Sensitivity of a global model to the uptake of N2O5 by tropospheric aerosol

The uptake of N₂O₅ on aerosol impacts atmospheric concentrations of NO_x and so O₃, OH, and hence CH₄. Laboratory studies show significant variation in the rate of uptake, with a general decline in the value of γ_N2O5 over the last decade as increasingly relevant tropospheric proxies have been studied. In order to understand the implication of this decline for tropospheric composition, a global model of tropospheric chemistry and transport (GEOS-Chem) is run with differing values of γ_N2O5 (0.0, 10×10^-6, 10×10^-4, 10^-3, 5×10^-3, 10^-2, 2×10×10^-2, 0.1, 0.2, 0.5, and 1.0). We identify three regimes in the model response. At low values of γ_N2O5, the model shows reduced sensitivity to the value of γ_N2O5 as heterogeneous uptake of γ_N2O5 does not provide a significant pathway to perturb NO_x burdens. At high values of γ_N2O5 the model again shows reduced sensitivity to the value of γ_N2O5, as NO_x loss through heterogeneous removal of γ_N2O5 is limited by the rate of production of NO₃, rather than the rate of heterogeneous uptake. At intermediate values of γ_N2O5 the model shows significant sensitivity to the value of γ_N2O5. We find regional differences in the model's response to changing γ_N2O5. Regions with high aerosol surface area and low temperatures show NO₃ production becoming rate limiting at lower γ_N2O5 values than regions with lower aerosol surface area and higher temperatures. The northern extra-tropics show significant sensitivity to the value of γ_N2O5 at values consistent with current literature (0.001-0.02), thus an accurate description of γ_N2O5 is required for adequate simulation of O₃ burdens and long-range transport of pollutants in this region.

Our model simulations also provide insight into the sensitivity of coupled chemistry-aerosol simulations to the choice of γ_N2O5. We find little change in the global sensitivity of NO_x, O₃ and OH to γ_N2O5 in the range 0.05 to 1.0, but a significant drop in sensitivity below this range. Thus simulations of the coupled impact of both chemistry and aerosol changes through time will be sensitive to the choice of γ_N2O5.