In June 2019 a stratospheric eruption occurred at Raikoke (48^∘ N, 153^∘ E). Satellite observations show the injection of ash and SO₂ into the lower stratosphere and an early entrainment of the plume into a cyclone. Following the Raikoke eruption, stratospheric aerosol optical depth (sAOD) values increased in the whole Northern Hemisphere and tropics and remained enhanced for more than 1 year, with peak values at 0.040 (short-wavelength, high northern latitudes) to 0.025 (short-wavelength, Northern Hemisphere average). Discrepancies between observations and global model simulations indicate that ash may have influenced the extent and evolution of the sAOD. Top of the atmosphere radiative forcings are estimated at values between −0.3 and $-\mathrm{0.4}\mathrm{W}{\mathrm{m}}^{-\mathrm{2}}$ (clear-sky) and of −0.1 to $-\mathrm{0.2}\mathrm{W}{\mathrm{m}}^{-\mathrm{2}}$ (all-sky), comparable to what was estimated for the Sarychev eruption in 2009. Almost simultaneously two significantly smaller stratospheric eruptions occurred at Ulawun (5^∘ S, 151^∘ E) in June and August. Aerosol enhancements from the Ulawun eruptions mainly had an impact on the tropics and Southern Hemisphere. The Ulawun plume circled the Earth within 1 month in the tropics. Peak shorter-wavelength sAOD values at 0.01 are found in the tropics following the Ulawun eruptions and a radiative forcing not exceeding −0.15 (clear-sky) and −0.05 (all-sky). Compared to the Canadian fires (2017), Ambae eruption (2018), Ulawun (2019) and the Australian fires (2019/2020), the highest sAOD and radiative forcing values are found for the Raikoke eruption.

The climate active trace-gas carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere. A missing source in its atmospheric budget is currently suggested, resulting from an upward revision of the vegetation sink. Tropical oceanic emissions have been proposed to close the resulting gap in the atmospheric budget. We present a bottom-up approach including (i) new observations of OCS in surface waters of the tropical Atlantic, Pacific and Indian oceans and (ii) a further improved global box model to show that direct OCS emissions are unlikely to account for the missing source. The box model suggests an undersaturation of the surface water with respect to OCS integrated over the entire tropical ocean area and, further, global annual direct emissions of OCS well below that suggested by top-down estimates. In addition, we discuss the potential of indirect emission from CS₂ and dimethylsulfide (DMS) to account for the gap in the atmospheric budget. This bottom-up estimate of oceanic emissions has implications for using OCS as a proxy for global terrestrial CO₂ uptake, which is currently impeded by the inadequate quantification of atmospheric OCS sources and sinks.

We show that a fire plume injected into the lower stratosphere at high northern latitudes during the Canadian wildfire event in August 2017 partly reached the tropics. The transport to the tropics was mediated by the anticyclonic flow of the Asian monsoon circulation. The fire plume reached the Asian monsoon area in late August/early September, when the Asian monsoon anticyclone (AMA) was still in place. While there is no evidence of mixing into the center of the AMA, we show that a substantial part of the fire plume is entrained into the anticyclonic flow at the AMA edge and is transported from the extratropics to the tropics, and possibly the Southern Hemisphere particularly following the north–south flow on the eastern side of the AMA. In the tropics the fire plume is lifted by ∼5 km in 7 months. Inside the AMA we find evidence of the Asian tropopause aerosol layer (ATAL) in August, doubling background aerosol conditions with a calculated top of the atmosphere shortwave radiative forcing of −0.05 W m⁻². The regional climate impact of the fire signal in the wider Asian monsoon area in September exceeds the impact of the ATAL by a factor of 2–4 and compares to that of a plume coming from an advected moderate volcanic eruption. The stratospheric, trans-continental transport of this plume to the tropics and the related regional climate impact point to the importance of long-range dynamical interconnections of pollution sources.

