Nighttime and daytime dark oxidation chemistry in wildfire plumes: An observation and model analysis of FIREX-AQ aircraft data

We use observations from two research aircraft, the NASA DC-8 and the NOAA Twin Otter, with a detailed chemical box model, including updated phenolic mechanisms, to analyze smoke sampled during midday, sunset, and nighttime. Aircraft observations suggest a range of NO3 production rates (0.1-1.5 ppbv h(-1)) in plumes transported during both midday and after dark. Modeled initial instantaneous reactivity toward BBVOCs for NO3, OH, and O-3 is 80.1 %, 87.7 %, and 99.6 %, respectively.

Initial NO3 reactivity is 10-10(4) times greater than typical values in forested or urban environments, and reactions with BBVOCs account for > 97% of NO3 loss in sunlit plumes (jNO(2) up to 4 +/- 10(-3) s(-1)), while conventional photochemical NO3 loss through reaction with NO and photolysis are minor pathways. Alkenes and furans are mostly oxidized by OH and O-3 (11 %-43 %, 54 %-88% for alkenes; 18 %-55 %, 39 %-76 %, for furans, respectively), but phenolic oxidation is split between NO3, O-3, and OH (26 %-52 %, 22%-43 %, 16 %-33 %, respectively).

Nitrate radical oxidation accounts for 26 %-52% of phenolic chemical loss in sunset plumes and in an optically thick plume. Nitrocatechol yields varied between 33% and 45 %, and NO3 chemistry in BB plumes emitted late in the day is responsible for 72 %-92% (84% in an optically thick midday plume) of nitrocatechol formation and controls nitrophenolic formation overall. As a result, overnight nitrophenolic formation pathways account for 56% +/- 2% of NOx loss by sunrise the following day.

In all but one overnight plume we modeled, there was remaining NOx (13 %-57 %) and BBVOCs (8 %-72 %) at sunrise.