Aerosol partitioning in natural mixed‐phase clouds

Abstract

In situ aerosol and cloud drop microphysical measurements at a high‐alpine site are used to investigate aerosol partitioning between cloud and interstitial phases in natural, mid‐latitude, mixed‐phase clouds. Measurements indicate a decrease in the activated aerosol fraction (FN) for particle diameters dP > 100 nm with cloud temperature from FN ∼ 0.54 in summer liquid‐phase clouds to FN ∼ 0.08 in winter mixed‐phase clouds. The latter may be attributed to the Bergeron‐Findeisen mechanism whereby ice crystals grow at the expense of liquid water drops, releasing formerly activated aerosols back into the interstitial phase. This provides a means to distinguish the indirect effects of aerosols on drops and ice crystals.

The aerosol-climate model ECHAM5-HAM

“…Seinfeld and Pandis, 1998) whilst for aerosols it is prescribed in the form of a size-and composition-dependent scavenging parameter R. R is defined as the fraction of the tracer in the cloudy part of the grid box that is embedded in the cloud liquid/ice water. Values of R for stratiform clouds follow measurements of interstitial and in-cloud aerosol contents of Henning et al (2004), with slight modifications, and for ice clouds are based on Feichter et al (2004). It should be noted that for the accumulation and coarse mode R is lower for mixed phase clouds than for liquid clouds due to the growth of ice crystals at the expense of water droplets as a result of the Bergeron-Findeisen process (Henning et al, 2004).…”

Analysis and quantification of the diversities of aerosol life cycles within AeroCom

“…These have sources that are often situated above the planetary boundary layer, leading to elevated residence times. Another sensitive, but not well-constrained process is the representation of aerosol scavenging by ice at low temperatures in the higher atmosphere (e.g., Hendricks et al, 2004;Henning et al, 2004) (see also Table 8). …”

Aerosol optical depth over the Arctic: a comparison of ECHAM-HAM and TM5 with ground-based, satellite and reanalysis data

“…In the following we also explore a simple change in the model wet scavenging parametrization introduced recently by Bourgeois and Bey (2011) to better reproduce the observed optical properties and concentrations of aerosols in the Arctic region. HAM parametrizes wet scavenging using the precipitation formation rate of the ECHAM5 cloud scheme and computing the fraction of tracer that is embedded in cloud water using simple size-dependent and cloud-type dependent scavenging coefficients, based on measurements from Henning et al (2004). Bourgeois and Bey (2011) explored reducing these coefficients, based on a re-evaluation of the results in Henning et al (2004) and following papers, leading to an increase in BC and sulfate lifetimes and to increased burdens in the Arctic, while the global and annual scavenged masses remained similar.…”

“…HAM parametrizes wet scavenging using the precipitation formation rate of the ECHAM5 cloud scheme and computing the fraction of tracer that is embedded in cloud water using simple size-dependent and cloud-type dependent scavenging coefficients, based on measurements from Henning et al (2004). Bourgeois and Bey (2011) explored reducing these coefficients, based on a re-evaluation of the results in Henning et al (2004) and following papers, leading to an increase in BC and sulfate lifetimes and to increased burdens in the Arctic, while the global and annual scavenged masses remained similar. We apply this modification using the same parameters as described in their paper.…”