Analytical Chemistry Seminar
Mechanistic Modeling of Reactive Soil Nitrogen Emissions on a Continental Scale, by Quazi Ziaur Rasool, Rice University
"Nitrogen is an essential building block of all proteins and thus an essential nutrient for all life, including crops. Biological Nitrogen Fixation is the natural source of soil nitrogen available for biogeochemical transformations. However, anthropogenic perturbation to nitrogen cycle through the combustion of fossil fuels and consistently increasing fertilization is now larger than natural sources in the United States and globally. Recent global nitrogen budgets estimate that soil reactive nitrogen (Nr) emissions (predominantly from biochemical transformations in soil) have increased by a factor of 2-3 from pre-industrial levels. These increases are especially pronounced in agricultural regions. These emissions from biogeochemical transformations can be in reduced (NH3) or oxidized (NO, HONO, N2O) form, depending on complex biogeochemical transformations of soil nitrogen reservoirs.
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 cascade of nitrogen from soil to the atmosphere due to the complexity of and uncertainty in its transport and transformation. The lack of understanding of these multimedia transport processes is due to the typical focus of research on specific media and the difficulty in parameterizing the anthropogenically fixed nitrogen and their input into the atmosphere, primarily through mineral fertilizer application to crops, the largest source of environmental reactive nitrogen.
This talk will focus on modeling of the exchange of gaseous nitrogen species between the soil and the atmosphere, with an emphasis on Nitrogen oxides (NO, HONO). Contemporary air quality models like U.S. EPA’s Community Mulitscale Air Quality (CMAQ) model, typically neglect soil emissions of HONO and N2O. Previous soil nitrogen parametrizations in CMAQ have focused on NO emissions only and in a manner inconsistent with how soil NH3 emissions (i.e. accounting for anthropogenically fixed nitrogen from fertilizer application and atmospheric deposition). Thus, there is a need to more mechanistically and consistently represent the soil N processes that lead to emissions to the atmosphere. The new mechanistic scheme addresses the spatial-temporal variability of different reactive nitrogen emissions from soil through complex transport and transformation of soil nitrogen pools in both agricultural and non-agricultural soils. The CMAQ model with a new mechanistic scheme for modeling reactive nitrogen emissions from soil will be described and evaluated against observations of atmospheric particulate matter and NOx emissions. The use of multimedia and biome-specific measurements to constrain model parameters, and how this can improve continental scale (Continental US) models will be presented. These findings will be presented with an emphasis on the sensitivity of the modeling system to different air-soil exchange parameterizations and how the representation of these emissions can be improved."