Model of Below-Cloud Scavenging of Moderately and Highly Soluble Gaseous Pollutants by Rain in Inhomogeneous Atmosphere Equation of mass balance for soluble trace gas in the gaseous and liquid phases: B. Krasovitov, T. Elperin and A. Fominykh the dissolved gas transferred by rain droplets: Department of Mechanical Engineering, Ben-Gurion University of the Negev, Beer- Sheva, Israel e u - velocity of a droplet. Abstract Results and discussion Description of the model 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. Feingold-Levin DSD: Integral mass balance of the dissolved gas in a droplet: Fig. 2. Evolution of ammonia (NH 3) distribution in the atmosphere due to equilibrium scavenging by rain. (1) where - mixed-average concentration of the dissolved gas in a droplet - concentration of a soluble gaseous pollutant in a gaseous phase R is the rain intensity (mm h– 1). Fig. 4. Dependence of scavenging coefficient vs. altitude for ammonia wash out (linear initial distribution of ammonia in the atmosphere ( ). Total concentration of soluble gaseous pollutant in gaseous and liquid phases reads: Gas absorption by falling droplets (2) SO 2 absorption of boiler flue gas HF absorption in the aluminum industry In-cloud scavenging of gaseous pollutants (SO 2, CO, NOx, NH 3) Fig. 5. Dependence of scavenging coefficient vs. altitude for ammonia wash out (linear initial distribution of ammonia in the atmosphere ( ). (3) Since Scavenging coefficient: then from (1) and (3) we obtain: Falling rain droplets (4) Air Fig. 6. Evolution of HNO 3 distribution in the atmosphere due to scavenging by rain. is the species in dissolved state Soluble gas Henry’s law: Vertical concentration gradient of soluble gases Fig. 7. Dependence of scavenging coefficient vs. rain intensity for NH 3 wash out at the later stage of rain (5) Description of the model Conclusions Boundary conditions Combining Eqs. (1) - (5) we obtain: Gaseous pollutants in atmosphere (7) (8) (6) (9) SO 2 and NH 3 – anthropogenic emission CO 2 – competition between photosynthesis, respiration and thermally driven buoyant mixing Fig. 3. Evolution of ammonia (NH 3) distribution in the atmosphere due to scavenging by rain. where Equilibrium scavenging (10) Solution of Eq. (10) with initial and boundary conditions (7)-(8) reads: Fig. 1. Aircraft observation of vertical profiles of CO 2 concentration (by Perez. Landa et al. , 2007) (11) For highly soluble gas: where (12) 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 heightdependent. 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 of the coefficient of diffusion. Scavenging of highly soluble gases is independent of gas solubility. References 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, 21332139. 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.
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