Aircraft measurements of biomass burning aerosol over West Africa during DABEX

Abstract

[1] This paper investigates the properties of biomass burning aerosols over West Africa using data from the UK FAAM aircraft during the Dust and Biomass-burning Experiment (DABEX). Aged biomass burning aerosols were widespread across the region, often at altitudes up to 4 km. Fresh biomass burning aerosols were observed at low altitudes by flying through smoke plumes from agricultural fires. The aircraft measured aerosol size distributions, optical properties, and vertical distributions. Single scattering albedo varied from 0.73 to 0.93 (at 0.55 mm) in aerosol layers dominated by biomass burning aerosol. We attribute much of this variation to the variable proportion of mineral dust and biomass burning aerosol. We estimate the single scattering albedo of aged biomass burning aerosol to be around 0.81 with an instrumental uncertainty of ±0.05. External mixing, and possibly internal mixing, between the biomass burning aerosol and mineral dust presents an additional source of uncertainty in this estimate. The size distributions of biomass burning aerosols were dominated by particles with radii smaller than 0.35 mm. A 20% increase of count mean radius was observed when contrasting fresh and aged biomass burning aerosols, accompanied by changes in the shape of the size distribution. These changes suggest growth by coagulation and condensation. Extinction coefficients, asymmetry parameters, and Angstrom exponents are calculated from Mie theory, using the lognormal fits to the measured size distributions and assumed refractive indices.

Measurements of aerosol properties from aircraft, satellite and ground‐based remote sensing: a case‐study from the Dust and Biomass‐burning Experiment (DABEX)

“…Aircraft measurements during DABEX showed that the biomass-burning aerosol was strongly absorbing, with a single-scattering albedo of around 0.81 at 550 nm (Johnson et al, 2008a). This compares with values of 0.85-0.90 measured during the Southern African Regional Science Initiative (SAFARI-2000) (Eck et al, 2003;Haywood et al, 2003a;Leahy et al, 2007).…”

“…The 0.35 µm cut-off radius between the fine and coarse mode corresponded to a local minimum (inflexion point) in volume size distributions (see Fig. 8b of Johnson et al, 2008a). For the purposes of modelling we assume all particles larger than 0.35 µm to be dust and all particles smaller than 0.35 µm to be biomass-burning aerosol.…”

“…Two log-normals were used to capture the fine mode (radii 0.05-0.35 µm) and three log-normals were used to capture the coarse mode (radii 0.35-10 µm). Based on detailed examination of aircraft data in Johnson et al (2008a), we believe the fine mode to be dominated by biomass-burning aerosol and the coarse mode to be dominated by dust. The 0.35 µm cut-off radius between the fine and coarse mode corresponded to a local minimum (inflexion point) in volume size distributions (see Fig.…”

“…A small adjustment was made to convert the absorption coefficient from 567 nm to 550 nm -the central wavelength of the nephelometer. This adjustment was estimated using Mie calculations based on the aircraft size distributions (Johnson et al, 2008a). No corrections are made for evaporation of water within the aerosol instruments because the ambient relative humidity was quite low (∼40%) and did not exceed 70% during the flight.…”

“…The PCASP is calibrated using latex spheres of known sizes that have a refractive index of 1.588 + 0.0i. Corrections for refractive index have been applied to the PCASP size bin radii following the method of Liu and Daum (2000) (see Johnson et al, 2008a for more details). The sizing algorithm of the PCASP instrument assumes spherical particles, whereas dust is known to exist in irregular shapes producing different scattering properties from spheres.…”