Based on a Monte Carlo simulation (see section 2.3.1) and considering uncertainties in observed GEM, estimated particle mass, estimated transport time, and the estimated GEM/SO2 emission ratio, we obtain empirical GEM scavenging rate constants of -0.046 (-0.071; -0.030) µg -1 m 3 h -1 and -0.0023 (-0.0038; -0.0014) µg -1 m 3 h -1 for PM0.7DMPS and PM0.95sulfate proxy, respectively (see Figure ). These rate constants should be seen as first approximations with large uncertainties, and they are likely biased low because the real interaction time between GEM and reactive plume constituents was most likely shorter than the plume transport time estimated in section 2.3.4. Nevertheless, we encourage the use of these rate constants as a first approximation to address particle-related GEM scavenging in aging volcanic plumes. In any case, the conditions for which they were obtained have to be kept in mind, i.e. for 3 -7 hours aged volcanic plumes, abundantly present and likely sulfate-dominated (mostly sulfuric acid) submicron volcanic particles, and for temperature and relative humidity ranges of ~10 -15 C and ~15 -20%, respectively. Any extrapolation from these conditions must be approached with care. 4). All fit parameters and their 95% confidence intervals were obtained with Monte Carlo simulations. The slope of the fit corresponds to the estimated interaction rate constant.