Modeling the Air-Vegetation-Soil Exchange of Reactive Nitrogen.

Nitrogen is an essential building block of all proteins and thus an essential nutrient for all life. However, in excess reactive nitrogen can lead to poor air or water quality, loss of biodiversity, and impact respiratory and cardiac health. Human activity has perturbed this cycle through the combustion of fossil fuels and synthesis of fertilizers. The anthropogenic contribution to this cycle is now larger than natural sources in the United States and globally. Reactive nitrogen in the atmosphere is a precursor for ozone and particulate matter formation and contributes to nutrient loading by being washed out by precipitation and the deposition of atmospheric nitrogen gases and aerosols. Until recently, little progress has been made in modeling of the nitrogen cycle in the environment due to the complexity of and uncertainty in its transport and transformation between soil, water and atmospheric media. Improving our understanding of these multimedia transport processes has been complicated by the typical focus of research on specific media and the difficulty in parameterizing the human dimension of anthropogenically fixed reduced nitrogen and input into the environment, primarily through mineral fertilizer application to crops. Here we will focus on measurements and modeling of the exchange of gaseous nitrogen species between the soil and the atmosphere, with an emphasis on ammonia. The U.S. EPA’s Community Mulitscale Air Quality (CMAQ) model will be described and evaluated against observations and the sensitivity of atmospheric particulate matter and nitrogen deposition CMAQ to NH3 emissions will be explored. Modeling of field scale measurements, the use of multimedia measurements to constrain model parameters, and how this can improve continental scale models will be presented. These findings will be presented with an emphasis on the sensitivity of the modeling system to air-soil exchange parameterizations and how the representation of these emissions can be improved.