Sensitivity of continental United States atmospheric budgets of oxidized and reduced nitrogen to dry deposition parametrizations

Abstract

Reactive nitrogen (Nr) is removed by surface fluxes (air-surface exchange) and wet deposition. The chemistry and physics of the atmosphere result in a complicated system in which competing chemical sources and sinks exist and impact that removal. Therefore, uncertainties are best examined with complete regional chemical transport models that simulate these feedbacks. We analysed several uncertainties in regional air quality model resistance analogue representations of air-surface exchange for unidirectional and bi-directional fluxes and their effect on the continental Nr budget. Model sensitivity tests of key parameters in dry deposition formulations showed that uncertainty estimates of continental total nitrogen deposition are surprisingly small, 5 per cent or less, owing to feedbacks in the chemistry and rebalancing among removal pathways. The largest uncertainties (5%) occur with the change from a unidirectional to a bi-directional NH3 formulation followed by uncertainties in bi-directional compensation points (1-4%) and unidirectional aerodynamic resistance (2%). Uncertainties have a greater effect at the local scale. Between unidirectional and bi-directional formulations, single grid cell changes can be up to 50 per cent, whereas 84 per cent of the cells have changes less than 30 per cent. For uncertainties within either formulation, single grid cell change can be up to 20 per cent, but for 90 per cent of the cells changes are less than 10 per cent.

Keywords: atmospheric deposition; dry deposition uncertainty; nitrogen budget.

Figure 1.

Maps of 2002 annual total nitrogen deposition (kg per hactare) for (a) unidirectional CMAQ and (b) bi-directional CMAQ; (c) total N deposition ratio.

Figure 2.

Ratio of sensitivity case to base case for aerodynamic resistance sensitivity (40% decrease): (a) increased dry Ox-N + Red-N deposition; (b) decreased wet Ox-N + Red-N deposition; (c) total N deposition ratio.

Figure 3.

Ratio of sensitivity case to base case for soil gamma sensitivity (50% increase): (a) increased NH₃ emissions; (b) increased wet + dry Red-N deposition; (c) total N deposition ratio.