Abstract
The effect of gravitational settling of cloud droplets and ice particles on the tropospheric distributions of soluble trace gases is examined, focusing in particular on nitric acid and hydrogen peroxide. For this study, we separate cloud condensate into two classes: large (rain, snow and graupel) and small (cloud water and cloud ice). The first class has typical fall speeds of several m/s and frequently reaches the ground. The redistribution and loss of soluble trace gases due to this type of precipitation is generally already included in global photochemical models (although the manner in which this is done varies widely between models). However, an additional redistribution can result from the gravitational settling of the second class, small cloud droplets or ice particles, whose mean fall speeds can reach 1 m/s. When we include this additional downward flux term in our model simulations, we find a significant impact on the distributions of the soluble trace gases HNO3 and H2O2. In the upper troposphere, the settling of ice particles leads to a strong reduction in the mixing ratios of these gases, due to the relatively high terminal velocities of ice crystals (generally > 10 cm/s). The impact of cloud droplet settling in the lower troposphere is also found to be significant in some regions, though it is generally smaller due to the slower settling velocities of liquid droplets; in addition, the settling loss in the lower troposphere is computed to be frequently overwhelmed by the supply of HNO3 and H2O2 via sublimating hydrometeors settling in from above. Our findings may help explain the overestimate of free tropospheric HNO3 found in several global photochemical modeling studies. The reductions computed for HNO3 and H2O2 also have an effect on the distributions of related trace gases, such as NOx and OH. Gravitational settling might also directly affect other soluble trace gases and aerosols. There are substantial uncertainties involved in computing the global impact of gravitational settling, such as the uptake of gases and aerosols onto and into ice particles and the high degree of variability and complexity of clouds. Results of sensitivity studies examining some of these uncertainties are presented. Even employing a low-end settling velocity for ice particles (5 cm/s) and a minimum ice uptake efficiency for HNO3 based on laboratory data, we still compute a notable impact on the upper tropospheric HNO3 and H2O2 distributions.
