A global view of the stratospheric background, volcanic and wildfire aerosol in the CALIOP era (2006–2023)

Abstract. This study deals with the stratospheric aerosol during the 17 years of lidar measurements with CALIOP. To obtain extinction from the backscattering measurements, we estimated the lidar ratios of the main aerosol injections into the stratosphere. The stratospheric background is estimated by making a subdivision of the stratosphere into nine parts, spanned by three latitude and altitude intervals, reaching background conditions individually at different times. The extracted background shows excellent agreement with solar occultation measurements in the volcanically quiescent period 1998–2000. Our results show that 70 % of the background aerosol in the deep Brewer-Dobson (dBD) branch is formed above 19 km altitude, indicating strong influence of carbonyl sulfide on the stratospheric background aerosol. The stratosphere was clearly affected by 15 volcanic eruptions and 5 wildfires. Their combined aerosol load affected Southern extratropics, tropics and Northern extratropics almost equally, and the altitude distribution shows that the shallow Brewer-Dobson branch was most affected (43 %) followed by the dBD (31 %) and lowermost stratosphere (26 %). The most important events in order of maximum AOD were the Hunga Ha’apai eruption (2022), Australian wildfires (2019-20) and the eruptions of Raikoke (2019), Sarychev (2009) and Nabro (2011). These events induced strong variability in the stratospheric aerosol optical depth (AOD), causing highly variable climate impact in the period studied with yearly average global effective radiative forcing ranging from -0.14 W/m² at background conditions to -0.4 W/m². CALIOP provided important data for stratospheric aerosol climatologies during its 17 years of operation.