In-cloud and Below-cloud Scavenging of Nitric Acid Vapor

Scavenging coefficients, A, for the removal of HNO?; 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 s1 is estimated for A. Evaluation of rainfall washout coefficients gives values of A ranging from 1.3 x 10m5 to 1.5 x 10. ‘s-t, 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, is estimated in the range l---8 x 10-6s-‘, representing comparable contributions from dry deposition and rainfall scavenging. Nitrogen oxides-NO and to a lesser extent NO,-are introduced into the atmosphere as by-products of fossil fuel combustion by electric power generation and other stationary facilities, by vehicles and by aircraft. A major fraction of these oxides of nitrogen is further oxidized by gas-phase reactions to NO, and in turn to nitric acid vapor, HNO,. The elementary reactions that comprise these processes are now rather well understood (Raulch et al., 1980, Levine and Schwartz, 1982). The formation rate of HNO, vapor has been estimated in model calculations to be as great as 0.2 ppb min’ for conditions associated with urban phot~hemi~l smog (Calvert and McQuigg, 1975) and NO, transformation rates (to HNO, and, to a lesser extent, PAN) have been reported as great as 0.14-0.24 hh’ in an urban plume (Spicer, 1980). Concentrations of HNO, as great as 5510ppb have been reported in regions influenced by the transport of air pollutants (Spicer, 1977; Okita and Ohta, 1979). However, measured HNO, concentrations in clean air are reported as substantially lower. Huebert and Lazrus (1979) report HNO, concentrations of 0.15-0.8 ppb in mid-latitudes, but substantially lower ( < 0.03 to 0.11 ppb) in the remote continental boundary layer. Similarly low values ( < 0.03 to 0.11 ppb) are reported for clean air by Kelly et al. (1980), who find nitric acid concentrations invariably less than NO, ( = NO + NO,) concentrations. Similar considerations pertain to HNO, in the stratosphere, where again this species is expected to be the principal chemical sink for nitrogen oxides. Mixing ratios of up to 1Oppb are reported for HNO, in the mid-stratosphere (25 km) (Lazrus and Gandrud, 1975; Harries et al., 1976), diminishing to substantially lower values at the tropopause. These considerations have led to heightened interest in HNO, removal processes, since in the absence of a rather fast removal rate of HNO, this species would be expected to accumulate toconcentrations substantially greater than are observed (Huebert and Lazrus, 1979). Rates of gas-phase free radical reactions (e.g. with HO to form NO,) appear to be slight and lead ultimately back to HNO,. Hence it appears that heterogeneous processes will dominate HNO, removal. Among these, reaction with aerosol particles appears to be a potentially important sink. However, as noted by Tang (1980) only basic particles would be effective in this regard, because of the high vapor pressure of HNO, above acidic salts. Hence this process is self-limiting in the absence of base such as NH,. These considerations, as well as the high solubiiity of HNO, in water (as the ions H+ and NO;) suggest that HNO, is removed from the atmosphere principally as HNO,, either by dry deposition (to the ocean or to the ubiquitous layer of water that coats virtually all surfaces) or by preoipitation scavenging (Chameides, 1975; Stedman et al., 1975). The importance of heterogeneous removal processes upon calculated concentration profiles of tropospheric HNO, has been emphasized by Fishman and Crutzen (1977) who have shown, by model calculations, an order of ~~itude decrease in HNO, concentrations as a consequence of an assumed heterogeneous removal rate of 2 x 10m6 s’ (mean residence time *:6 days) arbitrarily ascribed to HNO, in the lowest 6 km. Such tropospheric removal processes would appear to serve also as the ultimate removal mechanism for stratospheric HNO,, subsequent to transport of HNO,-containing stratospheric air across the tropopause (Fishman and Crutzen, 1977). The description of aqueous-phase HNO, removal rates is thus of interest in consideration of HNO, budgets and residence times (Rodhe and Grandell, 1972, Shnn et al.,