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 poorly 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 with in situ micrometeorology to simulate bidirectional fluxes. NH3 dry deposition was largest during the summer, with 47 % of annual net NH3 dry deposition occurring in June, July, and August. Because in situ, high time-resolution concentration and meteorological data are often unavailable, the impacts on estimated deposition from utilizing more commonly available biweekly NH3 measurements and ERA5 meteorology were evaluated. Fluxes simulated with biweekly NH3 concentrations, commonly available from NH3 monitoring networks, underestimated NH3 dry deposition by 45 %. These fluxes were strongly correlated with 30 min fluxes integrated to a biweekly basis (R2 Combining double low line 0.88), 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 a factor of 2.