Measurements of the amount of gas and ash being emitted from the volcano and their effects on rainwater are extremely important. They provide extra insight into the eruptive behaviour and also give valuable information on the health risk that the volcanic emissions may pose.
Ash is collected from several sites around the volcano weekly during light ashfall or after major ashfall events. Usually ash falls to the west and north-west of the volcano as a result of the prevailing winds. In Plymouth, ash thicknesses of up to five centimetres have fallen during a single event and, between 1995 and 1999, a total of more than 30 centimetres accumulated (see contoured map to the right).
COSPEC (Correlation Spectrometer) and DOAS (Differential Optical Absorption Spectrometer) measure sulphur dioxide in the volcanic plume. While the lava dome is erupting it gives a good indication of the rate of extrusion. On Montserrat rates of sulphur dioxide emission have been as high as 4000 tonnes per day, but more commonly have been under 1000 tonnes per day. The FTIR (Fourier Transform Infra Red spectrometer) measures proportions of different gases in the plume; changes in these proportions give clues to what is happening deep in the magma chamber. Diffusion tubes detect sulphur dioxide at ground level and provide an indication of likely human exposure.
Rainwater is collected at several sites around the volcano. The chemical composition of the water is highly dependent on the amount of gases being emitted from the volcano, and whether the site is underneath the volcano plume. Rain from two kilometres west of the volcano is very acidic with pH as low as 2. The ash is also acidic, and together with water can cause severe corrosion.