Source apportionment of reactive nitrogen in fog vs. rain using isotope ratios and Bayesian modeling (Ore and Eagle Mts., Central Europe)

Estimates of atmospheric inputs of reactive nitrogen (N_r) into ecosystems are often based on vertical deposition. We investigated to what extent horizontal deposition affects total atmospheric deposition of N_r in two sparsely populated mountain regions near the Czech-German-Polish borders in an era of easing pollution. Among three scenarios of horizontal-water contribution to rainfall (additional 5, 10 and 20%), the 10% scenario was considered the most realistic. Between 2023 and 2025, wet horizontal deposition of NH₄⁺-N plus NO₃^--N constituted as much as 66-77% of total wet inorganic N_r deposition. Because condensation of rain and fog droplets occurs at different altitudes, we hypothesized that the N_r-source mix in horizontal and vertical deposition would differ. N-isotope analyses and Bayesian modeling indicated larger dominance (up to 75%) of vehicle-exhaust derived NH₄⁺-N in fog than in rain. In contrast, the mix of NO₃^--N sources for fog and rain was nearly identical, with average contributions of six sources making up 9-24% each. Remote large NO_x sources originating in coal-burning powerplants and natural-gas burning industries played similarly weak roles as more local and/or diffuse sources related to residential coal and wood burning, traffic and biogenic/soil emissions.