Abstract
Smoke from Western North American wildfires reached the stratosphere in large amounts in August 2017. Limb-oriented satellite-based sensors are commonly used for studies of wildfire aerosol injected into the stratosphere (OMPS-LP (Ozone Mapping and Profiler Suite Limb Profiler) and SAGE III/ISS (Stratospheric Aerosol and Gas Experiment III on the International Space Station)). We find that these methods are inadequate for studies the first 1–2 months after such a strong fire event due to event termination (“saturation”). The nadir-viewing lidar CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) is less affected due to shorter path in the smoke, and, further, provides means that we could use to develop a method to correct for strong attenuation of the signal. After the initial phase, the aerosol optical depth (AOD) from OMPS-LP and CALOP show very good agreement above the 380 K isentrope, whereas the OMPS-LP tends to produce higher AOD than CALIOP in the lowermost stratosphere (LMS), probably due to reduced sensitivity at altitudes below 17 km. Time series from CALIOP of attenuation-corrected stratospheric AOD of wildfire smoke show an exponential decline during the first month after the fire, which coincides with highly significant changes in the wildfire aerosol optical properties. The AOD decline is verified by the evolution of the smoke layer composition, comparing the aerosol scattering ratio (CALIOP) to the water vapor concentration from MLS (Microwave Limb Sounder). Initially the stratospheric wildfire smoke AOD is comparable with the most important volcanic eruptions during the last 25 years. Wildfire aerosol declines much faster, 80–90 % of the AOD is removed with a half-life of approximately 10 days. We hypothesize that this dramatic decline is caused by photolytic loss. This process is rarely observed in the atmosphere. However, in the stratosphere this process can be studied with practically no influence from wet deposition, in contrast to the troposphere where this is the main removal path of sub-micron aerosol particles. Despite the loss, the aerosol particles from wildfire smoke in the stratosphere are relevant for the climate.
Highlights
Background stratospheric aerosol is composed of sulfuric acid, water, carbonaceous components, and minor extraterrestrial and tropospheric components (Murphy et al, 2007; Kremser et al., 2016; Martinsson et al, 2019)
We use an approach based on five satellite sensors to study the influence on the stratosphere of the great North American fire in August 2017
To explain differences in aerosol optical depth (AOD) between OMPS-LP and CALIOP, a comparison of extinction coefficients follows, where results from SAGE III/ISS are included in the comparison
Summary
Background stratospheric aerosol is composed of sulfuric acid, water, carbonaceous components, and minor extraterrestrial and tropospheric components (Murphy et al, 2007; Kremser et al., 2016; Martinsson et al, 2019). Volcanism is a strong source of the stratospheric sulfurous, carbonaceous and ash aerosol (Martinsson et al, 2009; Andersson et al, 2013; Friberg et al., 2014). Like that of Mt Pinatubo in 1991, affect the stratosphere for several years, causing global cooling of several tenths of degrees Kelvin (Kremser et al, 2016). These eruptions are scarce, only a few per century (Ammann et al, 2003; Stothers, 2007). The pyroCbs lifted smoke from the fire to the extratropical tropopause region, where absorption of radiation by black carbon (BC) in the smoke induced additional lift to 23 km altitude in 2 months (Yu et al, 2019)
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