Observations of ozone formation in southern African savanna and grassland fire plumes

Abstract

Open biomass burning is one of the largest sources of aerosols and reactive trace gases into the atmosphere, having a significant effect on earth's radiative budget and air quality. Biomass burning degrades air quality by increasing both particulate matter and ozone levels. Unlike aerosols, ozone is not directly emitted in fires but it is frequently formed in ageing plumes. This is not surprising, as many of the reactive trace gases emitted in biomass burning including nitrogen oxides, carbon monoxide and volatile organic compounds, act as precursors for ozone formation. Geographically, Africa is the most significant source region of biomass burning emissions. Here, we report rapid ozone formation in daytime savanna and grassland fire plumes observed in South Africa. We observed higher ozone production in more flaming cases for plumes fresher than 1.5h. However, in plumes aged >1.5h combustion characteristics have no effect, but ozone production is positively correlated with nitrogen oxides. Furthermore, the plumes with highest ozone production also had the strongest secondary aerosol formation during plume ageing. Additionally, we report emission factors of nitric oxide and sulphur dioxide for southern African savanna and grassland fires. SO2 emission factor was on average 1.1 g kg−1, which is two to three times higher than previous observations for savanna and grassland. On the other hand, the average NO emission factor (2.6 g kg−1) agrees well with previous observations.