North American Wildfires are Changing the Atmospheric Carbon Monoxide Seasonal Cycle

Abstract:

Fires in the Pacific Northwest (PNW) have been found to counteract the reductions in man-made aerosol pollution for North America, both locally and for the Inter-mountain West. Carbon monoxide (CO) is co-emitted with aerosols from fires, and with a lifetime of about 2 months, is a valuable tracer for transported pollution. We investigate the impact of increased PNW fires on CO and find a change in the observed seasonal cycle of CO concentrations for recent years.

We analyze CO and aerosol optical depth (AOD) measurements taken from the NASA/Terra satellite for three North American regions: PNW, Central USA and the Northeast. CO is from Measurements of Pollution in the Troposphere (MOPITT) and AOD is measured by the Moderate Resolution Imaging Spectroradiometer (MODIS). We find that although CO has been decreasing globally over the last 2 decades, August CO in the PNW region is increasing. These trends in August alter CO seasonality for large regions of North America between two time periods: 2002 to 2011 and 2012 to 2018. Seasonality differences in the PNW CO are clear, with the growth of a second peak in August for the recent time period, in addition to the usual April maximum. The second CO seasonal peak is coincident with an increase in the PNW AOD maximum. Fire-driven impact on seasonality is supported by investigating multiple fire and anthropogenic emission inventories. The downwind impact shown for CO also has implications for other air-quality relevant trace gases from fire. Overall, our results show that wildfires in the PNW are an increasingly important source of pollution affecting North America.