An Eulerian model for scavenging of pollutants by raindrops |
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| Institution: | 1. Department of Mathematics and Statistics, University of Strathclyde, Livingstone Tower, 26 Richmond Street, Glasgow G1 1XH, UK;2. Department of Statistics, Visva-Bharati University, Santiniketan, West Bengal 731 235, India;3. Department of Biomathematics and Game Theory, University of Warsaw, ul. Banacha 2, 02-097 Warszawa, Poland;4. Agricultural and Ecological Research Unit, Indian Statistical Institute, 203, B.T. Road, Kolkata 700 108, India;1. School of Natural and Built Environments, University of South Australia, Adelaide, Australia;2. School of Art, Architecture and Design, University of South Australia, Adelaide, Australia;1. Federal University of Pernambuco (UFPE), Av. Prof. Luís Freire 1000, Cidade Universitária, CEP: 50740-540 Recife, Pernambuco, Brazil;2. Federal University of Sergipe (UFS), Av. Marechal Rondon, s/n Jardim Rosa Elze, São Cristóvão, SE CEP: 49100-000, Brazil;3. National Institute for Space Research (INPE), Av. dos astronautas, 1758, Jd Granja, CEP: 12227-010 São José dos Campos, SP, Brazil;1. Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK;2. Computational Fluid Dynamics Unit, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou Str., Zografou Campus, 15780 Athens, Greece;3. Metropolitan College, School of Engineering, 74 Sorou Str., Marousi, Athens 15125, Greece |
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| Abstract: | An Eulerian model for simulating the coupled processes of gas-phase depletion and aqueousphase accumulation of the pollutant species during a rain event has been formulated. The model is capable of taking into account any realistic vertical profile of pollutant species concentrations and time-dependent initial aqueous-phase concentrations at the cloud base. The model considers the processes of single species absorption and dissociation in the aqueous phase. The coupled partial differential equations constituting the model are discretized into a set of ordinary differential equations by using the Galerkin method with chapeau functions as the basis functions. These equations are solved to obtain the pollutant concentrations of the gas phase and raindrops as well as the pH of raindrops as a function of time and distance below cloud-base.Simulations are performed for scavenging of gaseous HNO3, H2O2, SO2, formaldehyde and NH3. For the case of highly soluble HNO3 and H2O2, raindrops are far from equilibrium with the gas phase and their capacity for absorption of these gases is undiminished even as they reach ground level. The gas-phase concentrations for these species decrease exponentially with time and the washout is determined primarily by the rain intensity and mass-transfer coefficient of the gaseous species to the raindrops. The pollutant species concentrations in raindrops are an almost linear function of the distance below the cloud base. For the simulation conditions considered in this study, the half-life periods of these gases for removal from the atmosphere range from 15 to 40 min.For SO2 and formaldehyde, the aqueous-phase concentrations approach equilibrium as the drops fall to ground level and the gas-phase concentrations show large gradients in the vertical. Half-life periods for SO2 range from 1.3 to 13 h depending on the initial raindrop pH and rain intensity. For formaldehyde, the half-life ranges from 19 to 63 min.Solubility of NH3 is a strong function of the raindrop pH. As NH3 is absorbed, the raindrop pH increases and NH3 solubility decreases. For pre-acidified drops (pH = 4.6), ammonia solubility is very high and the drops are far from equilibrium with the gas phase throughout the falling period. The half-life for ammonia ranges from 11 min to over 3 h in our simulations. |
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