Ozone formation from West African wildfires

Abstract:

Tropospheric ozone (O₃) can adversely affect human health and environmental ecosystems and it is therefore vitally important to understand its formation pathways from both natural and anthropogenic precursors. Wildfires are an important source of these precursors (both VOCs and NO_x) and it is likely that the prevalence of wildfires will increase in a warming climate. Wildfires have been shown to contribute to elevated O₃ at air quality monitoring sites, so it is therefore important to better understand the emissions, photochemistry and impacts of these fires. Here we present airborne measurements of O₃ and its precursors in wildfire plumes in Senegal and Uganda.

Measurements of O₃, CO, NO_x, CH₄, CO₂, C₂H₆ and a wide range of VOCs were made from the UK FAAM large research aircraft. Measurements were made directly in the wildfire plumes, in more aged background haze in regions of biomass burning and over the Atlantic Ocean following several days of transport. Measurements of a range of O₃ enhancement ratios (ΔO₃ / ΔCO) were observed, ranging from 0.05 when sampling within 1-2 hours transport time from the fires, to 0.3 when sampling up to 150 hours from the Senegalese fires. The VOC and NO­_x composition of the plumes is also investigated with enhancement ratios of different VOCs and NO_x to CO calculated in order to derive emission ratios that could be used to provide emission estimates of VOCs from wildfires.

The measurements are then compared against predictions from two model at a horizontal resolution of 25x25km. Firstly, the assimilation mode of the GEOS Composition Forecasting (GEOS-CF) model using QFED biomass burning emissions. And secondly, a regional grid within the offline chemical transport model GEOS-Chem using GFAS biomass burning emissions.