Last reviewed 5 November 2013

Measuring air pollution emitted from discrete or so-called point sources is relatively easy; the technologies and methods to do this are mature and widely applied. Characterising diffuse or so-called fugitive emissions, however, is a lot harder. Rick Gould reports.

Characterising fugitive emissions is especially difficult when it comes to identifying the sources of dust emissions, particularly for respirable particulate matter, or PM10. While a combination of continuous reference monitoring and total-dust measurements, coupled with meteorological data and complex modelling, can identify sources of fugitive emissions, using such techniques is expensive and resource-intensive. At the same time, while passive samplers are comparatively inexpensive, they only provide a total-dust measurement over a given period of time, and can then provide only a little information on the origin of dust, even when a large number of passive samplers are used around a suspected site.

However, a team from the Environment Agency and Lancaster University’s Environment Centre (LEC) has developed and applied a new type of simple, passive monitor that provides valuable information on the direction of dust emissions, as well as other types of pollutant. Known as Directional Passive Air Samplers (DPAS), the Environment Agency and the LEC have, among other things, now used this patented system to identify the sources of fugitive dust emissions from a metal reprocessing site in an area of poor air quality. Furthermore, after discovering which specific activities were responsible for the largest emissions, using DPAS showed that subsequent controls on the processes responsible for the dust emissions resulted in a reduction of environmental impacts of between 40% and 80%, depending on the activity targeted. So this article describes the principle behind DPAS, how it compares to more conventional types of monitoring, and how it could change both the monitoring and control of air pollution.

A new fish in the sea of monitoring methods

People seeing the DPAS system for the first time typically comment that it looks like a fish or shark. The system consists of two main parts: the lower section is circular, comprising 12 equally-spaced segments of 30° each, and is fixed to a mount; the upper section looks like a streamlined sea creature with a prominent fin, has openings at either end, and is free to rotate after it has been fitted to the lower, fixed, circular section. In simple terms, the sampler is a passive device that depends on the wind passing through it; any pollutants passing through the sampler are then deposited into the segments as they pass through the body of the device. Each segment is filled with an appropriate medium for capturing the target pollutant, while the large fin ensures that the “mouth” of the sampler is always in the direction of the wind. The system has been tested successfully with both gaseous pollutants and dust.

For example, field tests reported in 2010 (“Field Testing of a New Flow-through Directional Passive Air Sampler Applied to Monitoring Ambient Nitrogen Dioxide”, Journal of Environmental Monitoring) showed that the sampler produced a similar pattern of results to a continuous reference analyser, when sampling for nitrogen dioxide. The continuous analyser may have been more accurate and precise, but on the other hand, the directional sampler is much simpler, far easier to operate and considerably less expensive. In simple terms, on a like for like basis, the sampler cost around 5% of the cost of the continuous analyser; therefore the research showed that the sampler had great potential as a complementary technique to conventional and expensive reference monitoring.

Then in 2011, the team reported on wind-tunnel tests (“Design and Laboratory Testing of a New Flow-through Directional Passive Air Sampler for Ambient Particulate Matter”, Journal of Environmental Monitoring) that showed that the sampler also had significant potential as a means of measuring fugitive dust emissions. The researchers reported that the air passing through the sampler at least halved its speed, thereby encouraging dust to settle from the air stream. At the time, the team optimistically wrote that “it may be possible to relate collected amounts of particulate matter simply to ambient concentrations and wind velocities”. So did their optimising prove founded? A subsequent and more recent investigation at a steel works showed that it did.

Fishing for fugitive sources at a steel works

Scunthorpe is home to one of Europe’s largest steel works. It also has significant problems with air quality, and often breaches the EU’s limit value for ambient PM10. While it has been intuitively clear that the steel works is likely to be a contributor to ambient levels of PM10, identifying the exact sources has been much harder. While using conventional, continuous reference monitors, meteorological data and modelling for identifying sources of emissions is cumbersome and resource-intensive to operate, not to mention expensive, DPAS, on the other hand, is simple and relatively inexpensive.

So the team set up a number of DPAS around a metal reprocessing site, which was a suspected source of the fugitive emissions. The team divided the study area into six sectors, and placed samplers along a plane either side of a suspected source. This meant that each pair of samplers would capture emissions both upwind and downwind of the site, and that the net dust deposited on a pair of samplers could be used to identify the direction of the origin of the fugitive dust.

The sectors were chosen to target specific activities at the site, such as road transport of materials, hoppers, processing belts and drop-bailing. Using pairs of samplers to determine the net deposition of dust allowed the team to discount road transport of slag as a significant source. On the other hand, the other processes were significant to varying degrees, so the operator of the site — which worked closely with the Environment Agency and the LEC during this research — was able to target further controls on the processes that were strongly implicated in significant releases of dust. When the operator had applied a variety of controls, a further investigation using DPAS showed that, for each process targeted, the deposition of dust downwind was reduced by at least 40% for one of the processes, and typically between 70% and 80% for four other activities.

Currently, the team is investigating how the samplers can be used to sample and measure releases of ammonia and radionuclides. Ammonia, in particular, is released in significant quantities from agricultural activities; at the same time, the releases of ammonia are largely diffuse, making the exact sources harder to identify, and hence control. However, as with the investigation at the steel works, using DPAS to measure ammonia is showing promise.