Biomass burning contribution to ambient air particulate levels at Navrongo in the Savannah zone of Ghana

Abstract

The concentrations of airborne particulate matter (PM) in Navrongo, a town in the Sahel Savannah Zone of Ghana, have been measured and the major sources have been identified. This area is prone to frequent particulate pollution episodes due to Harmattan dust and biomass burning, mostly from annual bushfires. The contribution of combustion emissions, particularly from biomass and fossil fuel, to ambient air particulate loadings was assessed. Sampling was conducted from February 2009 to February 2010 in Navrongo. Two Gent samplers were equipped to collect PM₁₀ in two size fractions, coarse (PM_10-2.5) and fine (PM_2.5). Coarse particles are collected on a coated, 8-μm-pore Nuclepore filter. Fine particle samples were sampled with 47-mm-diameter Nuclepore and quartz filters. Elemental carbon (EC) and organic carbon (OC) concentrations were determined from the quartz filters using thermal optical reflectance (IMPROVE/TOR) methods. Elements were measured on the fine-particle Nuclepore filters using energy-dispersive x-ray fluorescence. The average PM_2.5 mass concentration obtained at Navrongo was 32.3 μg/m³. High carbonaceous concentrations were obtained from November to March, the period of Harmattan dust and severe bush fires. Total carbon was found to contribute approximately 40% of the PM_2.5 particulate mass. Positive matrix factorization (PMF) suggested six major sources contributing to the PM_2.5 mass. They are two stroke engines, gasoline emissions, soil dust, diesel emissions, biomass burning, and resuspended soil dust. Biomass combustion (16.0%) was identified as second most important source next to soil dust at Navrongo.

Implications:

Black carbon has now been recognized as playing a major role in radiation forcing, and one major source of biomass burning is slash-and-burn agriculture. Such practices are used in Sub-Saharan Africa and the time for field burning corresponds to the time when there is also substantial transport of Saharan dust to West Africa. This paper shows the major role of biomass burning on fine particle concentrations in northern Ghana and provides useful data on the EC concentrations in this area.

Acknowledgment

The authors gratefully acknowledge the technical staff of the Center for Air Resources Engineering and Science at Clarkson University, Potsdam, NY, who contributed to the elemental and carbonaceous analyses. They also acknowledge the authorities of UDS at Navrongo campus for their cooperation during the sampling.