Aging of Soot

Work in collaboration with Heike & Bernhard Vogel

An important example for the complexity of aerosol-climate interactions is the aging process of soot. Soot particles contribute both to the direct and indirect climate effect. While freshly emitted soot is initially hydrophobic and externally mixed, it can be transferred into an internal mixture by coagulation, condensation or photochemical processes. These aging processes affect the hygroscopic qualities and hence the growth behavior, the optical properties and eventually the lifetime of the soot particles.

Our understanding of these processes is still fragmentary. The aging process of soot is therefore one of the key uncertainties concerning the burden and effect of black carbon. We addressed this problem with 3D simulations with the coupled mesoscale model KAMM/DRAIS, which provides a highly resolved boundary layer and allows for an explicit treatment of the aging process of soot by coagulation and condensation. Based on the results of the simulations, the time scale on which soot is transferred from an external to an internal mixture is derived. The focus is on continental conditions in an industrialized environment, and two different meteorological scenarios are investigated, that is a summer and a winter episode. The figures below shows time-height sections for the aging timescales in hours for summer (top) and winter (bottom).

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Generally, the derived aging time scales are smaller than most of the value currently used in global climate models, a few of hours during daytime. A considerable variability in space and time is found suggesting that using one constant value to describe the aging process may be an oversimplification.

Current work

We are currently investigating the role of secondary organic aerosol for the aging of soot.

Publications

N. Riemer, M. West, R.A. Zaveri, R.C. Easter [2009] Estimating soot aging time scales with a particle-resolved aerosol model, Journal of Aerosol Science, 2009, submitted, preprint at arXiv:0903.0029v1

N. Riemer, H. Vogel, B. Vogel [2004] Soot aging time scales in polluted regions during day and night, Atmospheric Chemistry and Physics 4, 1885-1893, SRef-ID: 1680-7324/acp/2004-4-1885. (pdf)

N. Riemer, H. Vogel, B. Vogel, F. Fiedler [2003] Modeling aerosols on the mesoscale-γ: Treatment of soot aerosol and its radiative effects, Journal of Geophysical Research 108, 4601, DOI: 10.1029/2003JD00 3448. (pdf)