Variability and diel dependence of O3 – NOx – VOC chemistry in western wildfire plumes: Results from the NOAA Twin Otter during FIREX-AQ

Abstract:

North American air quality predictions rely on an accurate understanding of the variability in photochemical ozone production from western wildfire smoke plumes. A suite of chemical measurements including ozone, nitrogen oxides, and volatile organic compounds (VOCs) as well as photolysis rates were made from the NOAA Twin Otter research aircraft as a part of the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) experiment. This research aircraft sampled nine unique fire complexes in five western states. Typical research flight days included flights during the afternoon, evening and night. In general, observed ozone production in the sampled plumes is rapid, reaching maximum ozone within an hour downwind. A 0-D box modeling tool was developed to further probe the chemistry driving ozone production in these plumes. This tool was used to calculate a campaign wide ozone isopleth that allows for a comparison of each individual fire complex in a common framework. The analysis shows the sensitivity of ozone production to initial NO_x and VOC emissions. However ozone isopleths are not capable of describing the temporal transition of NO_x and VOC sensitive chemical regimes. We use a radical budget approach to probe the temporal evolution of the chemistry. Afternoon photochemical plumes display a rapid transition from VOC sensitive chemistry to NO_x sensitive chemistry, mainly driven by radical production from photolysis of HCHO and HONO emitted directly from the fire. Evening photochemical plumes exhibit a slower transition from NO_x sensitive chemistry to VOC sensitive chemistry when compared to afternoon plumes, with a larger portion of the radical production from Alkene ozonolysis than photolysis reactions prevalent in afternoon plumes.