Satellite Limb Observations of Unprecedented Forest Fire Aerosol in the Stratosphere. Issue 16 (23rd August 2019) - Bourassa, Adam E.
Abstract: Intense forest fires in western North America during August 2017 caused smoke plumes that reached the stratosphere. While this phenomenon has often been observed, this particular event caused increases in stratospheric aerosol extinction at higher altitudes with greater magnitude than previously observed in the satellite record. Here we use multiple satellite limb sounding observations, which provide high sensitivity to thin aerosol layers and good vertical resolution, to show that enhancements in aerosol extinction from the fires reached as high as 23 km in altitude and persisted for more than 5 months. Within 1 month, the aerosol is observed to cover latitudes from 20°N to 60°N, which is essentially the northernmost limit of the observations. At midlatitudes between 15‐ and 20‐km altitudes, the sustained level of median aerosol extinction measured at 750 nm increased by almost an order of magnitude, from approximately 10 −4 km −1 to nearly 10 −3 km −1 . Agreement between limb scatter and occultation measurements is generally within 20% despite potential bias due to modified aerosol shape and composition. Plain Language Summary: Aerosol particles in the upper atmosphere are most often formed by explosive volcanic eruptions that reach high altitudes. Here the aerosol can last for many months, or even years, and scatter sunlight back to space causing global cooling. In addition to volcanic eruptions, intense wildfires can sometime burn hot enough to generate atmospheric convection that carries smoke particles high into the atmosphere. In this study we use satellite measurements that observe the vertical structure of the atmosphere with high sensitivity to thin aerosol layers to report on the long‐lasting stratospheric effect of western North American wildfires from the summer of 2017. Aerosol from these fires was known to reach the stratosphere and circulate throughout the Northern Hemisphere. However, these satellite measurements, which have a record that stretches almost 40 years, show that this fire generated a stratospheric aerosol cloud almost 10 times thicker than background levels and lasted for more than 5 months. This is the largest impact from wildfires ever observed in the 40‐year satellite record. Key Points: Satellite limb measurements show that forest fires in August 2017 created a layer of persistent high‐altitude aerosol in the stratosphere The magnitude and extent of the aerosol has not been observed before in the satellite limb sounding era from a forest fire event Agreement between limb scatter and occultation measurements is generally within 20% despite potential bias due to modified aerosol shape and composition