Total Organic Carbon Loss and Changes in Bulk Chemical Composition Seen in Vertically Stratified Aged African Biomass Burning Plumes over the Southeast Atlantic Ocean during ORACLES.

Southern Africa produces roughly 30% of the Earth’s biomass burning (BB) aerosol particles. Organics, nitrates, sulfates, and black carbon (BC) are lofted into the free troposphere and transported over the Southeast Atlantic Ocean (SEA) along with other chemical species. Considerable uncertainty remains in the chemical composition of the aerosol and the chemical reactions that occur during the long transport times (2 days to 15 days) over the SEA, especially for organic aerosol (OA). The lifetime and properties of OA have been difficult to study on time scales greater than 48 hours. As part of the NASA ORACLES (ObseRvations ofAerosols aboveCLouds and their intEractionS) September 2016, August 2017, and October 2018 airborne field campaigns, an Aerosol Mass Spectrometer (AMS) sampled the chemical composition and chemical structure of the aerosol in this region. Observations from all three years indicate that the organic aerosol is highly oxidized with an organic matter to organic carbon ratio of ~2.5. Average modified combustion efficiency (MCE=(DCO₂)/( DCO+DCO₂)) for both the 2016 and 2017 campaigns range between 0.975-0.999, consistent with highly efficient flaming fires. The smoke was also consistently vertically stratified, with chemically-distinguishable upper (> 3500m) and lower (1500m-3500m) plumes based on OA, NH_4, NO₃, and SO₄. The layered structure of the plume can be explained by relative age, as opposed to differences in source regions. The lower plume is defined by a lower NO₃:OA (1:12) ratio, and a higher SO₄:OA ratio than the upper plume. This is consistent with increasing SO₂ oxidation into H₂SO₄ and displacement of HNO₃ in the particulate phase as the plume ages. The inference of a younger upper plume and an older lower plume is also consistent with f44 measurements, a common aging indicator derived from the mass ratio of the CO₂⁺ ion to total organic ions. Furthermore, an experimental plume age module in the Weather Research and Forecasting (WRF) model also indicates a clear relationship between the model age prediction and the f44 measurements. The age from the WRF model and changes in the organic carbon (OC) to black carbon ratio indicate a mean lifetime of OC of approximately 6 days. Few previous studies have been able to document such aged smoke plumes. These observations are unique because of the chemical uniformity of the sources and the range of plume ages. One important consequence is that the single-scattering-albedo of the upper, fresher, plume is typically higher than in the lower plume.