The evaluation of polluted gas leaks from valves, pumps and piping of industrial plants is a problem difficult to solve for health & safety and environment responsibles.
The fugitive emissions from big chemical and petrochemical areas can be really dangerous, in case of failures and relevant incidents; their diffusion in atmosphere can harm workers and people living nearby.
Fugitive emissions are the main emission factor and represent the most frequent cause of harassment for people living nearby.
Besides incidental factors often permanent factors occur such as ventilation of tanks and emissions from vents, flares, sewages or small stacks. Environmental regulations and prescriptions to industries at risk of major accident require the adoption of more and more efficient monitoring systems. An ideal monitoring system should be configured as a barrier placed next to the industrial perimeter (fence-line), so that it might include all the plant at risk of accident or leaks, and be capable of detecting a large variety of gas compounds.
At present the technique closer to the ideal one is the area monitoring system (open-path monitoring system) produced by OPSIS A.B.
SARTEC has realized, in Italy two important applications for at risk industrial area monitoring (Termoli and Marghera), based on DOAS OPSIS. In these applications, it is possible to intercept and measure immissions caused by possible gas leaks from industrial plant by using some optical paths placed in strategic positions. The different optical paths can be complete with a single spectrophotometer, but it is also possible to connect two or more paths two a single analyzer to keep the monitoring system costing down. The open-path DOAS OPSIS systems can be integrated with some sensors and spot analyzers of gas and organic compounds (such as VOS, IPA etc) to increase the efficacy of the monitoring. By combining the chemical measures with wind velocity and direction, using specialized software, the position and intensity of pollutant sources can be determined.
The open-path DOAS technology
The analytical techniques that can be used in open-path monitoring systems are:

Each technique is characterized by specific advantages and disadvantages; DOAS systems are those that have the best compromise between quality and reliability of measures from one side, and cost and construction complexity to the other side. The DOAS spectrophotometric systems produced by OPSIS can operate both in the UV range (UV DOAS) and in the IR range (IR DOAS); the UV DOAS method is the most used because it allows to monitor both ubiquitous pollutants (SO2, NOx, O3, NH3) and organic compounds (aromatic hydrocarbons, aldehydes, phenol, sulphurate compounds), with low detection limit of few ppb.
The measuring principle of the OPSIS systems is based on the Lambert-Beer equation that establish the relation between the quantity of absorbed light and the number of molecules of gas present on the optical path. The open-path UV-DOAS monitoring system is an excellent instrument  to evaluate air quality and to signal possible gas leaks from industrial sites. By using this technique it is possible to realize a strategic network of measure paths near the industrial perimeter, to quickly detect and report, every possible abnormal polluting emission. The main advantages of the technique are:

By using correctly the open-path DOAS and the appropriate software for monitoring and meteorological data elaboration it is possible to define the position of the sources and quantify the related emissions. Following this it is possible to plan adequate measures to reduce fugitive emissions, with positive consequences of environment and on production efficiency. Each ton of VOC dispersed from a petrochemical plant with leaks is a waste of products and raw materials; the leak’s reduction can justify the investment of a DOAS open-path system.