As a consequence of extreme heat and drought, record-breaking wildfires developed and ravaged south-eastern Australia during the fire season 2019–2020. The fire strength reached its paroxysmal phase at the turn of the year 2019–2020. During this phase, pyrocumulonimbus clouds (pyroCb) developed and injected biomass burning aerosols and gases into the upper troposphere and lower stratosphere (UTLS). The UTLS aerosol layer was massively perturbed by these fires, with aerosol extinction increased by a factor of 3 in the visible spectral range in the Southern Hemisphere, with respect to a background atmosphere, and stratospheric aerosol optical depth reaching values as large as 0.015 in February 2020. Using the best available description of this event by observations, we estimate the radiative forcing (RF) of such perturbations of the Southern Hemispheric aerosol layer. We use offline radiative transfer modelling driven by observed information of the aerosol extinction perturbation and its spectral variability obtained from limb satellite measurements. Based on hypotheses on the absorptivity and the angular scattering properties of the aerosol layer, the regional (at three latitude bands in the Southern Hemisphere) clear-sky TOA (top-of-atmosphere) RF is found varying from small positive values to relatively large negative values (up to −2.0 W m⁻²), and the regional clear-sky surface RF is found to be consistently negative and reaching large values (up to −4.5 W m⁻²). We argue that clear-sky positive values are unlikely for this event, if the ageing/mixing of the biomass burning plume is mirrored by the evolution of its optical properties. Our best estimate for the area-weighted global-equivalent clear-sky RF is $-\mathrm{0.35}\pm \mathrm{0.21}$ (TOA RF) and $-\mathrm{0.94}\pm \mathrm{0.26}$ W m⁻² (surface RF), thus the strongest documented for a fire event and of comparable magnitude with the strongest volcanic eruptions of the post-Pinatubo era. The surplus of RF at the surface, with respect to TOA, is due to absorption within the plume that has contributed to the generation of ascending smoke vortices in the stratosphere. Highly reflective underlying surfaces, like clouds, can nevertheless swap negative to positive TOA RF, with global average RF as high as +1.0 W m⁻² assuming highly absorbing particles.

Every year during the Asian summer monsoon season from about mid-June to early September, a stable anticyclonic circulation system forms over the Himalayas. This Asian summer monsoon (ASM) anticyclone has been shown to promote transport of air into the stratosphere from the Asian troposphere, which contains large amounts of anthropogenic pollutants. Essential details of Asian monsoon transport, such as the exact timescales of vertical transport, the role of convection in cross-tropopause exchange, and the main location and level of export from the confined anticyclone to the stratosphere are still not fully resolved. Recent airborne observations from campaigns near the ASM anticyclone edge and centre in 2016 and 2017, respectively, show a steady decrease in carbon monoxide (CO) and increase in ozone (O₃) with height starting from tropospheric values of around 100 ppb CO and 30–50 ppb O₃ at about 365 K potential temperature. CO mixing ratios reach stratospheric background values below ∼25 ppb at about 420 K and do not show a significant vertical gradient at higher levels, while ozone continues to increase throughout the altitude range of the aircraft measurements. Nitrous oxide (N₂O) remains at or only marginally below its 2017 tropospheric mixing ratio of 333 ppb up to about 400 K, which is above the local tropopause. A decline in N₂O mixing ratios that indicates a significant contribution of stratospheric air is only visible above this level. Based on our observations, we draw the following picture of vertical transport and confinement in the ASM anticyclone: rapid convective uplift transports air to near 16 km in altitude, corresponding to potential temperatures up to about 370 K. Although this main convective outflow layer extends above the level of zero radiative heating (LZRH), our observations of CO concentration show little to no evidence of convection actually penetrating the tropopause. Rather, further ascent occurs more slowly, consistent with isentropic vertical velocities of 0.7–1.5 K d⁻¹. For the key tracers (CO, O₃, and N₂O) in our study, none of which are subject to microphysical processes, neither the lapse rate tropopause (LRT) around 380 K nor the cold point tropopause (CPT) around 390 K marks a strong discontinuity in their profiles. Up to about 20 to 35 K above the LRT, isolation of air inside the ASM anticyclone prevents significant in-mixing of stratospheric air (throughout this text, the term in-mixing refers specifically to mixing processes that introduce stratospheric air into the predominantly tropospheric inner anticyclone). The observed changes in CO and O₃ likely result from in situ chemical processing. Above about 420 K, mixing processes become more significant and the air inside the anticyclone is exported vertically and horizontally into the surrounding stratosphere.