Estimated washout coefficients for sulphur dioxide, nitric oxide, nitrogen dioxide and ozone

Abstract

The washout coefficient of a gas in air is the fraction of it removed in unit time by rain below cloud base. The ‘apparent’ coefficients were estimated by statistically comparing hourly ground-level concentrations just before and at the onset of heavy, non-frontal rain. The concentrations were obtained from 5 y of continuous monitoring at a rural site. The coefficient (s −1) estimated for SO 2 was (2.61 ± 0.14) × 10 −5 times the rate of rainfall (mm h −1). This is completely consistent with a previously published value derived from the data from 10 rural sites for one year, and both estimates are consistent with published values of dissolved SO 2 in rainwater at another rural site, giving some confidence in the technique. The values of the coefficients estimated for NO and NO 2 were about 40 and 80%, respectively of that for SO 2. Some nitrite is found in rainwater, but not enough to explain the washout found. However, the analysers used would measure HNO 3 aerosols as NO 2, and these are very soluble. In addition, fast reactions are known which can convert oxides of nitrogen into soluble nitrates, present in sufficient concentration in rainwater. The statistical process leads to the coefficient estimated for O 3 being negative, implying that O 3 is produced at the time of rain. This is most probably due to the strong winds and turbulence which accompany heavy rain, giving replenishment of low-level O 3 from upper air levels where O 3 is normally produced. No systematic changes in the other gas concentrations accompany the changes in O 3 values.