Dr Thomas Christopher in the process of monitoring the instrument measuring the sulphur dioxide flux
April 23, 2021
Scientists measure SO2 in Soufriere gas plume

Crucial measurements of the Sulphur Dioxide flux in the gas plume streaming out of the volcano were conducted for the first time ever at La Soufrière last week.

 Sulphur Dioxide or SO2, is one of the volatiles which come from a volcano when it is still active, and as long as it is present, it signals that the eruption has not ended. When the eruption shuts down, Hydrogen Sulphide is the major sulphur species.  

 Dr Thomas Christopher, a gas specialist from Montserrat, working at the University of the West Indies(UWI)-Seismic Research Centre(SRC), based at the Montserrat Volcano Observatory (MVO), is the scientist who is conducting the measurements which give him data as to the masses of the gas being produced by the ton every day.  

 The coastguard assists his daily efforts while navigating the debris laden sea around the flanks of the volcano. On Sunday, April 18 vessel SVG-09, and leading seaman Davis; AB Browne; AB Mofford ;and petty officer Clint Lewis, were assigned to the task.  

 “Simply put what it’s doing is comparing clear sky to gassy sky, and working out how much sulphur is in there based on the difference between the clear and the gassy one,” Christopher explained as it relates to the ultraviolet spectroscopy. 

 As he was speaking a coastguard officer had already followed the routine and secured the part of the instrument that points towards the sky, and plume.  

 On April 9, La Soufrière started off with frequent explosions and ash venting, but by the following Monday the time between explosive events began to increase.  

 “…During the explosions you don’t have a horizontal plume, you have vertical gas going up, so it wasn’t until Tuesday (April 13) when I saw the plume then I decided okay, it was worthwhile,” the scientist revealed.  

 The gas travels like a river from the volcano, and the coastguard vessel travels until Christopher finds it, and then they scan it at least another five times. With the instrument, “you’re seeing all the light coming through the plume, you’re making a slice through the plume and creating a face of it, and then you can basically integrate that over time to work out how much gas is coming out based on how much gas you have.” 

 The figures that he has processed following the scans vary, with the first being about 800 tons of SO2 a day, and another being 460 tons.  

 This difference is expected, Christopher said, because other than the magma degassing, “there’s deposits on the ground which are degassing as well- the deposits from the explosions. So they are adding to how much gas you’re seeing…” 

 It is important for him to collect the data every day because the volcano “huffs and puffs” throughout. “…We’re measuring for an hour, maybe half an hour, so that’s a very small snapshot. You are presenting data in tons per day, but you’re not actually measuring for a whole day, you’re measuring for a fraction of a day so you know that even that data is representative.”  

 Therefore, “You wanna get as much measurements in so you can see where your baseline is, because there would be noise within your baseline It would vary but somewhere in the middle is where your true value really is so the idea now is to get the tempo up really quickly and get a baseline going.”  

 There are plans to, given the chance, set up on the flanks of the volcano, a permanent station to measure the Sulphur Dioxide flux.  

 The instrument will have the impression that it is moving back and forth under the gas plume, and this information will be delivered live to the scientists.  

 This particular trip also included the use of the multi-GAS instrument, which collects data on the ratios of hydrogen sulphide, sulphur dioxide, carbon dioxide and water. It uses a very different method, sucking in the gases to the instrument, writing a file, and then pumping it back out.  

 “The gasses are coming from different parts of the system, so if I can tell how much of any type is coming up; it will give me an idea of where the degassing is more prominent in the system,” the scientist outlined.