Use of a large aircraft to measure composition and chemistry of wildfires.

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. Instrumented research aircraft provide one of the best methods for studying emissions of VOCs and NOx from wildfires. Aircraft provide the flexibility to sample close to fires, allowing for calculation of emission factors, as well as further afield to study the chemical processing of fire plumes.

 

Here we present measurements of O₃ and its precursors taken from the UK large atmospheric research aircraft. Flights sampling wildfires in the Amazon rainforest in Brazil, scrublands in Senegal, wetlands in Uganda and moorland peat fires in the UK are reported, with measurements of O3, CO, NOx, CH4, CO2, C2H6 and a wide range of VOCs sampled directly in the plume and in more aged air up to 5 days from the source. Measurements of a range of O₃ enhancement ratios (DO3 / DCO) are observed, ranging from 0.05 when sampling within 1-2 hours transport time from all 4 types of fire, to 0.3 when sampling up to 100 hours away from the Senegalese fires. VOC composition of the plumes is also investigated. Ratios of different VOCs to CO are examined to derive emission ratios that are used to provide emission estimates of VOCs from wildfires. OH reactivity calculations in the plumes are used to assess the potential contribution of different VOCs to O₃ formation. In addition, measurements of aged air from fires in sub-Saharan Africa are compared against values calculated by the GEOS Composition Forecasting (GEOS-CF) system, a global atmospheric model with 25 km resolution, focusing on the model’s ability to capture ozone from biomass burning.