Diode laser spectrometer for NO2 quantification: Absolute laser spectroscopic and direct NO2 concentration measurements for atmospheric monitoring within the EMPIR project MetNO2

Nitrogen dioxide (NO₂) is an atmospheric pollutant that needs to be accurately measured for air quality control. The standard reference method (SRM, as laid down in EN 14211:2012 [1]) for NO₂ emissions is based on chemiluminescence, where NO₂ is only indirectly measured. Due to the fact that NO₂ is the only air pollutant that is indirectly measured and because of some shortcomings in SRM-based measurements, there are attempts to develop methods also for direct NO₂ quantifications that are accurate and reliable [2, 3]. Laser spectroscopic techniques such as direct tunable diode laser absorption spectroscopy (dTDLAS [4]), which has been demonstrated for direct and absolute measurements of a variety of atmospheric molecules (H₂O, NH₃, CO₂ and CO) [4-7], provide excellent options for direct atmospheric NO₂ measurements. Based on the experience with other species, a test method for direct NO₂ measurements based on dTDLAS was found to be a promising alternative as compared to the SRM.

We present a measurement method based on dTDLAS for direct and absolute NO₂ concentration measurements compatible to [8] and complying with metrological principles of SI-traceability. The approach was realized by two independent, newly developed mid infrared (ICL, QCL) laser spectrometers (one aiming at compact and field-deployable system integration). Results of directly measured NO₂ concentrations are presented, addressing traceability to the SI, to demonstrate the capability of the measurement method. Guide to the expression of uncertainty in measurement (GUM) compliant uncertainty budgets are reported to show the current data quality. A first principles laser spectroscopic system which does not need calibration by gaseous reference material and which is validated for concentration results that are directly traceable to the SI shall be referred to as an “optical gas standard”, (OGS). We present validations in the concentration range 100 µmol/mol to 1000 µmol/mol. A discussion on current limitations and potentials for an upscaling of these new NO₂ systems to be operated as OGSs towards ambient air concentrations will be part of this presentation, too.

This work has received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. PTB is member of the European Metrology Network for Climate and Ocean Observation (https://www.euramet.org/european-metrology-networks/climate-and-ocean-observation/).

References

[1] European Standard: “Ambient air - Standard method for the measurement of the concentration of nitrogen dioxide and nitrogen monoxide by chemiluminescence”, EN 14211:2012

[2] EMPIR project 16ENV02, “Metrology for Nitrogen Dioxide (MetNO2)”, http://em-pir.npl.co.uk/metno2/

[3] P. Morten Hundt, Michael Müller, Markus Mangold, Béla Tuzson, Philipp Scheidegger, Herbert Looser, Christoph Hüglin, Lukas Emmenegger, Atmos. Meas. Tech., 11, 2669–2681 (2018)

[4] J. A. Nwaboh, Z. Qu, O. Werhahn, V. Ebert, Appl. Opt. 56, E84-E93 (2017)

[5] B. Buchholz, N. Böse, V. Ebert, Appl. Phys. B 116, 883-899, (2014)

[6] J.A. Nwaboh, J. Hald, J.K. Lyngsø, J.C. Petersen, O. Werhahn, Appl. Phys. B 110:187–194 (2013)

[7] A. Pogány, O. Werhahn, V. Ebert, Imaging and Applied Optics 2016, DOI: 10.1364/3D.2016.JT3A.15

[8] Werhahn O, Petersen J C (eds.) 2010 TILSAM technical protocol V1_2010-09-29 (http://www.euramet.org/fileadmin/docs/projects/934_METCHEM_Interim_Report.pdf)