The impact of precipitation scavenging on the transport of trace gases: A 3-dimensional model sensitivity study

With the global Chemistry-Transport model MATCH sensitivity simulations were performed to determine the degree to which especially upward transport of gases from the earth's surface is limited by convective and large-scale precipitation scavenging. When only dissolution of species in the liquid phase is taken into account, mixing ratio reductions in the middle and upper troposphere by approximate to 10% are calculated for gases with a Henry's Law constant H of 10(3) mol/l/atm. The removal increases to approximate to 50% for H = 10(4) mol/l/atm, and to 90% for H = 10(5) mol/l/atm. We also consider scavenging by the ice phase, which is generally much less efficient than by the aqueous phase. In fact, rejection of gases from freezing water droplets may be a source of trace gas at higher altitudes. H2O2 and the strong acids (H2SO4, HNO3, HCl, HBr, HI) have such large solubilities that they become largely removed by precipitation. When significant concentrations of these gases and sulfate aerosol exist above the liquid water domain of the atmosphere, they have likely been produced there or at higher altitudes, although some could have come from trace gas rejection from ice particles or from evaporating hydrometeors. Several other gases are affected by precipitation, but not strongly enough to prevent fractional transfer to the middle and upper troposphere: e.g., HNO4, HNO2 at pH less than or equal to 5, CH2O, the organic acids at pH less than or equal to 6, CH3SOCH3, HOCl, HOBr, and HOI. NH3 is largely removed by liquid phase scavenging at pH less than or equal to 7 and SO2 at pH greater than or equal to 7. At pH less than about 6, upward transport of SO2 should largely depend on the efficiency of oxidation processes in the water droplets by O-3 and H2O2. Most gases have solubilities which are too low for significant precipitation scavenging and aqueous phase oxidation to occur. This holds, e.g., for O-3, CO, the hydrocarbons, NO, NO2, HCN, CH3CN, CH3SCH3, CH3O2H, CH3CHO and higher aldehydes, CH3OH and higher alcohols, peroxyacetylnitrate (PAN), CH3COCH3 and other ketones (note that some of these are not listed in Table I because their solubilities are below 10 mol/l/atm). Especially for the short-lived gases, transfer from the boundary layer to the middle and upper troposphere is actually promoted by the enhanced upward transport that occurs in clouds.