Abstract
During the 3 years of the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) campaign, the NASA Orion P-3 was equipped with a 2D stereo (2D-S) probe that imaged particles with maximum dimension (D) ranging from 10 < D < 1280 μm. The 2D-S recorded supermicron-sized aerosol particles (SAPs) outside of clouds within biomass burning plumes during flights over the southeastern Atlantic off Africa’s coast. Numerous SAPs with 10 < D < 1520 μm were observed in 2017 and 2018 at altitudes between 1230 and 4000 m, 1000 km from the coastline, mostly between 7–11° S. No SAPs were observed in 2016 as flights were conducted further south and further from the coastline. Number concentrations of refractory black carbon (rBC) measured by a single particle soot photometer ranged from 200 to 1200 cm−3 when SAPs were observed. Transmission electron microscopy images of submicron particulates, collected on Holey carbon grid filters, revealed particles with potassium salts, black carbon (BC), and organics. Energy-dispersive X-ray spectroscopy spectra also detected potassium, a tracer for biomass burning. These measurements provided evidence that the submicron particles originated from biomass burning. NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) 3 d back trajectories show a source in northern Angola for times when large SAPs were observed. Fire Information for Resource Management System (FIRMS) Moderate Resolution Imaging Spectroradiometer (MODIS) 6 active fire maps showed extensive biomass burning at these locations. Given the back trajectories, the high number concentrations of rBC, and the presence of elemental tracers indicative of biomass burning, it is hypothesized that the SAPs imaged by the 2D-S are examples of BC aerosol, ash, or unburned plant material.
Highlights
Global biomass burning (BB) emits on average 2.5 Pg yr−1 of carbon aerosol, with Africa producing 49 % of these global emissions
We report observations of long-range transport of supermicron-sized aerosol particles (SAPs) originating from biomass burning in central Africa using measurements obtained aboard the NASA Orion P-3 research aircraft as part of the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign (Redemann et al, 2021; Zuidema et al, 2016)
No SAPs were detected during the 2016 Intensive Observation Periods (IOPs)
Summary
Global biomass burning (BB) emits on average 2.5 Pg yr−1 of carbon aerosol, with Africa producing 49 % of these global emissions (van der Werf et al, 2006). This class of soot aggregates have been observed in the emissions from the Kuwait oil fires in 1991 and were characterized by chain lengths up to 5 μm (Weiss et al, 1992) It is unknown whether these supermicron-sized aerosols can be transported long distances from their source regions. We report observations of long-range transport of supermicron-sized aerosol particles (SAPs) originating from biomass burning in central Africa using measurements obtained aboard the NASA Orion P-3 research aircraft as part of the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign (Redemann et al, 2021; Zuidema et al, 2016). Remote sensing analysis of ORACLES has determined that there were no systematic differences in aerosol properties found between the air above low-level clouds and above nearby clear-sky areas during the daytime (Shinozuka et al, 2020)
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