Aging of biomass burning aerosols over West Africa: Aircraft measurements of chemical composition, microphysical properties, and emission ratios

Abstract

This paper investigates the physical and chemical characteristics of biomass burning aerosol over West Africa using data from the UK Facility for Airborne Atmospheric Measurements aircraft. Measurements of biomass burning aerosol were made during the Dust and Biomass‐burning Experiment (DABEX) and Dust Outflow and Deposition to the Ocean (DODO) field experiments in January and February 2006. Layers of aged biomass burning aerosols were observed throughout the region, and fresh biomass burning aerosols were encountered during the penetration of smoke plumes at low altitudes. Vertical profiles of aerosol properties across the region are shown. Measurements from an Aerodyne Quadrupole Aerosol Mass Spectrometer (Q‐AMS) show changes in chemical composition between fresh and aged biomass burning aerosols, over a region spanning thousands of kilometers. These data represent the first time that continental‐scale variability in biomass burning aerosol composition has been observed. However, an almost linear relationship between organic aerosol mass concentration and carbon monoxide concentration was observed across the region. A net carbon loss occurs over the aerosol lifetime in the region owing to a combination of chemical processing and repartitioning of organic mass to the gas phase. Evolution of the number size distribution was observed, with coagulation concluded to be the dominant process involved, a finding supported by coagulation box modeling. Regional‐scale emission ratios for organic (0.041) and black carbon (0.0072) with respect to CO have been estimated over West Africa, one of the largest sources of biomass burning globally. Biomass burning emissions from the West African Sahel are poorly represented in the literature, and these results represent important continental‐scale emissions. They are in good agreement with literature values from other regions.