Constraining Lower Tropospheric Aerosol Scavenging using Heavy Water Isotope Ratios in Vapor

The isotopic composition of atmospheric water vapor can provide constraints on relative amounts of air mass mixing and precipitation that an air parcel has experienced over an integrated trajectory, which is not easy to determine using conventional thermodynamic variables alone. There has been a recent expansion in the number of data sets of water vapor isotopic composition and a substantially improved set of theories and models for using them to constrain moisture sources and the occurrence of past precipitation in air masses. Here we present an aircraft in-situ data set which provides a spatially (vertical and horizontal) and temporally detailed perspective on both the boundary layer (BL) and lower troposphere (LT) isotopic composition. Observations were obtained as part of the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign which involved three, month-long aircraft in-situ sampling periods in the southeast Atlantic LT and marine BL during times of heavy aerosol loading from African continental sources. Isotopic compositions for water D/H and 18O/16O supplemented a suite of other thermodynamic, trace gas, and aerosol variables. Using ORACLES measurements, the D/H isotope ratio combined with H 2 O, carbon monoxide and black carbon concentrations within a mass balance analysis is performed to simultaneously evaluate aerosols scavenging due to precipitation and dilution due to air mass mixing. The numerical model enables a quantitative estimate of the relative contributions of mixing and precipitation that sampled LT air parcels have undergone since origination over the African continent using the D/H isotope ratio. We present estimates of the black carbon scavenging coefficient, defined here as the ratio of black carbon mass per liquid water mass removed from a parcel due to precipitation. Variations in the scavenging coefficient in terms of initial African BL concentrations and relative proportions of air mass mixing and precipitation experienced by the BL are discussed.