While adjusting well to the observed GEM depletions, the first-order relationship from the previous section has limited practical usability for longer times of plume aging because it attains equilibrium (i.e., no more GEM scavenging) only when all GEM has been consumed or all particles have been removed from the atmosphere. This would likely lead to unrealistically large aerosol-related GEM scavenging, considering that the observed GEM depletions occurred in a matter of hours within the plume and that volcanic submicron particles can remain suspended in the atmosphere for weeks or even months at high altitudes . Instead, it appears more likely that volcanic aerosol has a limited capacity to scavenge GEM, for example because reactive chemicals are consumed or adsorption sites are occupied. In this section, we investigate the volcanic GEM depletion events in view of an active-site-limited reactive uptake process, which fulfils the above consideration. Note that all estimates in this section are based on GEM-depleted observations only (n = 10; see red points in Figure ). All estimates and confidence intervals are based on Monte Carlo simulations (see section 2.3.1) and consider uncertainties in observed GEM, estimated particle mass, and the estimated GEM/SO2 ER.