, and the cation-to-anion ratio increased from 0.92 ± 0.02 to 1.35 ± 0.06, due to a dramatic decline in measured anions. As correlations between wet deposition pH and the anions SO42− and NO3− weakened, correlations between wet deposition pH and the cations NH4+ and Ca2+ strengthened, suggesting that these cations are playing a larger role in controlling precipitation pH today. In recent years (2021–2022), NH4+ was significantly correlated with bulk deposition pH in all land uses except the urban site, with the strongest correlation at the rural/agricultural site, where the pH was 5.96 ± 0.49. Similarly, Ca2+ was strongly and significantly correlated with bulk deposition pH across all land uses. For Ca2+, the urban site was distinct from the other sites due to the high pH (6.58 ± 0.44) and Ca2+ concentrations at that site, likely driven by high levels of particulate matter deposition. Our results indicate that land use plays a role in the strength and dominant process of acid neutralization, with Ca-rich dust more important in urban areas and reduced N more important in agricultural regions. As precipitation pH increases and land use changes across the country, base cations and reduced N will play an increasingly important role in regulating precipitation pH in the U.S., with important implications for ecosystems continuing to recover from acid rain.