1. Effect of Rain Scavenging on Altitudinal Distribution of Soluble Gaseous Pollutants in the Atmosphere B. Krasovitov, T. Elperin, A. Fominykh Department of Mechanical Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel A. Vikhansky School of Engineering and Material Science, Queen Mary, University of London, Mile End Road, London E1 4NS, UK Abstract We analyze precipitation scavenging of soluble hazardous gases from the atmosphere by rain droplets. The developed model is valid for low gradients of soluble gaseous pollutants in a gaseous phase and is suitable for predicting scavenging of moderately soluble gases, e.g., sulfur dioxide (SO 2 ), ammonia (NH 3 ) etc. from the atmosphere. Using the equation of mass balance for soluble gaseous species in gaseous and liquid phases we derived a nonstationary convective-diffusion equation for evaluating the amount of precipitation required for scavenging of various soluble gaseous pollutants from the atmosphere and determined transient altitude distribution of these gases in the atmosphere during rain fall. Numerical solution of the derived equation with the appropriate initial and boundary conditions showed that soluble gas in the atmosphere is washed down by precipitation and is smeared by diffusion. Using the suggested model we analyzed the temporal evolution of the vertical profiles of NH 3 and SO 2 in the atmosphere caused by their washout. We calculated also scavenging coefficient. It was showed that the magnitude of scavenging coefficient varies with time and altitude and depends on the vertical distribution of soluble gaseous pollutants in the atmosphere, parameter of gas solubility and on the rain intensity. In addition, we suggest simple analytical formulas for “equilibrium scavenging” of moderately soluble gases and for scavenging of highly soluble gases, such as HNO 3 or H 2 O 2 by rain. Gas absorption by falling droplets Description of the model Results and discussion Conclusions References Vertical concentration gradient of soluble gases SO 2 absorption of boiler flue gas HF absorption in the aluminum industry In-cloud scavenging of gaseous pollutants (SO 2, CO 2, CO, NOx, NH 3 ) Soluble gas is the species in dissolved state Henry’s Law: Gaseous pollutants in atmosphere SO 2 and NH 3 – anthropogenic emission CO 2 – competition between photosynthesis, respiration and thermally driven buoyant mixing Fig. 1. Aircraft observation of vertical profiles of CO 2 concentration (by Perez- Landa et al., 2007) Description of the model Fig. 3. Evolution of ammonia (NH 3 ) distribution in the atmosphere due to scavenging by rain. It is shown that the magnitude of scavenging coefficient at the ground increases with time whereas the value of scavenging coefficient in the below-cloud atmosphere immediately adjacent to the cloud decreases with the amount of precipitation. It is shown that scavenging coefficient in the atmosphere is height- dependent. Scavenging of soluble gas begins in the upper atmosphere and scavenging front propagates downwards with “wash down” velocity and is smeared by diffusion. It is found that scavenging coefficient strongly depends on the initial distribution of soluble trace gas concentration in the atmosphere. Calculations performed for linear distribution of the soluble gaseous species in the atmosphere show that the scavenging coefficient increases with the increase of soluble species gradient. It is shown that the process of equilibrium scavenging is independent on coefficient of diffusion. Scavenging of highly soluble gases is independent on gas solubility. Elperin, T., A. Fominykh, and B. Krasovitov (2009) Effect of altitude concentration gradient of soluble gaseous pollutants on their scavenging by falling rain droplets, Journal of the Atmospheric Sciences, 66, No. 8, 2349–2358. Elperin, T., A. Fominykh, and B. Krasovitov (2010) Scavenging of soluble trace gases by falling rain droplets in inhomogeneous atmosphere, Atmospheric Environment, 44, 2133  2139. T. Elperin, A. Fominykh and B. Krasovitov (2011) Uptake of soluble gaseous pollutants by rain droplets in the atmosphere with nocturnal temperature profile, Atmospheric Research, 99, 112–119. Perez-Landa, G., P. Ciais, G. Gangoiti, J. L. Palau, A. Carrara, B. Gioli, F. Miglietta, M. Schumacher, M. Millian, and M. J. Sanz (2007) Mesoscale circulations over complex terrain in the Valencia coastal region, Spain – Part 2: Modeling CO 2 transport using idealized surface fluxes, Atmospheric Chemistry and Physics, 7, 1851–1868. (1) (5) April 3 – 8, 2011, Vienna, Austria Falling rain droplets Air Integral mass balance of the dissolved gas in a droplet: Total concentration of soluble gaseous pollutant in gaseous and liquid phases reads: where  solubility parameter  mass transfer coefficient in a gaseous phase  concentration of a soluble gaseous pollutant in a gaseous phase  mixed-average concentration of the dissolved gas in a droplet  characteristic diffusion time (2) where u - velocity of a droplet. The total flux of the dissolved gas transferred by rain droplets: (3) Since then from (1) and (3) we obtain: (4) Equation of mass balance for soluble trace gas in the gaseous and liquid phases: where Boundary conditions (7) (8) (9) Fig. 2. Evolution of ammonia (NH 3 ) distribution in the atmosphere due to equilibrium scavenging by rain. Fig. 4. Dependence of scavenging coefficient vs. altitude for ammonia wash out (linear initial distribution of ammonia in the atmosphere ( ). Fig. 5. Dependence of scavenging coefficient vs. altitude for ammonia wash out (linear initial distribution of ammonia in the atmosphere ( ). Fig. 7. Dependence of scavenging coefficient vs. rain intensity for NH 3 wash out at the later stage of rain Feingold-Levin DSD: R is the rain intensity (mm h –1 ). Scavenging coefficient: Fig. 6. Evolution of HNO 3 distribution in the atmosphere due to scavenging by rain. Combining Eqs. (1) - (5) we obtain: where (6) European Geosciences Union General Assembly 2011 Equilibrium scavenging (11) For highly soluble gas: (12) (10) Solution of Eq. (10) with initial and boundary conditions (7)-(8) reads: