The effect of random precipitation times on the scavenging rate for tropospheric nitric acid

Abstract

Studies of the effect of averaging the continuity equation for soluble species over wet and dryperiods of a precipitation cycle have shown that the effective scavenging rate which must beused in such averaged models is less than the classical value obtained by a simple average ofthe scavenging rate over a cycle. This is a consequence of the fact that for an intermittent lossprocess such as precipitation wet periods correspond to depressed and dry periods to enhancedsoluble species concentration. There is thus an inherent negative correlation betweendepartures from the mean of the scavenging rate and soluble species concentration which isnot accounted for in the averaging process. This study shows that if the source of the solublespecies is time dependent, then periods of positive correlation between scavenging rate andspecies concentration may also result from temporal averaging. This can result when the endof a precipitation period is coincident with the diminution or termination of the source. Aspecific example considered is the late afternoon rainout of nitric acid occurring just prior tothe night time cessation of its chemical production. Under some assumed precipitationscenarios, the positive correlation may dominate, leading to an effective scavenging rateexceeding the classical value. A model is developed to take account of the effect of randomprecipitation times on the nitric acid scavenging rate. These calculations give effectivescavenging rates about a factor of 2 to 3 greater than those of some other published models.For small wet fractional periods, typically less than 0.1, the effective scavenging rate mayexceed the classical value slightly, although the details of this calculation depend on the meanprecipitation period and on some of the model’s statistical assumptions. DOI: 10.1111/j.1600-0889.1988.tb00298.x