Deposition of Particulate Elemental Carbon from the Atmosphere
Elemental carbon is removed from the atmosphere by both precipitation scavenging and dry deposition at the Earth’s surface. Measurements of the wet removal flux may be obtained from optical or chemical analyses of the insoluble material in precipitation samples. Sampling considerations include losses to the walls of the sampling system, contamination from fugitive dust, pollen, or biological growth in the sample, and extraction of the carbon on to a suitable filter medium. An estimate of the dry deposition flux may be obtained by exposing a clean surface to the atmosphere but not to precipitation, washing with water subsequent to collection, and then analyzing in the same manner as precipitation samples. This may not be representative of the true flux because of the wide variety of surfaces to which deposition may occur, but does provide an estimate of the contribution of dry deposition to the precipitation samples.
Preliminary samples collected in Washington State suggest that precipitation scavenging is an important removal mechanism, and that atmospheric lifetimes for elemental carbon are comparable to those reported for sulfate aerosols. This implies that the removal mechanisms for the two aerosol types are similar, and is consistent with the hypothesis that sulfates and elemental carbon are mixed within the same particles in the atmosphere.
KeywordsElemental Carbon Deposition Velocity Settling Velocity Deposition Flux Precipitation Sample
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