Up in smoke: Extensive and intense wildfires in the Pacific Northwest of the United States in 2017 injected large quantities of smoke into the stratosphere. Yu et al. used satellite observations and modeling to characterize the history and chemistry of that smoke. The smoke rose to altitudes between 12 and 23 kilometers within 2 months owing to solar heating of black carbon. The smoke then remained in the stratosphere for more than 8 months. Photochemical loss of organic carbon resulted in a smoke lifetime 40% shorter than expected. Science , this issue p. 587

Ozone forms in the Earth's atmosphere from the photodissociation of molecular oxygen, primarily in the tropical stratosphere. It is then transported to the extratropics by the Brewer–Dobson circulation (BDC), forming a protective ozone layer around the globe. Human emissions of halogen-containing ozone-depleting substances (hODSs) led to a decline in stratospheric ozone until they were banned by the Montreal Protocol, and since 1998 ozone in the upper stratosphere is rising again, likely the recovery from halogen-induced losses. Total column measurements of ozone between the Earth's surface and the top of the atmosphere indicate that the ozone layer has stopped declining across the globe, but no clear increase has been observed at latitudes between 60° S and 60° N outside the polar regions (60–90°). Here we report evidence from multiple satellite measurements that ozone in the lower stratosphere between 60° S and 60° N has indeed continued to decline since 1998. We find that, even though upper stratospheric ozone is recovering, the continuing downward trend in the lower stratosphere prevails, resulting in a downward trend in stratospheric column ozone between 60° S and 60° N. We find that total column ozone between 60° S and 60° N appears not to have decreased only because of increases in tropospheric column ozone that compensate for the stratospheric decreases. The reasons for the continued reduction of lower stratospheric ozone are not clear; models do not reproduce these trends, and thus the causes now urgently need to be established.

Australian wildfires burning from December 2019 to January 2020 injected approximately 0.9 Tg of smoke into the stratosphere; this is the largest amount observed in the satellite era. A comparison of numerical simulations to satellite observations of the plume rise suggests that the smoke mass contained 2.5% black carbon. Model calculations project a 1 K warming in the stratosphere of the Southern Hemisphere midlatitudes for more than 6 months following the injection of black‐carbon containing smoke. The 2020 average global mean clear sky effective radiative forcing at top of atmosphere is estimated to be −0.03 W m⁻² with a surface value of −0.32 W m⁻². Assuming that smoke particles coat with sulfuric acid in the stratosphere and have similar heterogeneous reaction rates as sulfate aerosol, we estimate a smoke‐induced chemical decrease in total column ozone of 10–20 Dobson units from August to December in mid‐high southern latitudes.

Significance: Nonvolcanic stratospheric aerosols account for 20% of the radiative forcing of the entire atmospheric aerosol system since 1850. The Asian summer monsoon (ASM) effectively pumps Asian pollutants to the upper troposphere and lower stratosphere, leading to enhanced aerosol formation. Our in situ measurements combined with modeling work show that the aerosol formed within the ASM anticyclone is exported to the entire Northern Hemispheric stratosphere. On an annual average basis, we estimate that ∼15% of the Northern Hemisphere column stratospheric aerosol surface area originates from the Asian summer monsoon anticyclone region. This surface area is comparable to the modeled contribution from the sum of the small volcanic eruptions over the period 2000 to 2015.

Significance: Large wildfires have been observed to inject smoke into the stratosphere, raising questions about their potential to affect the stratospheric ozone layer that protects life on Earth from biologically damaging ultraviolet radiation. Multiple observations of aerosol and NO₂concentrations from three independent satellite instruments are used here together with model calculations to identify decreases in stratospheric NO₂concentrations following major Australian 2019 through 2020 wildfires. The data confirm that important chemistry did occur on the smoke particle surfaces. The observed behavior in NO₂with increasing particle concentrations is a marker for surface chemistry that contributes to midlatitude ozone depletion. The results indicate that increasing wildfire activity in a warming world may slow the recovery of the ozone layer.