Abstract. 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 -0.4Wm-2 (clear-sky) and of −0.1 to -0.2Wm-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 rate constant of the reaction OH + CH 3 O 2 → products (R1). has been measured at 294K by simultaneous coupling of Laser Induced Fluorescence (LIF) and cw-Cavity Ring Down Spectroscopy (cw-CDRS) to laser photolysis. OH radicals were generated by 248nm photolysis of either O 3 in the presence of H 2 O or H 2 O 2 , CH 3 O 2 radicalswere generated simultaneously by photolysis of CH 3 I and their absolute concentrations were obtained by cw-CRDS. OH decays were measured under excess CH 3 O 2 concentrations and a very fast rate constant of k 1 = (2.8±1.4)×10 -10 cm 3 s -1 was found independent of pressure at 50 and 100 Torr helium.