Detection of mixing and precipitation scavenging effects on biomass burning aerosols using total water heavy isotope ratios during ORACLES

Abstract. The interaction between biomass burning aerosols and clouds remains challenging to accurately determine in part because of difficulties in using direct observations to account for influences of scavenging and dilution separately from sources. The prevalence of mixing versus 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 distinct, and can be treated as separate analyses, although connections are made where relevant. Aircraft in-situ measurements of total water heavy isotope ratios to are used to assess of air parcel precipitation history. Subsequently, the isotopic fingerprint of precipitation influence is used to identify the prevalence of wet scavenging of black carbon aerosols. In situ observations 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. Regression of the aircraft data onto a simple model of convective detrainment is used to develop a metric of precipitation history. The approach is supported by complementary analysis using the ratio of black carbon to carbon monoxide (BC / CO). The method is expanded to test the entrainment and precipitation influences on marine boundary layer air. This is more difficult than the lower FT analysis since signals are more subtle, and limited by imperfect knowledge of the water and isotope ratios of the entrained airmass at cloud-top. Nonetheless, lower cloud condensation nuclei concentrations occur in the sub-cloud layer coincident with isotope ratio evidence of precipitation, indicating aerosol scavenging in the 2016 and 2018 IOPs. For the 2017 IOP, with the highest sub-cloud CCN concentrations, there is no connection between precipitation signals and CCN concentrations, likely indicating the importance of the different geographic sampling and air mass history in that year. These findings demonstrate the value of leveraging the isotope ratio signals of precipitation history that is distinct from the signature of dilution effects to constrain BC scavenging coefficients in a manner which can guide in model parameterizations, and ultimately lead to improvements in the accuracy of simulated BC concentrations and lifetimes in climate models.