Sensitivity of Simulated Ammonia Fluxes in Rocky Mountain National Park to Measurement Time Resolution and Meteorological Inputs

Abstract. Gaseous ammonia (NH₃) is an important precursor for secondary aerosol formation and contributes to reactive nitrogen deposition. NH₃ dry deposition is rarely quantified due to the complex bidirectional nature of NH₃ atmosphere-surface exchange and lack of high time-resolution in situ NH₃ concentration and meteorological measurements. To better quantify NH₃ dry deposition, measurements of NH₃ were made above a subalpine forest canopy in Rocky Mountain National Park (RMNP) and used in situ micrometeorology to simulate bidirectional fluxes. NH₃ dry deposition was largest during the summer, with 48 % of annual net NH₃ dry deposition occurring in June, July, and August. A net annual dry deposition estimated using measured 30-minute NH₃ concentrations and in situ meteorological data, accounted for 6 % of total RMNP reactive inorganic N deposition. Because in situ, high-time resolution concentration and meteorological data are often unavailable, the impact on estimated deposition from more commonly available input data was evaluated. Fluxes simulated with biweekly NH₃ concentrations, commonly available from NH₃ monitoring networks, underestimated NH₃ dry deposition by 29 %. These fluxes were strongly correlated with 30-minute fluxes integrated to a biweekly basis (R² = 0.89) indicating that a correction factor could be applied to mitigate the observed bias. Application of an average NH₃ diel concentration pattern to the biweekly NH₃ concentration data removed the observed low bias. Annual NH₃ dry deposition from fluxes simulated with reanalysis meteorological inputs exceeded simulations using in situ meteorology measurements by 59 %.