To estimate variation of PM0.95 sulfate on the 29 th of April we construct a PM0.95 sulfate proxy (PM0.95sulfate proxy). We assume that volcanic PM0.95 sulfate can be expressed as a function of observed SO2, a clear volcanic plume tracer and precursor for secondary sulfate, and the DMPS-derived particle volume (PV0.7DMPS), which is proportional to PM0.7. We then constrained the so-defined PM0.95sulfate proxy by comparison to filter-based sulfate measurements, i.e. considering that PM0.95sulfate proxy should be equal to measured PM0.95 sulfate if averaged over the same sampling period. We assign an elevated relative uncertainty of 50% (k = 2) to PM0.95 sulfate proxy , because it was not directly measured but indirectly determined. More details about PM0.95sulfate proxy are given in Appendix A.