Discovery of Supermicron-size Soot Particles over the South East Atlantic Ocean

During the 2017 Observations of Aerosols Above Clouds and their Interactions (ORACLES) field campaign, the NASA P3-Orion was equipped with a 2D-Stereo (2D-S) probe that contains 128 diodes that are 10 µm in size nominally measuring particles with sizes ranging from 5 µm < D < 1280 µm. The 2D-S probe captured suspended aerosols within a biomass burning plume during 6 research flights over the South East Atlantic Ocean at times when no liquid water was detected by the King liquid water probe. Over 165 suspended aerosols sized between 10 µm and 1.52 mm were observed at altitudes ranging from 1500 m to 2500 m between 8-9°S along 5°E, 1000 km from the coastline. Concentrations (500-1200 cm^-3) of refractory black carbon (rBC) measured by a single particle soot photometer at the same time suggest these supermicron-sized particles are composed of rBC, also referred to as soot. Transmission electron microscopy (TEM) images of submicron particulates were collected on TEM Holey carbon grid filters concurrently with the probes. These TEM images reveal particles with crystalline structures as well as carbonaceous particles. Energy-dispersive X-ray spectroscopy (EDS) spectra from the filters revealed elevated levels of potassium (K) and oxygen (O) indicating the particles are of biomass burning origin. NOAA HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) models for three-day back trajectories show a source location in northern Angola for the time periods with the large supermicron-sized particles. FIRMS (Fire Information for Resource Management System) MODIS 6 (Moderate Resolution Imaging Spectroradiometer) active fire maps showed extensive biomass burning at these source locations. Given the source location, the high concentrations of rBC, and the TEM-identified carbonaceous particles, it is hypothesized that the supermicron-sized particles imaged by the 2D-S are examples of large soot superaggregates previously seen in biomass burning smoke. The radiative impact of these superaggregates on the radiative budget is quantified by determining the absorption of near-infrared radiation associated with the computed aggregated concentrations in order to determine the effects of the suspended aerosols on water and energy budgets. This analysis of these particles will thus help ascertain the impacts those particulates have on climate, air pollution, and cloud radiative forcing.