On the evaporation of ammonium sulfate solution

Abstract

Aqueous evaporation and condensation kinetics are poorly understood, and uncertainties in their rates affect predictions of cloud behavior and therefore climate. We measured the cooling rate of 3 M ammonium sulfate droplets undergoing free evaporation via Raman thermometry. Analysis of the measurements yields a value of 0.58 ؎ 0.05 for the evaporation coefficient, identical to that previously determined for pure water. These results imply that subsaturated aqueous ammonium sulfate, which is the most abundant inorganic component of atmospheric aerosol, does not affect the vapor-liquid exchange mechanism for cloud droplets, despite reducing the saturation vapor pressure of water significantly.aerosol ͉ clouds ͉ aqueous ͉ inorganic T he vapor-liquid exchange dynamics of water underlie vital processes in biology, engineering, and atmospheric science. The evaporation and condensation rates of water are particularly important in the formation of cloud particles and are among the largest unknowns in assessing the impact of indirect aerosol effects on the radiative balance in the atmosphere (1). Model studies suggest that evaporation rates Ͻ10% of the maximum rate determined by gas kinetic theory for atmospherically relevant aqueous systems would indicate kinetic control over cloud growth processes, with implications for cloud and aerosol models (2, 3). Attempts to quantify the evaporation and condensation rates of water have yielded values spanning three orders of magnitude, although the most recent values converge to a single order of magnitude (4-10). This uncertainty contributes to the wide variability in cloud model predictions for the anthropogenic effect on size and number of cloud condensation nuclei (CCN) and the corresponding radiative impact on the global system (1,3,11,12).Measuring evaporation and condensation rates of liquid water is difficult because of the complex heat and mass transfer processes occurring in such experiments. The temperature of the liquid surface must be accurately determined and, in most cases, both evaporation and condensation must be properly accounted for. In addition, it has been suggested that some of the earlier measurements of these rates, which typically involved static liquid surfaces, yielded artificially low rates because of the contamination of the liquid with surface impurities (6). Such impurities may well be present in atmospheric conditions; indeed, field measurements of aerosol growth rates appear to indicate such a slowing effect (13,14). In our previous work, we addressed the problem of accurately modeling the heat and mass transfer processes through our measurements of the free evaporation of pure H 2 O and pure D 2 O (4, 5, 10). We also studied relative evaporation rates of isotopomers in mixtures (10,15). In those studies of pure solutions, condensation was negligible, allowing the evaporation process to be modeled accurately and then directly related to the cooling rate of the droplets. We found that evaporation for both pure H 2 O and pure D 2 O occurred at Ϸ60%...

Influence of organic films on the evaporation and condensation of water in aerosol

“…This conclusion is broadly consistent with a recent analysis of a wide range of growth kinetic measurements made on ambient aerosols at various geographic locations, which concluded that the mass accommodation coefficient must be larger than 0.1 for cloud condensation nuclei (36). It is also consistent with laboratory-based measurements that have shown that the presence of surface-active ions or low-molecular-weight organic compounds (e.g., acetic acid) have little to no effect on the evaporation coefficient (16,17). However, it must be remembered that tropospheric temperatures are largely lower than studied here.…”

“…For example, Saykally and coworkers (15) have investigated ion adsorption at water surfaces and the resulting impact on mass transfer. Although ammonium sulfate was not found to influence the evaporation kinetics of water (16), subtle effects due to the presence of more surface active perchlorate ions were observed (17).…”

Effect of Surface Active Ions on the Rate of Water Evaporation

“…In our previous studies, the resulting curve was linear, 2,3,15 but in this study we observed a slight curvature and found a quadratic fit to reproduce the calibration data more accurately, especially the lower temperatures. Figure 1b shows a sample calibration curve with both linear and quadratic fits.…”

“…Unlike our previous measurements of ammonium sulfate solution, no icicles were observed to grow into the interaction region for sodium perchlorate solutions, meaning that interference from solid icicles both directly and indirectly (by providing a collision surface and enabling condensation back on the droplets) was not an issue. 15 This is likely due to a difference in salt crystal structure upon freezing of the solution.…”

A coupled observation – modeling approach for studying activation kinetics from measurements of CCN activity

“…Both sub-saturated (e.g. Chuang, 2003Chuang, , 2006Sjogren et al, 2007;Drisdell et al, 2009;Tong et al, 2011) and supersaturated (e.g. Winkler et al, 2004Winkler et al, , 2006Ruehl et al, 2008Ruehl et al, , 2009Lance et al, 2009;Asa-Awuku et al, 2009Shantz et al, 2010) conditions have been used.…”