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Detecting Precipitation and Aerosol Removal from the African Boundary Layer during ORACLES using Water Vapor Heavy Isotope Ratios
  • Dean Henze,
  • David Noone
Dean Henze
Oregon State University

Corresponding Author:[email protected]

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David Noone
The University of Auckland
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The NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) project provides an extensive data set of aircraft in-situ meteorological, aerosol, and total water heavy isotope ratio measurements in the southeast Atlantic (SEA) lower troposphere over the months of Sept. 2016, Aug. 2017, and Oct. 2018. These months are during southern hemisphere spring, which is the agricultural burning season in subtropical southern Africa. During this season, biomass burning aerosol (BBA) loaded air in the African planetary boundary layer is carried out over the SEA by lower troposphere easterly flow. The goal of ORACLES was to study the effects of aerosol loading in the lower troposphere and as they mix into the semi-permanent cloud deck over the SEA. During ORACLES, water isotope ratio measurements D/H and 18O/16O were taken using Picarro brand cavity ring-down spectroscopic analyzers integrated into our Water Isotope System for Precipitation and Entrainment Research (WISPER). With over 300 hrs (~140,000 linear km at the typical aircraft speed) of 1 Hz sampling between 22°S to the equator and from 70 m to 6 km, this is a large and vertically resolved isotope dataset. In this presentation, we first use the coupled measurements of total water concentration and its heavy isotope ratio D/H to distinguish BBA plumes which have experienced prior precipitation vs those which have not. The findings are supported with lagrangian back-trajectories, MERRA surface temperatures and moisture, and isoCAM surface isotope values, which are combined to constrain simple isotope models. We find that BBA air sampled in 2016 experienced almost no precipitation, while BBA air and moisture in 2017 follow a model of convective outflow and most of the moisture loss is due to precipitation. Finally, we show the strong agreement between D/H evidence of precipitation amount and an indicator of aerosol removal via precipitation: the ratio of black carbon to carbon monoxide. In contrast, no correlation between total water concentration and aerosol removal is found, demonstrating the additional information contained in the isotope ratio measurements. This also provides an example of isotope ratio measurements supplementing other variables during a multifaceted field project to address science questions.