We constrained the mean GEM/SO2 ER from Piton de la Fournaise in the April/June 2018 eruptive period by combining GEM passive sampler observations with Multi-GaS SO2 observations from the summit (see section 2.2.2.3). We obtain an upper limit for the GEM/SO2 ER of 2. and). On the other hand, we use the lowest ER reported in the literature so far (1.5•10 -7 ) 63 as a conservative lower limit for the ER from Piton de la Fournaise. This is described in detail in Appendix C.