Mineral dust and global tropospheric chemistry: Relative roles of photolysis and heterogeneous uptake

We investigate the influence of mineral dust on tropospheric chemistry in the present climate at the global scale. The analysis examines the effects of dust on photolysis and heterogeneous uptake, operating independently and together. In numerical experiments the size-resolved, time-varying mineral dust distribution predicted by the global Dust Entrainment and Deposition (DEAD) model perturbs the gas phase species in a global chemical transport model (University of California at Irvine (UCI) CTM). We find that the photolysis perturbation dominates limited regions in the low to middle troposphere, while heterogeneous uptake dominates the rest of atmosphere. Coupling of the photochemical and heterogeneous effects of dust is weak in the global mean but moderate in dusty regions, where coupling is sometimes responsible for more than 20% of local O-3 changes. Ozone and odd-nitrogen concentrations are perturbed in opposite directions by photolysis and heterogeneous chemistry, resulting in a weak net change. However, both processes decrease the concentrations of OH and HO2. The global mean change due to dust is -0.7% for tropospheric O-3, -11.1% for OH, -5.2% for HO2, and -3.5% for HNO3. Large seasonal signals are present near dust source regions. Over the North African region and tropical Atlantic Ocean downwind, OH decreases by -66.8%, six times more than the global mean reduction. Interestingly, net photolysis-induced annual mean O-3 changes are greater in the Southern Hemisphere than in the Northern Hemisphere, where significantly more dust and O-3 precursors reside. In polar regions, O-3 change is dominated by transported O-3 and is not sensitive to local dust concentration. O-3 change due to photolysis depends not only on dust vertical structure but also on the availability of O-3 precursors. O-3 change due to heterogeneous reactions on dust is sensitive to dust vertical structure, mainly through the influence of temperature on uptake rates.