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

The uptake of N2O5 on aerosol impacts atmospheric concentrations of NOx and so O-3, OH, and hence CH4. Laboratory studies show significant variation in the rate of uptake, with a general decline in the value of gamma(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 gamma(N2O5) (0.0, 10(-6), 10(-4), 10(-3), 5 x 10(-3), 10(-2), 2 x 10(-2), 0.1, 0.2, 0.5, and 1.0). We identify three regimes in the model response. At low values of gamma(N2O5), the model shows reduced sensitivity to the value of gamma(N2O5) as heterogeneous uptake of N2O5 does not provide a significant pathway to perturb NOx burdens. At high values of gamma(N2O5) the model again shows reduced sensitivity to the value of gamma(N2O5), as NOx loss through heterogeneous removal of N2O5 is limited by the rate of production of NO3, rather than the rate of heterogeneous uptake. At intermediate values of gamma(N2O5) the model shows significant sensitivity to the value of gamma(N2O5). We find regional differences in the model's response to changing gamma(N2O5). Regions with high aerosol surface area and low temperatures show NO3 production becoming rate limiting at lower gamma(N2O5) values than regions with lower aerosol surface area and higher temperatures. The northern extra-tropics show significant sensitivity to the value of gamma(N2O5) at values consistent with current literature (0.001-0.02), thus an accurate description of gamma(N2O5) is required for adequate simulation of O-3 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 gamma(N2O5). We find little change in the global sensitivity of NOx, O-3 and OH to gamma(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 gamma(N2O5).