The chemical fractionation of atmospheric aerosol as a result of snow crystal formation and growth

Abstract

The relationships between the physical and chemical properties of mixed-phase clouds were investigated at Storm Peak Laboratory (3220m MSL) located near the continental divide in northwestern Colorado. Interstitial aerosol particles, cloud droplets and snow crystals were concurrently collected when the laboratory was enveloped by a precipitating cloud. All samples were analyzed for trace elements, soluble anions, electrical conductivity and acidity.The results show average trace constituent concentration ratios of cloud water to snow water range from 0.4 to 26. All but six of the 32 elements and ions measured had ratios greater than one. This result suggests a chemical species dependency of in-cloud aerosol particle scavenging processes. Evidence of a decrease of in-cloud aerosol particle scavenging efficiency by snow due to increases in aerosol concentration is also presented.Differences between the chemical composition of cloud water and snow water are manifested most strongly when snow crystals grow by vapor deposition. In-cloud scavenging efficiencies by snow crystals for most aerosol particle chemical species are dependent on the growth of the snow crystals by accretion of cloud droplets. This chemical fractionation of the atmospheric aerosol by snow crystal formation and growth should be most active where narrow, continental cloud droplet size distributions and low liquid water contents are prevalent, enhancing the probability of snow crystal growth by diffusion.

Scavenging of acidic gases (HCOOH, CH3COOH, HNO3, HCl, and SO2) and ammonia in mixed liquid‐solid water clouds at the Puy de Dôme mountain (France)

“…This occurs either by collision of liquid droplets with ice particles followed by shock freezing (timing process) for large enough ice crystals or by diffusion of water vapor to the ice because of the lower vapor pressure of ice with respect to water (Wegener-Bergeron-Findeisen mechanism [Pruppacher and Klett, 1997]). Borys et al [1988] demonstrated that nucleation of cloud droplets followed by ice crystal r'mm•g is the main process in taking up aerosols in mixed clouds, the collision of ice particles with interstitial aerosols re,naming an inefficient process (Figure 1). During the freezing of supercooled liquid droplets, solutes present as salts will be segregated by the ice leading to an hfi•omogeneous distribution in the formed ice particle, but the bulk chemistry of rimed droplets will not differ from that of supercooled droplets.…”

Surface Snow, Firn and Ice Core Composition in Polar Areas in Relation to Atmospheric Aerosol and Gas Concentrations: Critical Aspects

“…The riming process becomes efficient only if droplet diameter is larger than 10 µm and crystal diameter is greater than about 50, 150 and 300 μm, when considering dendrites, hexagonal plates and columns, respectively [29] [30]. Riming is likely to be common during the relatively warm summer season in coastal areas of Antarctica, whereas at the South Pole where temperatures are lower, riming is less likely, i.e.…”

A three‐dimensional regional modeling study of the impact of clouds on sulfate distributions during TRACE‐P

“…2. Gases may be absorbed into water drops and subsequently get incorporated into ice through the riming process [ Diehl et al , 1998; Borys et al , 1988; Mitchell and Lamb , 1989]. This process is included in the model cloud microphysical transformation.…”