Effect of Wildfire Aerosols on NO2 Photolysis and Ozone Production at the Mt. Bachelor Observatory

Abstract:

In this study, we have two goals: to quantify the effect of biomass burning aerosols on j_NO2 photolysis and to look at O₃ formation in biomass burning plumes as it relates to j_NO2 photolysis. Wildfire plumes were observed during the summer of 2015 at the Mt. Bachelor Observatory, a high-elevation (2.8 km a.s.l.) mountaintop site located in central Oregon. These plumes were identified using the following criteria: (1) 5-minute ambient aerosol scattering σ_sp ≥ 20 Mm^-1 for at least two hours, (2) 5-minute CO ≥ 150 ppbv for at least two hours, (3) strong correlation (r² ≥ 0.70) between σ_sp and CO, and (4) consistent air mass back trajectories indicating transport over known fire locations.

We measure nitrogen oxides using a chemiluminescence detector and j_NO2 photolysis using a diode array actinic flux spectroradiometer. We also measure O₃ using two techniques: (a) UV method with a cavity ring-down spectrometer and (b) chemiluminescence method with a custom-made instrument. We compare fire event observations between these two procedures to prove consistency.

Based on these measurements, we quantify a lower bound for the HO₂ and RO₂ radical concentrations in wildfire plumes. We then look at plume and non-plume data and examine deviations from the photostationary state. Finally, we use the TUV model v5.2 to simulate clear-sky conditions and therefore quantify the reduction/enhancement in j_NO2 values and O₃ production due to wildfire aerosols. This gives us insight into the photochemical environment in biomass burning plumes, which until now, remains poorly understood.