The effect of fractional cloudiness on the oxidation of SO₂

Abstract

We have investigated the effect of fractional cloudiness on the oxidation of SO₂ in the marine boundary layer. We address the question to which extent the budget of SO₂ is affected by the way cloudwater content and cloudcover are resolved in mesoscale meteorological models. In these models, the cloudwater and cloud cover are calculated simply from the grid-cell average temperature and total water content (all-or-nothing), or, mostly, a subgrid parameterization (fractional cloudiness) is used. Such a subgrid parameterization is considered much more realistic, but does not allow for a direct implementation of cloud chemistry. For both approaches, we have simulated the SO₂ concentration with a two-layer model, which contains the basic production and loss mechanism of SO₂ in a cloudy marine boundary layer. We found that the SO₂ concentration calculated using the subgrid parameterization to calculate the cloud parameters may deviate up to a factor of two as compared to that calculated using an all-or-nothing cloud parameterization. Calculations performed in a similar way for 5 observed cases of fractional cloudiness show maximum deviations between -85% and + 33%. These observed cases have been selected from the GATE, Puerto Rico '72, ASTEX, FIRE and Semamphore measurement campaigns. Meteorological chemical mesoscale models which can take fractional cloudiness into account in the simulation of cloud chemistry are expected to give an important improvement of the quantification of the sulfur budget. Consequently, the results of meteorological mesoscale models which treat SO₂ aqueous phase chemistry with all-or-nothing cloud parameters should be treated with care. Moreover, we conclude that fractional cloudiness can have a major effect on the oxidation of SO₂ in the MBL even for a relatively small cloud cover. Parameters critical to the effect are the over-all lifetime of SO₂ in the cloud and the rate of exchange between cloudy and non-cloudy air.