Measurements of the gas emissions from the volcano have been carried out throughout the eruption. These measurements have constituted an important input to the assessment of volcanic risk. Sulphur dioxide (SO₂) and hydrogen chloride (HCl) are the species routinely monitored, primarily by remote sensing techniques.

Measurements of the gas and ash emissions from the volcano have added to the volcanic risk assessment, and have also been important in mitigating against potential health problems.

Two methods of monitoring gas emissions have been used regularly throughout the eruption. The first method uses a Correlation Spectrometer (also called a Cospec) to measure the daily output of sulphur dioxide from the volcano. Using ultraviolet light from the sun, passing through the volcanic plume, the concentration of this gas can be measured. High levels of sulphur dioxide were measured during periods of high extrusion rate of the lava dome.

Sulphur dioxide has also been measured using diffusion tubes that are left at sites around the volcano for about 2 weeks. These give a measure of the level of gas at ground level, and can be directly related to the gas levels for people living or working around the volcano. One other method of measuring gas concentrations has been occasionally on Montserrat: Fourier Transform Infrared spectroscopy or FTIR. Using this method the ratios of different gases can be measured, for example, the ratio chlorine to sulphur dioxide.

In addition to gas measurements, the chemistry of rain water has been monitored through the eruption. At sites directly west of the volcano the pH of the rainwater reached as low as 2, i.e., very acidic. This is a result of the rain passing through the gaseous volcanic plume. Ash has also been collected from several sites around the volcano, leading to estimates of the mass of ash erupted for individual volcanic events. For example, on 17 September 1996, it is estimated that 600,000 tonnes of ash fell to the west of the volcano.

Environmental Monitoring

The environmental effects of the eruption have been routinely monitored throughout the eruption. This has included the measurement of ambient sulphur dioxide (SO₂) concentrations at ground level, the monitoring of fine dust particles in the air, and the measurement of the pH of rainwater.