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Detection of mixing and precipitation scavenging effects on biomass burning aerosols using total water heavy isotope ratios during ORACLES
  • Dean Henze,
  • David Noone
Dean Henze
Oregon State University

Corresponding Author:[email protected]

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David Noone
Waipapa Taumata Rau
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The prevalence of mixing vs. precipitation processes in biomass burning aerosol (BBA) laden air over the southeast Atlantic is assessed during three intensive observation periods during the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign. Air in the lower free troposphere (FT) and marine boundary layer (MBL) are treated as separate analyses, although connections are made where relevant. The study, centering on aircraft in-situ measurements of total water heavy isotope ratios, has two main objectives. The first is to gauge whether the atmospheric hydrology, and in particular precipitation, can be constrained primarily through visual assessment of the aircraft isotope ratio data plotted against total water concentration, similarly to several previous studies. However, regression of the data onto a simple model of convective detrainment is also used and alludes to the possibility of a precipitation data product derived from isotope ratios. The second objective is to connect variations in aerosol concentrations to the hydrology as diagnosed by the isotope ratio measurements, and determine whether aerosol variations are attributable to wet scavenging. First, joint water concentration (q) and H2O/HDO isotope measurements (δD) in the lower FT are combined with satellite and MERRA-2 data into simple analytical models to constrain hydrologic histories of BBA-laden air originating over Africa and flowing over the southeast Atlantic. We find that even simple models are capable of detecting and constraining the primary processes at play. Further, a strong correlation between isotopic evidence of precipitation in lower FT air masses and an in-situ indicator of wet scavenging of black carbon – the ratio of black carbon to carbon monoxide (BC/CO) – is shown. In comparison, the correlation between BC/CO and the water concentration itself is low. Since wet scavenging is the primary removal mechanism of black carbon, these findings suggest that isotope measurements could support studies constraining the lifetime of black carbon in the FT. Next, the ability of measurements interpreted with simple analytical models in (q, δD) space to distinguish cloud-top entrainment vs. precipitation signals in the MBL is tested. This proves more difficult than the lower FT analysis since signals are smaller. We find that the largest obstacle to this goal is the (q, δD) values of the entrained airmass at cloud-top. We also compare cloud condensation nuclei (CCN) concentrations in the sub-cloud layer to the isotopic measurements. In 2016 and 2018 IOPs, lower CCN concentrations coincide with isotope ratio evidence of precipitation, indicating aerosol scavenging. However, a more complex model simulating water, isotope ratios, and aerosols would be necessary to achieve more definitive conclusions. For the 2017 IOP, with the highest sub-cloud CCN concentrations, there is no connection between precipitation signals and CCN concentrations.