Comparison of NO2 measurement using CAPS technology

With European (EN) and United States (US EPA) regulatory approval of instruments utilising Cavity Attenuated Phase Shift (CAPS) technology for direct NO2 measurements, it is important that intercomparisons with older style chemiluminescent analysers are performed to understand differences and ensure valid comparisons of long term data sets are possible and accurate.

This study compares a new analyser based on CAPS technology with a chemiluminescent analyser in a near roadside application with a view to investigating the differences in performance as well as its advantages and disadvantages.

NO2 concentrations were measured in a suburb of Melbourne, with vehicular traffic as a significant emission source. In general, the comparison results show a high degree of correlation between both technologies, as have similar studies.

We look at the data and relate it back to specifications that can be misleading without an understanding of the technologies behind the measurements. Both instruments have a similar response time, but the effect on the data is not the same; one can miss entire episodes while the other will average very short events.

The analyser based on CAPS technology may initially appear more “noisy” at low concentrations, but a closer inspection of the data reveals this is actually an accurate measurement compared to the smoothed chemiluminescent response.

Most ambient stations have used Nitric Oxide (NO) and Ozone (O3) to calibrate chemiluminescent based analysers. However with direct Nitrogen Dioxide (NO2) measurement, such calibration is no longer ideal and an NO2 gas standard seems to be the suitable option; other alternatives are discussed.

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