New seasonal pattern of pollution emerges from changing North American wildfires

Wildfires in Northwest America have been increasing, expanding both the extent and duration of the wildfire season. This increase is linked to climate change through drought severity and fuel dryness, with models predicting further increases under accelerating climate change. Human activities also influence wildfire occurrence through land use changes, ignitions, and land management policies. Wildfire smoke is detrimental to human health, both locally and downwind. Understanding local and transported contributions to poor air quality from wildfire pollution is critical for optimizing health system responses and mitigating future health impacts. Increasing wildfires have already degraded air quality in the USA, particularly regarding fine particulate matter (organic aerosols). While North American anthropogenic aerosol emissions have been decreasing, improving air quality in the East, wildfire pollution is driving an upward trend in aerosols in the Northwest. The influence of Pacific Northwest (PNW) wildfires on other atmospheric pollutants and their downwind impacts require further investigation. Carbon monoxide (CO), emitted from fires during incomplete combustion, is a valuable tracer for tracking atmospheric transport of large pollution sources like wildfires. With a lifetime of weeks to months, it helps understand distributions of other wildfire-related species, such as tropospheric ozone. Globally, atmospheric CO has been decreasing, but a recent slowdown in the Northern Hemisphere, particularly in summer, may be linked to climate-driven increases in high-latitude wildfires in the North American PNW.

The authors cite numerous studies linking increasing wildfires in the western US to climate change and human activities. Studies are referenced demonstrating the negative impact of wildfire smoke on human health, particularly regarding particulate matter air quality. Existing research highlights the decreasing trend in anthropogenic aerosol pollution in the eastern US, contrasting with the increasing trend in wildfire-related aerosols in the Pacific Northwest. The use of carbon monoxide (CO) as a tracer for wildfire emissions and its atmospheric lifetime are also established in the literature, linking it to the study of other species such as tropospheric ozone.

This study examined the atmospheric impact of PNW wildfire emissions between 2002 and 2018 across three North American regions: the PNW, Central USA, and Northeast. Satellite-measured CO data from the Measurements of Pollution In The Troposphere (MOPITT) instrument were used to investigate how CO abundance during peak PNW burning months differed from other months. Seasonal pattern changes in regional CO abundance were analyzed to highlight local and downwind impacts. The analysis also incorporated aerosol optical depth (AOD) data from the Moderate Resolution Imaging Spectroradiometer (MODIS). Multiple fire and anthropogenic emission inventories (FINN, GFED, QFED, Zheng reanalysis, CAMS-GLOB-ANT) were utilized to support the attribution of observed changes to PNW wildfires. The study employed ordinary least squares analysis for spatial trend plots and calculated regional time series with detrending to account for the Northern Hemisphere background CO trend. Independent t-tests were used to compare monthly averages of seasonal cycles between two time periods (2002–2011 and 2012–2018). The study also included preliminary analysis of mortality data for Colorado to investigate a potential link between transported PNW wildfire pollution and respiratory deaths. The data used in this study are publicly available from NASA and other repositories, and the code is available on GitHub.

The study revealed an upward trend in August CO concentrations over large regions of North America between 2002 and 2018, with the strongest increase in the PNW. This August increase contrasts with decreasing CO trends in other months, reflecting a shift in the seasonal pattern. The spatial pattern of the August CO trend mirrors the trend in August AOD, suggesting a common source (wildfires). Time series analysis showed an emergence of an August CO peak after 2011, coinciding with a strengthening of the August AOD peak in the PNW. This is further supported by the spatial and temporal co-evolution of AOD and CO in August, and consistent with peak burning periods in the PNW. The emergence of the August CO peak is observed across all three regions of interest, indicating substantial downwind transport of wildfire pollution. Analyzing seasonal cycles from 2002-2011 and 2012-2018, the study showed a clear difference in CO patterns. Before 2011, all regions showed a photochemically-driven maximum in April, consistent with the global pattern. After 2011, an additional summer peak emerges in August, particularly prominent in the PNW. This bimodal pattern is present in both column and surface layer CO, indicating surface air quality impacts. Changes in the CO seasonal cycle coincided with a recent strengthening of the August AOD peak, particularly in the PNW and Central USA, suggesting wildfire emissions as the driver. Analysis of four fire emission inventories (FINN, GFED, QFED, Zheng reanalysis) consistently showed peak fire CO emissions in the PNW during August, an order of magnitude higher than in other regions. These inventories further support enhanced August fire CO emissions in the PNW during 2012–2018 compared to 2002–2011. Anthropogenic emission inventories (CAMS-GLOB-ANT, Zheng reanalysis) showed no increase in CO emissions during the later time period, suggesting that anthropogenic emissions are not driving the observed changes in CO. Global modeling with CAM-chem confirmed that increases in August CO after 2011 are linked to fire emissions from the PNW. A preliminary analysis suggests a correlation between the increased August CO linked to PNW wildfire emissions and increased respiratory mortality in Colorado.

The findings demonstrate a significant alteration of the CO seasonal cycle across North America due to increased PNW wildfires, resulting in elevated atmospheric pollution in August. This increase impacts surface air quality both locally and downwind, posing potential health risks. The observed changes highlight the significant influence of PNW wildfires on North American air quality, especially given the predicted increase in wildfire activity under accelerating climate change. The study’s use of multiple emission inventories strengthens the attribution of the observed changes to wildfires, while preliminary mortality data suggests a link between transported wildfire pollution and health impacts. This emphasizes the need to consider the impacts of transported pollution in assessments of public health risks.

This research establishes a clear link between increasing PNW wildfires and significant alterations in the seasonal pattern of atmospheric CO across large areas of North America. The emergence of a secondary CO peak in August, impacting surface air quality, necessitates a broader consideration of the far-reaching consequences of wildfire emissions on human health. Future research should focus on detailed epidemiological studies to quantify the health impacts and conduct more comprehensive analyses of the interplay between emission trends and variability driven by climate and weather factors.

The study acknowledges that year-to-year variability in transport to downwind regions may influence atmospheric variability. A complete quantification of the role of wildfire emissions compared to dynamic changes in transport is left for future research. Additionally, the study's mortality analysis is preliminary and requires further epidemiological investigation to establish causality definitively. The study focuses primarily on CO as a tracer gas, and while it implies impacts on other pollutants (ozone, PM2.5), direct measurements of those pollutants and their health impacts would strengthen the conclusions.