A Simplified Model of Predicting SO2 Absorption by Single Atmospheric Raindrops with Chemical Dissociation and Internal Circulation

Received: August 13, 2011
Revised: September 28, 2011
Accepted: September 28, 2011
Download Citation: RIS|BibTeX|https://doi.org/10.4209/aaqr.2011.08.0130  

Cite this article:
Chen, W.H., Chen, Y.Y. and Hung, C.I. (2011). A Simplified Model of Predicting SO2 Absorption by Single Atmospheric Raindrops with Chemical Dissociation and Internal Circulation. Aerosol Air Qual. Res. 11: 860-872. https://doi.org/10.4209/aaqr.2011.08.0130

ABSTRACT

A simplified model of predicting chemical SO₂ absorption by single freely falling raindrops with internal circulation in the atmosphere is developed in the present study. By multiplying a modification factor α into the model of interfacial velocity established from creeping flow, it is found that the relative error between the simplified model and the two-phase simulation method is less than 4%. Accordingly, the simplified model enables us to simulate the atmospheric SO₂ absorption process with less computational effort and without losing accuracy. The simulated results indicate that the dissociation of H₂SO₃ governs the mass transfer process and the concentration of HSO₃⁻ is by far larger than those of SO₃²⁻ and H₂SO₃. As a result, the chemical absorption takes a much longer period of time to achieve the uptake process. Specifically, for the raindrop radius in the range of 200–500 μm, the absorption time of chemical absorption is larger than that of physical absorption by the factors of 70–290. From the perspective of characteristic time, mass diffusion is the controlling mechanism for SO₂ absorption. When chemical absorption is carried out, the absorption period is 28–33 folds of the characteristic time of mass diffusion, implying that the former is always larger than the latter by over an order of magnitude.

Keywords: Model; Raindrop; Below-cloud scavenging; Sulfur dioxide (SO2) uptake; Transient; Chemical absorption; Mass diffusion number