Portable open path ambient air quality monitor developed with Agency co-operation

In the UK, as in most developed countries, air quality measurement has been made by a combination of networks of fixed monitoring stations and extrapolating data from these relatively few points to useful national pictures by modelling techniques and emission factor estimates.

Solution

The Environment Agency selected the following determinands for widespread monitoring because of their potential impacts on human and environmental health: sulphur dioxide, nitric oxide, nitrogen dioxide, benzene and ozone. The target lower limit of detection should be one tenth of their recommended environmental quality standards, in the order of 5ppb. An initial review by the Environment Agency of commercially available open-path UV interferometric spectrometers, suggested that the more powerful long path length instruments that can reach these low limits of detection are highly sensitive to alignment accuracy and, although suitable to permanently sited applications, would not be practicable for mobile monitoring.

The Siemens Falcon was selected for extended evaluation because its relatively small size, freedom from mains electrical power and relative ease of alignment were conducive to transportable operation. An additional advantage for investigative air quality studies was the ease with which it could be uprated to monitor additional determinands.

Acceptable sensitivity could be obtained from the instrument if the data is integrated over some 15 minutes.

Benefits

The UV Falcon instrument is a conventional, dual ended (ie separate transmitter and receiver) long path FTUV spectrometer system in concept, but it utilises a novel, highly sensitive but also mechanically robust spectrometer design. This spectrometer is based on Wollaston prisms and produces an interferogram in the spatial domain that can be simply detected by a 1024 element NMOS array detector. This arrangement benefits from having no moving parts, which affords great reliability and robustness. Due to an efficient optical design, relatively low intensity light sources are utilised thus affording the low power consumption needed for battery operation as well as ensuring an eye safe beam.

The analyser will measure up to 20 gases simultaneously in real time including the gases of interest to the Environment Agency.

The range of gases that the instrument can measure may be easily expanded, with additional gases being provided as software files that are simply downloaded to the instrument by the user.

Site trials showed the system to be rugged in use, easily set up and capable of operating over an eight-hour day using battery power alone.

The instrument operates in a fully flexible manner, and is able to be (quickly and reliably) deployed in virtually any situation and to achieve easily reproducible levels of performance.

Future applications

A paper by S Sutton of Siemens Environmental Systems and T M Long of the Environment Agency reports that other Agency research is indicating that air pollutants can be generated biogenically, that is by natural processes, as well as anthropogenically. The precursors of ozone, in particular (nitrogen oxides and volatile organic compounds), can be formed in large quantities from land and marine vegetation and soil micro-organisms. Biogenic emissions are the result of complex processes with high diurnal and seasonal variation, and a considerable amount of monitoring data will be required to characterise these temporal emissions and their interactions in order to develop an understanding of biogenic emission factors. At present these data can only be supplied by expensive on-site instrumentation, such as gas chromatrograph/mass spectrometer systems which are impractical for widespread monitoring.

Portable version

A portable-reference version of the Siemens Falcon is stated to be well suited to carry out such work cost-effectively, particularly since it can be reconfigured easily to measure additional gaseous species that become relevant during the investigations. In addition to monitoring anthropogenic air quality at a local level, the modified Siemens Falcon thus has conisiderable potential as a tool for the cost-effective characterisation of biogenic air pollutant emissions and the impact of air pollutants on the natural environment.