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Zusammenfassung:
The combination of influences of black carbon on radiative properties of the atmosphere, and on the characteristics of clouds is central to properly evaluating and quantifying cloud-aerosol interactions. Global and regional modeling of black carbon distribution requires accounting for the removal by wet deposition, which is complicated by the stochastic nature of rainfall production within cloud systems, which is difficult to quantify. Measurements of wet scavenging in most settings are necessarily inferential. Measurements made before and after some type of event, or measuring time series of black carbon, require methods to attribute concentration changes to the likelihood of precipitation and the proportion of dilution. Here we use the stable isotope ratios of water vapor paired with black carbon measurements to uniquely identify the history of precipitation influencing air parcels, and thus provide a near direct method for estimating wet scavenging. The significance of the approach is that black carbon loss from scavenging can be separated from reduction in black carbon concentration due to dilution. Using data from the NASA ORACLES aircraft campaign, the relationship between precipitation in convective regimes near equatorial Africa are found to be key drivers of wet deposition. Along with new estimates of scavenging efficiency for this domain, the result removes ambiguity in interpreting black carbon concentrations in regions where dilution is suspected. Moreover the isotopic approach allows a real time monitor of dilution fraction to be available within a sampling mission, than can be used to support optimal sampling strategies to meet aerosol observational objectives.
