Expanding number of Western US urban centers face declining summertime air quality due to enhanced wildland fire activity

Abstract

Combining multiple sources of information on atmospheric composition, wildland fire emissions, and fire area burned, we link decadal air quality trends in Western US urban centers with wildland fire activity during the months of August and September for the years 2000–2019. We find spatially consistent trends in extreme levels (upper quantile) of fine particulate matter (PM2.5), organic carbon, and absorption aerosol optical depth centered on the US Pacific Northwest during the month of August. Emerging trends were also found across the Pacific Northwest, western Montana, and Wyoming in September. Furthermore, we identify potential wildfire emission ‘hotspots’ from trends in wildfire derived PM2.5 emissions and burned area. The spatial correspondence between wildfire emissions hotspots and extreme air quality trends, as well as their concomitant spatial shift from August to September supports the hypothesis that wildfires are driving extreme air quality trends across the Western US. We derive further evidence of the influence of wildland fires on air quality in Western US urban centers from smoke induced PM2.5 enhancements calculated through statistical modeling of the PM2.5-meteorology relationship at 18 Western US cities. Our results highlight the significant risk of increased human exposure to wildfire smoke in August at these Western US population centers, while also pointing to the potential danger of emerging trends in Western US population growth, wildfire emissions, and extreme air quality in September.

Wildfire plumes in the Western US are reaching greater heights and injecting more aerosols aloft as wildfire activity intensifies

“…In the Klamath Mountains, Sierra Nevada, and Southern Rockies, we find hinge fit trends (2017/2018–2020) of 99.0, 201.2, and 256.1 m/year, respectively, representing enhancements by a factor of 9–15 relative to results from linear regression (2003–2020). The presence of positive, significant (p < 0.1), and potentially accelerating trends throughout much of the mountainous Pacific Northwest and California, as well as the Southern Rockies, overlaps with previously identified potential wildfire emissions “hotspots” 17 .…”

“…Model evaluation is performed via comparison to the MISR plume heights dataset 64 . Emphasis is placed on August and September plumes given previously-identified wildfire emissions trends 17 and the computational/storage expense of generating regional-decadal scale model inputs.

Figure 1 ( a ) Results for linear regressions trend analyses (2003–2020) of the maximum August–September plume top height for Western US ecoregions and Canadian ecoprovinces based on ~ 4.6 million plumes.

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“…Over recent decades, Western US wildfire activity has become increasingly intense, with many mountainous regions demonstrating increases in annual fire area burned 14 – 16 , annual fire area burned at high-severity 14 , the number of large wildfires 16 , wildfire emissions of aerosol 17 , and in some cases high-altitude burning 18 . Similarly, portions of British Columbia demonstrate increases in wildfire emissions 17 .…”

“…Over recent decades, Western US wildfire activity has become increasingly intense, with many mountainous regions demonstrating increases in annual fire area burned 14 – 16 , annual fire area burned at high-severity 14 , the number of large wildfires 16 , wildfire emissions of aerosol 17 , and in some cases high-altitude burning 18 . Similarly, portions of British Columbia demonstrate increases in wildfire emissions 17 . This shift in wildfire activity has been, in part, attributed to a changing climate, with strong links drawn to increasing temperatures and aridity 14 , 18 – 21 , reduced summertime precipitation 22 , and a declining snowpack 23 , 24 .…”

“…Despite remaining uncertainties as to the exact governing equations of the wildfire plume rise and its expression in a modeling framework 44 , climate driven trends toward enhanced wildfire activity 14 – 24 and related degradation of Western US air quality 17 , 49 , 50 highlight the need for investigation into the evolving state of wildfire aerosol injection and the associated vertical mass distribution. At present, the temporal and spatial limitations of satellite sampling by space-borne sensors—i.e., the Multi-angle Imaging SpectroRadiaometer (MISR) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)—prohibit a purely observational investigation of this topic.…”