volcanic eruptions are less than 10% of CO2 emissions released during non-eruptive volcanic activity. The 15 June 1991 eruption of Mount Pinatubo (VEI 6) in the Philippines released a total of 18 ± 4 Tg of SO2. Such large VEI 6 eruptions are rare and only occur once every 50 – 100 years. The 2010 eruptions of Eyjafjallajökull (VEI 4) in Iceland emitted a total of 5.1 Tg CO2. VEI 4 eruptions occur about once per year. For comparison, human burning of fossil fuels and production of cement released 36,300 Tg CO2 into the atmosphere in 2015. Therefore, the amount of CO2 emitted due to human activity is 600 times the amount of CO2 annually released by volcanoes. However, some recent volcanic CO2 emission estimates are higher than Fischer et al (2019). The estimates of Burton et al. (2013) of 540 Tg CO2/year and of Werner et al. (2019) of 220 - 300 Tg CO2/year take into account diffuse CO2 emissions from volcanic regions. Even considering the highest estimate of volcanic CO2 emissions of 540 Tg CO2/year, current CO2 emission by human activity of 36,300 Tg CO2/year is 67 times higher. Sensing, collection and measurement Volcanic gases were collected and analysed as long ago as 1790 by Scipione Breislak in Italy. The composition of volcanic gases is dependent on the movement of magma within the volcano. Therefore, sudden changes in gas composition often presage a change in volcanic activity. Accordingly, a large part of hazard monitoring of volcanoes involves regular measurement of gaseous emissions. For example, an increase in the CO2 content of gases at Stromboli has been ascribed to injection of fresh volatile-rich magma at depth within the system. Volcanic gases can be sensed (measured in-situ) or sampled for further analysis. Volcanic gas sensing can be: within the gas by means of electrochemical sensors and flow-through infrared-spectroscopic gas cells outside the gas by ground-based or airborne remote spectroscopy e.g., Correlation spectroscopy (COSPEC), Differential Optical Absorption Spectroscopy (DOAS), or Fourier Transform Infrared Spectroscopy (FTIR). Sulphur dioxide (SO2) absorbs strongly in the ultraviolet wavelengths and has low background concentrations in the atmosphere. These characteristics make sulphur dioxide a good target for volcanic gas monitoring. It can be detected by satellite-based instruments, which allow for global monitoring, and by ground-based instruments such as DOAS. DOAS arrays are placed near some well-monitored volcanoes and used to estimate the flux of SO2 emitted. The