Spatially resolved oxidation chemistry of reactive organic compounds in wildfire plumes

The emissions and evolution of wildfire smoke was investigated with a variety of high time-resolution measurements during two recent flight-based measurement campaigns, aboard the NSF/NCAR C-130 during WE-CAN in 2018 and aboard the NOAA Twin Otter during FIREX-AQ in 2019. Using measurements of a variety of oxidized organic compounds as well as nitrous acid (HONO, often the dominant oxidant source) from an iodide-adduct chemical ionization mass spectrometer (I-CIMS), we illustrate the spatial heterogeneity of oxidation chemistry that can be found in daytime wildfire plumes. In general, the dilution-corrected concentrations of highly reactive organic compounds such as phenol and catechol react away fastest on the edges and top of plumes, where oxidation products are likewise enhanced. This is likely due to higher oxidant concentrations on plume edges/top due to enhanced photolysis of oxidant sources such as HONO in more dilute smoke. We use the measurements of such reactive compounds to estimate oxidant concentrations and gradients within plumes. By comparing with measurements from other high time-resolution instruments, we investigate how the spatially variable oxidant concentrations affect physico-chemical processes such as secondary aerosol formation and brown carbon evolution. Measurements of plume evolution during nighttime from the FIREX-AQ campaign are used to illustrate the contrasting spatially resolved plume evolution that occurs in the absence of OH radicals. Lastly, we investigate how this spatially resolved chemistry could impact the interpretation of plume evolution when using spatially resolved measurements versus transect-averaged measurements to compare against models.