Long-Path Monitoring of Atmospheric Pollution

Gas monitoring has traditionally been conducted in the form of 'point monitoring', i.e. as a measurement at a single location in space. This is very suitable for intrinsically localized gas problems, such as measuring individual worker exposures or concentrations within pipes in a process plant. However, point monitoring is inadequate for measuring poorly mixed gases over substantial areas, such as fugitive emissions from industrial plant or leakages from chemical storage tanks, since it will be a matter of chance whether a particular plume of gas happens to intersect the measuring point. Such a large-area monitoring of gaseous pollutants is particularly suited to the new technique of long-path optical measurement. The technique employs a beam of, usually, infrared or ultraviolet radiation as the actual gas sensor, which is sent on a path that can be from a few metres to a few kilometres in length. The wavelength of the radiation is chosen such that the beam is absorbed by the particular gas to be measured and not by any other gas in the atmosphere. By measuring the reduction in the intensity of the beam due to its passage through the atmosphere the -average concentration of the target gas can be continuously monitored. The main technical advantages of such long-path monitors over traditional point sensors are: