Entrainment of free tropospheric aerosols as a regulating mechanism for cloud condensation nuclei in the remote marine boundary layer
- European Commission, Ispra (Italy)
A box model has been developed to study the formation of cloud condensation nuclei from dimethyl sulfide (DMS), in the unpolluted marine boundary layer (MBL). Chemical and microphysical processes include gas phase chemistry of DMS and SO2, formation and growth of H2SO4-H2O aerosols, in-cloud SO2 oxidation, and scavenging of SO2 and H2SO4 molecules by sea salt, cloud drops, and precipitation. Meteorological processes include exchange between the MBL and the overlying free troposphere (FT) and horizontal mixing after particle removal by precipitation. The description of the FT aerosol is based on the notion of new particle production in the upper troposphere (for which there is experimental evidence) and the notion that these particles evolve into a self-preserving aerosol (for which there is no clear experimental evidence yet). We argue that FT aerosols are expected to be self-preserving in areas of large-scale subsidence such as the subtropics. The model results are in reasonable agreement with summer observations of DMS, SO2, non-sea-salt (nss) SO4, condensation nuclei (CN), and cloud condensation nuclei (CCN) at Cape Grim. They show that entrainment of FT aerosols in the MBL quenches new particle formation within the MBL. A sensitivity/uncertainty analysis shows that most of the variation in the number concentration of MBL aerosols is due to the variation in parameters linked to the FT aerosol, whereas most of the variations in the MBL nss-sulfate mass is due to variations in the DMS flux. We conclude that FT-MBL exchange is likely to be an important mechanism that can explain both the observed levels of CN and CCN (active in straitform clouds) in the MBL and their lack of short-term variability.
- OSTI ID:
- 57388
- Journal Information:
- Journal of Geophysical Research, Vol. 100, Issue D2; Other Information: PBD: Feb 1995
- Country of Publication:
- United States
- Language:
- English
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