3) There is insufficient BrO to explain appreciable GEM oxidation by bromine chemistry, even in daytime plumes. MAX-DOAS observations at Maïdo captured the volcanic plume passages on the 29 th of April after sunrise. No measurements are available before sunrise, since the instrument relies on scattered solar photons as a light source. While observed SO2 column densities were strongly enhanced (maximum: 1.08 ± 0.05 × 10 18 molec cm -2 ), only very minor enhancements in the BrO column densities were observed (see supplementary Figure ). BrO/SO2 molar ratios for the daytime plumes on the 29 th of April were (1.28 ± 0.07)•10 -5 (N = 2), at the lower end of reported values in volcanic plumes . We estimate the GEM lifetime with regard to bromine-initiated oxidation as roughly 36 hrs in the daytime plume (see supplementary Table ). This estimate assumes 1 ppmv SO2, 40 ppbv O3, BrO/SO2 = 1.2•10 -5 , and Br/BrO = 0.1, corresponding to an average Br radical concentration of 1.2 pptv inside the plume. The GEM + Br reaction, thermal decomposition of Br-GEM adduct, and scavenging of Br-GEM + O3 were treated using reaction rate constants from Shah et al. ( ) (temperature = 288 K; pressure = 800 hPa).