Ozone chemistry in western U.S. wildfire plumes

Abstract

Wildfires are a substantial but poorly quantified source of tropospheric ozone (O₃). Here, to investigate the highly variable O₃ chemistry in wildfire plumes, we exploit the in situ chemical characterization of western wildfires during the FIREX-AQ flight campaign and show that O₃ production can be predicted as a function of experimentally constrained OH exposure, volatile organic compound (VOC) reactivity, and the fate of peroxy radicals. The O₃ chemistry exhibits rapid transition in chemical regimes. Within a few daylight hours, the O₃ formation substantially slows and is largely limited by the abundance of nitrogen oxides (NO_x). This finding supports previous observations that O₃ formation is enhanced when VOC-rich wildfire smoke mixes into NO_x-rich urban plumes, thereby deteriorating urban air quality. Last, we relate O₃ chemistry to the underlying fire characteristics, enabling a more accurate representation of wildfire chemistry in atmospheric models that are used to study air quality and predict climate.

Fig. 1.. Simplified scheme to illustrate the factors influencing O₃ formation in wildfire plumes.

Wildfires emit oxidant precursors, NO_x, and an enormous diversity of VOCs. In the near field, OH produced via photolysis of HONO initiates VOC oxidation, which proceeds in the presence of NO_x and leads to efficient O₃ formation. After a few hours, the HONO has been consumed and NO_x has been both diluted sufficiently and converted to PANs and ${\text{NO}}_3^{-}$ such that the O₃ formation slows by several orders of magnitude. In this simplified scheme, the width of arrows having the same color represents the relative importance of competing pathways.

Fig. 2.. Single transect analysis (STA) examines the differences in plume composition across individual transects of the wildfire plumes.

In (B), the flight track on 3 August 2019 is colored by OH exposure, which is lower in plume center than edges, as a result of high aerosol optical extinction in plume center. In (A), the dilution-corrected O_x formation (i.e., ΔO_x/ΔCO) is illustrated in one near-field transect.

Fig. 3.. Production and fate of OH.

(A) shows that the OH exposure correlates with the amount of HONO loss [f_HONO,lost = 1 − (ΔHONO/ΔCO)/(ΔHONO/ΔCO)_max] for the 3 August 2019 Williams Flats Fire. The correlation indicates that OH is produced mainly by HONO photolysis in the near field. The color represents the relative contribution of HONO photolysis to total HO_x production rate (denoted as f_jHONO). (B) shows that OVOCs, alkanes/alkenes, and furans are the major contributors to total VOCR based on the average of transects included in the O_x chemical closure analysis.

Fig. 4.. The measurements of ROOH and RONO₂ from propene oxidation are used to diagnose the RO₂ fate.

The ROOH is not produced in the transect with high [NO] (A) but produced in the transect with low [NO] (B). The signals of both RONO₂ and ROOH are divided by the branching ratio of the corresponding RO₂ reaction (i.e., α). The ROOH signal is multiplied by a factor of 4 to be shown in the same scale as RONO₂. The shaded area represents the 25th to 75th percentile. ppb, parts per billion.

Fig. 5.. The RO₂ fate transitions from an RO₂ + NO–dominated regime to a mixed regime with increasing importance of RO₂ + HO₂.

(A) The f_{RO2 + NO} decreases as smoke transports in the William Flats Fire sampled on 2 different days. The data points are colored by the fire radiative power (FRP) measured at the estimated time of smoke emission. (B) A large fraction of VOCs is oxidized in the mixed regime. The max ΔVOCR/ΔCO is represented by the average ΔVOCR/ΔCO of observations with the top 1% [CO] during the fire sample. The downwind distance is estimated on the basis of the aircraft position and the burned area. The dashed lines are provided as a visual aid. The ellipses represent the uncertainty range.

Fig. 6.. The evolution of the partitioning of NO_y species.

(A) shows measurements of the 3 August 2019 Williams Flats Fire. As smoke ages, the NO_x and HONO emitted from fires are converted to PAN and NO₃⁻. (B) shows that the fraction of NO_x loss to PAN (f_PAN) across each transect increases with smoke age, which results from evolving CH₃CHO/NO₂ as discussed in the text. Each data point represents one transect, and the transects from the same fire sampling patterns have the same color. The black line is provided as a visual aid. The numbers in parentheses represent the index of a set of crosswind transects in a flight.

Fig. 7.. The predicted and measured O_x production show reasonable agreement.

The ellipses represent the uncertainty range (Supplementary Materials, section S9). The slope and intercepts are obtained from a York fit.

Fig. 8.. Parameterization of the O₃ + NO₂ production.

The measured production of O₃ + NO₂ (P_{O3 + NO2}) across individual transects exhibits positive correlation with the span of OH exposure (ΔOH exposure) and MCE, as shown in (A) and (B), respectively. Thirty-nine transects are selected for this analysis (Supplementary Materials, section S4). (C) Comparison between predicted and measured P_{O3 + NO2} for individual transects. (D) The emission ratios (ERs) of NO₂ to CO derived from the field [i.e., a + b×max(0,MCE-c)] and measured in the 2016 FIREX FireLab are plotted as a function of MCE.