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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 5 | Copyright
Atmos. Meas. Tech., 11, 2669-2681, 2018
https://doi.org/10.5194/amt-11-2669-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 07 May 2018

Research article | 07 May 2018

Mid-IR spectrometer for mobile, real-time urban NO2 measurements

P. Morten Hundt1, Michael Müller1, Markus Mangold1,a, Béla Tuzson1, Philipp Scheidegger1, Herbert Looser1,2, Christoph Hüglin1, and Lukas Emmenegger1 P. Morten Hundt et al.
  • 1Laboratory for Air Pollution and Environmental Technology, Empa, 8600 Dübendorf, Switzerland
  • 2Institute for Aerosol and Sensor Technology, FHNW, 5210 Windisch, Switzerland
  • apresent address: IRsweep AG, 8093 Zürich, Switzerland

Abstract. Detailed knowledge about the urban NO2 concentration field is a key element for obtaining accurate pollution maps and individual exposure estimates. These are required for improving the understanding of the impact of ambient NO2 on human health and for related air quality measures. However, city-scale NO2 concentration maps with high spatio-temporal resolution are still lacking, mainly due to the difficulty of accurate measurement of NO2 at the required sub-ppb level precision. We contribute to close this gap through the development of a compact instrument based on mid-infrared laser absorption spectroscopy. Leveraging recent advances in infrared laser and detection technology and a novel circular absorption cell, we demonstrate the feasibility and robustness of this technique for demanding mobile applications. A fully autonomous quantum cascade laser absorption spectrometer (QCLAS) has been successfully deployed on a tram, performing long-term and real-time concentration measurements of NO2 in the city of Zurich (Switzerland). For ambient NO2 concentrations, the instrument demonstrated a precision of 0.23ppb at one second time resolution and of 0.03ppb after 200s averaging. Whilst the combined uncertainty estimated for the retrieved spectroscopic values was less than 5%, laboratory intercomparison measurements with standard CLD instruments revealed a systematic NO2 wall loss of about 10% within the laser spectrometer. For the field campaign, the QCLAS has been referenced to a CLD using urban atmospheric air, despite the potential cross sensitivity of CLD to other nitrogen containing compounds. However, this approach allowed a direct comparison and continuous validation of the spectroscopic data to measurements at regulatory air quality monitoring (AQM) stations along the tram-line. The analysis of the recorded high-resolution time series allowed us to gain more detailed insights into the spatio-temporal concentration distribution of NO2 in an urban environment. Furthermore, our results demonstrate that for reliable city-scale concentration maps a larger data set and better spatial coverage is needed, e.g., by deploying more mobile and stationary instruments to account for mainly two shortcomings of the current approach: (i) limited residence time close to sources with large short-term NO2 variations, and (ii) insufficient representativeness of the tram tracks for the complex urban environment.

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NO2 is a pollutant that regularly exceeds its limit values in European cities. We developed a compact, mobile laser spectrometer that measures NO2 concentrations and installed it on a tram in Zurich. Mobile operation resulted in NO2 concentration data with high spatio-temporal resolution. The data were validated against fixed air-quality monitoring sites and provided detailed insights into the spatio-temporal concentration distribution of NO2 in an urban environment.
NO2 is a pollutant that regularly exceeds its limit values in European cities. We developed a...
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