To evaluate how much GEM has been scavenged in the plume, it is necessary to estimate how much GEM would have been present in the absence of scavenging. This is simply the GEM background concentration (𝐺𝐸𝑀 𝑏𝑎𝑐𝑘𝑔𝑟𝑜𝑢𝑛𝑑 ) plus an additional volcanic contribution due to GEM emissions from the vent (𝐺𝐸𝑀 𝑣𝑜𝑙𝑐𝑎𝑛𝑖𝑐 ).