5C: WE-CAN – example: planned analyses

The NSF-funded WE-CAN ground-based and airborne field campaign aimed to systematically characterize the emissions and first 24 hours of smoke plume evolution from western U.S. wildfires. 21 different wildfire plumes were sampled.

While preliminary data from the experiment is still being compiled and only available for a subset of the observations, some striking differences between individual fire plumes emerged during the experiment. For example, the ozone production efficiency in plumes from different fires exhibited a very large variability, correlated in part with the initial availability of NOx in the freshly emitted plumes.

To illustrate this finding, figure 1 compares the plume chemistry in two different fires, the Bear Trap (a) and Taylor Creek (b) fires. The Taylor Creek fire was flown on July 30, 2018 and was burning in heavy timber in southwestern Oregon. The Bear Trap fire was flown on August 9, 2018, and was burning brush and timber in eastern Utah. Both plumes were measured repeatedly, moving the aircraft downwind across the plume in a lawnmower pattern, covering plume ages between less than 1 hour and about 3 hours.

Ozone production was much more vigorous in the Taylor Creek fire plume. The fresh Taylor Creek plume produced an excess of more than 30 ppb of ozone over background in the first hour after emission, with ozone production continuing to increase during plume aging. Over 130 ppb were produced over background after 3 hours of chemical aging. In contrast, the Bear Trap plume shows an excess of ozone around 20 ppb after ~1h, which does not increase with chemical aging. While the mixing ratios of typical fire VOC tracers in the initial plume crossing were comparable, NOx in the Taylor Creek fire was much higher initially compared to the Bear Trap fire, by a factor of about 10.

Chemical box modeling will be used to examine the differences in the chemistry of these fire plumes by comparing the modeling results with the large number of primary and secondary species measured on board the C-130 aircraft. Results from data analysis like this example, together with other outcomes from the WE-CAN campaign, will improve models and other scientific tools used to predict wildfire influence on local and regional air quality in the western United States and elsewhere.

Plume chemistry in wildfires.Plume chemistry in wildfires.
Figure 1. Comparison of plume chemistry in the Bear Trap (a) and Taylor Creek (b) fires. Left panels show plume crossings at ~1h after emission, right panels show plume crossings after ~3 hours of aging. Ozone production is far more vigorous in the Taylor Creek fire, which is likely due to the much larger initial NOx availability in the fresh plume.