In-cloud and below-cloud scavenging of Nitric acid vapor

Scavenging coefficients, A, for the removal of HNO 3 vapor from the atmosphere by both cloud and rain drops have been calculated for assumed models of drop-size distribution. For cumulus clouds a value of 0.2 s −1 is estimated for A. Evaluation of rainfall washout coefficients gives values of A ranging from 1.3 × 10 −5 to 1.5 × 10 −3s −1 , depending upon the rainfall rate, upon the drop-size distribution function employed, and, strongly upon the lower limit of the raindrop size employed in the calculations. The concentration of soluble gas dissolved within a falling drop per unit fall distance is found to be a function of drop size, with the smaller drops accumulating the greater concentration. The long-term average rate of heterogeneous removal of HNO 3 is estimated in the range 1–8 × 10 −6s −1 , representing comparable contributions from dry deposition and rainfall scavenging.