Distributions of trace gases and aerosols during the dry biomass burning season in southern Africa

Vertical profiles in the lower troposphere of temperature, relative humidity, sulfur dioxide (SO₂), ozone (O₃ , condensation nuclei (CN), and carbon monoxide (CO), and horizontal distributions of twenty gaseous and particulate species, are presented for five regions of southern Africa during the dry biomass burning season of 2000. The regions are the semiarid savannas of northeast South Africa and northern Botswana, the savanna-forest mosaic of coastal Mozambique, the humid savanna of southern Zambia, and the desert of western Namibia. The highest average concentrations of carbon dioxide (CO₂), CO, methane (CH₄), O₃, black particulate carbon, and total particulate carbon were in the Botswana and Zambia sectors (388 and 392 ppmv, 369 and 453 ppbv, 1753 and 1758 ppbv, 79 and 88 ppbv, 2.6 and 5.5 μg m^-3, and 13.2 and 14.3 μg m^-3). This was due to intense biomass burning in Zambia and surrounding regions. The South Africa sector had the highest average concentrations of So₂, sulfate particles, and CN (5.1 ppbv, 8.3 μg m^-3, and 6400 cm^-3, respectively), which derived from biomass burning and electric generation plants and mining operations within this sector. Air quality in the Mozambique sector was similar to the neighboring South Africa sector. Over the arid Namibia sector there were polluted layers aloft, in which average SO₂, O₃, and CO mixing ratios (1.2 ppbv, 76 ppbv, and 310 ppbv, respectively) were similar to those measured over the other more polluted sectors. This was due to transport of biomass smoke from regions of widespread savanna burning in southern Angola. Average concentrations over all sectors of CO₂ (386 ± 8 ppmv), CO (261 ± 81 ppbv), SO₂, (2.5 ± 1.6 ppbv), O₃ (64 ± 13 ppbv), black particulate carbon (2.3 ± 1.9 μg m^-3), organic particulate carbon (6.2 ± 5.2 μg m^-3), total particle mass (26.0 ± 4.7 μg m^-3), and potassium particles (0.4 ± 0.1 μg^-3), were comparable to those in polluted, urban air. Since the majority of the measurements in this study were obtained in locations well removed from industrial sources of pollution, the high average concentrations of pollutants reflect the effects of widespread biomass burning. On occasions, relatively thin (∼0.5 km) layers of remarkably clean air were located at ∼3 km above mean sea level, sandwiched between heavily polluted air. The data presented here can be used for inputs to and validation of regional and global atmospheric chemical models.