Development of a new semi-empirical parameterization for below-cloud scavenging of size-resolved aerosol particles by both rain and snow

Abstract

Abstract.A parameter called the scavenging coefficient is widely used in aerosol chemical transport models (CTMs) to describe below-cloud scavenging of aerosol particles by rain and snow. However, uncertainties associated with available size-resolved theoretical formulations for span one to two orders of magnitude for rain scavenging and nearly three orders of magnitude for snow scavenging. Two recent reviews of below-cloud scavenging of size-resolved particles recommended that the upper range of the available theoretical formulations for should be used in CTMs based on uncertainty analyses and comparison with limited field experiments. Following this recommended approach, a new semiempirical parameterization for size-resolved has been developed for below-cloud scavenging of atmospheric aerosol particles by both rain ( rain ) and snow ( snow ). The new parameterization is based on the 90th percentile of values from an ensemble data set calculated using all possible "realizations" of available theoretical formulas and covering a large range of aerosol particle sizes and precipitation intensities (R). For any aerosol particle size of diameter d, a strong linear relationship between the 90th-percentile log 10 ( ) and log 10 (R), which is equivalent to a power-law relationship between and R, is identified. The log-linear relationship, which is characterized by two parameters (slope and y intercept), is then further parameterized by fitting these two parameters as polynomial functions of aerosol size d. A comparison of the new parameterization with limited measurements in the literature in terms of the magnitude of and the relative magnitudes of rain and snow suggests that it is a reasonable approximation. Advantages of this new semiempirical parameterization compared to traditional theoretical formulations for include its applicability to belowcloud scavenging by both rain and snow over a wide range of particle sizes and precipitation intensities, ease of implementation in any CTM with a representation of size-distributed particulate matter, and a known representativeness, based on the consideration in its development, of all available theoretical formulations and field-derived estimates for (d) and their associated uncertainties.

The importance of interstitial particle scavenging by cloud droplets in shaping the remote aerosol size distribution and global aerosol-climate effects

“…The gravitational collection of particles below clouds by precipitation is typically included in global aerosol models and has been investigated in earlier studies of collection efficiency (Greenfield, 1957;Klett and Davis, 1973;Lin and Lee, 1975;Schlamp et al, 1976;Wang et al, 1978;Hall, 1980), parameterizations (Slinn, 1984;Jung and Lee, 1997;Croft et al, 2009;Wang et al, 2014) and recent reviews , so we will not focus on these effects in this paper. Brownian coagulation of cloud droplets and interstitial particles still occurs in precipitating clouds, but the collecting cloud droplets will only be wet scavenged/deposited if they are converted into a precipitation drop.…”

Relative contributions of gaseous oxidized mercury and fine and coarse particle‐bound mercury to mercury wet deposition at nine monitoring sites in North America

“…In contrast, scavenging of particle-bound mercury by snow is efficient particularly at higher latitudes [Amos et al, 2012]. Particle scavenging by snow is theoretically more efficient than rain because of the larger surface area of snow [Wang et al, 2014;Zhang et al, 2015]. Figure 4 illustrates that %GOM contributions to Hg wet deposition were greater than %FPBM and %CPBM at all sites in the summer (June-August), indicating the predominance of gas scavenging of Hg.…”

“…The importance of CPBM in the dry deposition budget was demonstrated in field data and modeling results [Fang et al, 2012]. Considering that particle scavenging by rain (size-resolved scavenging coefficient) and particle dry deposition (size-resolved dry deposition velocity) are greater for coarse particles than accumulation mode particles [Petroff and Zhang, 2010;Wang et al, 2014], CPBM may contribute more than FPBM to the wet deposition budget. …”

A landscape-scale approach to examining the fate of atmospherically derived industrial metals in the surficial environment

“…Therefore the calculation of wet deposition is repeated using upper and lower values of the mean ± the standard error of previously published values for Z (Z = 460 and 305 respectively), providing an estimated range for the flux of pollutant elements from the atmosphere to the surface. As the relative scavenging efficiency of snow is largely unknown (Wang et al, 2014), a single Z value is applied to the total precipitation amount. Likewise, the application of a single mean value to all trace elements is necessitated by the dearth of available data for many of the pollutants considered in this study.…”