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Mechanistic representation of soil N in CMAQ v5.1
Citation:
Rasool, Q., J. Bash, E. Cooter, AND D. Cohan. Mechanistic representation of soil N in CMAQ v5.1. 2017 CMAS Conference, Chapel Hill, NC, October 23 - 25, 2017.
Impact/Purpose:
Air quality models like CMAQ typically neglect soil emissions of HONO and N2O. Even with our previous update of the soil NO scheme in CMAQ, emissions estimation remained parametric and inconsistent with soil NH3 emissions. Thus, there is a need to more mechanistically and consistently represent the soil N processes that lead to emissions to the atmosphere.
Description:
Recent global nitrogen (N) budgets estimate that soil reactive N 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. The reactive N emissions from biogeochemical transformations can be in reduced (NH3) or oxidized (NO, HONO, N2O) form, depending on Complex biogeochemical transformations of soil N reservoirs. Air quality models like CMAQ typically neglect soil emissions of HONO and N2O. Even with our previous update of the soil NO scheme in CMAQ, emissions estimation remained parametric and inconsistent with soil NH3 emissions. Thus, there is a need to more mechanistically and consistently represent the soil N processes that lead to emissions to the atmosphere. Our updated approach estimates soil NO, HONO and N2O emissions by incorporating detailed agricultural fertilizer inputs from EPIC and CMAQ modeled N deposition into the soil N pool. EPIC addresses the nitrification, denitrification and volatilization rates along with soil N pools for agricultural soils only. The dynamic soil properties and nutrient (C and N) data for non-agricultural areas are hence used from global soil property and nutrient database available in literature. The EPIC-CMAQ framework utilizes bi-directional exchange only for NH3, which will be extended to other species. The NO and N2O emissions from nitrification and denitrification are computed mechanistically using the N sub-model of DAYCENT. These mechanistic definitions use soil water content, temperature, NH4+ and NO3− concentrations, gas diffusivity and labile C availability as dependent parameters at various soil layers. Soil HONO emissions will be estimated from the total nitrification NOx emission estimated from DAYCENT N sub-model. The model sets the ratio of HONO vs NO emissions based on soil moisture, pH and land use definitions modeled at sub-grid scales. We implement the scheme in CMAQ for the continental US. Comparison of the model estimated emission rates from the new mechanistic scheme with other existing schemes and measurements in the literature will be discussed.
