Hazardous air pollutants present a more serious threat to human health and the surrounding environment than traditional ambient air pollutants, such as carbon monoxide, because their impacts tend to be wide-ranging and complex. Also unlike the traditional ambient air pollutants, which are relatively few in number and relatively well-known, there is a multitude of potentially hazardous air pollutants that can cause problems, even in trace amounts. Typically, detection of these hazardous pollutants requires unique, sophisticated, and expensive techniques for each individual pollutant of concern.

NIWA operates a DOAS 2000 open path monitoring system, which offers significant advantages over traditional point monitoring equipment for air quality measurements:

• it provides the ability to continuously measure many pollutants that haven’t been able to be measured before in New Zealand, such as some hazardous pollutants
• multiple pollutants can be measured simultaneously so correlations can be determined for concentrations of hazardous pollutants with more traditional ambient air pollutants.
• monitoring over a wide area (an open path up to 1 km) provides a more representative picture of air quality and typical urban exposures

What is “open path” monitoring?

“Open path” monitoring of air quality works on the principle of differential optical absorption spectroscopy (DOAS). A high-powered light source emits a known spectrum of either ultraviolet (UV) or infrared (IR) light via a telescope across an open path containing air pollutants and the spectrum of the light reflected back is analysed. Different pollutants absorb at different wavelengths of the spectrum and exhibit characteristic absorption “fingerprints”. From the difference between the spectra emitted and that received, the concentration of those pollutants within the path can be determined.

Some pollutants have better absorption fingerprints in the IR (such as carbon monoxide, carbon dioxide, and hydrogen sulphide) whilst others perform better in the UV (such as benzene, formaldehyde, and nitrogen oxides). The path lengths that can be used range from 20m up to 1km, depending on the concentrations of the pollutants in the path and the strength of the received signal.

Which pollutants can we measure?

With the DOAS 2000 open path system, which utilises a xenon lamp as a concentrated UV source, we can measure the following pollutants:

• nitrogen oxides (including NO₂, NO, HNO₂, NO₃)
• sulphur dioxide (SO₂)
• ozone (O₃)
• ammonia (NH₃)
• volatile organic compounds (including benzene, toluene, formaldehyde, xylenes)

What kind of applications can it be used for?

Recently, the open path system has been used for a traffic emissions study in Nelson but it has a wide range of other applications due to its capabilities, portability and ease of operation. Some of these include:

• investigating and quantifying emissions from previously unmeasured sources e.g. landfills, industrial plants, vehicles
• determining actual and typical human exposures to pollution sources
• tracking plumes for dispersion modelling studies
• validating emissions inventory techniques and emission factors
• measuring emission fluxes for modelling and process research

Contact

Lou Reddish [ l.reddish@niwa.co.nz ]