16ENV05 MetNO2: Metrology for nitrogen dioxide

• Forschungsstelle: METAS
• Projektnummer: F-5232.30086

Schlüsselwörter (Englisch)

Nitrogen dioxide, measurement infrastructure, dynamic methods, nitrogen oxides, laser spectroscopy, monitoring networks, nitric acid, urban air quality, atmospheric composition, human health

Kurzbeschreibung (Englisch)

More accurate measurements of nitrogen dioxide (NO2) are needed in order to understand population level exposure, to improve air quality models and emission inventories, to better discern long-term trends in NO2 concentrations and to enforce air quality and vehicle emission legislation. This is essential for the timely evaluation of air pollution mitigation policies, and to improve our understanding of the influence of anthropogenic emissions on the climate system. This project will achieve the necessary accuracy by developing capabilities for the direct measurement of NO2 using newly available selective NO2 techniques and direct calibration with more accurate and stable primary reference standards of NO2.

Nitrogen dioxide (NO2) is the air pollutant considered to have one of the greatest impacts on human health. A major source of NO2 in cities is from fossil fuel combustion in motor vehicles. Diesel powered vehicles emit twenty times more NO2 compared to petrol powered vehicles. In Europe, NO2 in the air we breathe is becoming a massive issue due to large increases in diesel vehicle ownership, resulting from government driven tax incentives, in conjunction with emission standards not delivering the expected reductions under real world driving conditions. This was highlighted by the Volkswagen emissions scandal, recent health reports linking NO2 exposure with adverse health outcomes and the continuing breach of European Union (EU) legislation (2008/50/EC), which sets both annual and hourly limit values (NO2 concentration maxima), in the majority of EU member states. NO2 emissions in Europe are not decreasing fast enough and lower NO2 concentrations will be needed in the future to improve the quality of life for European citizens and to reduce the economic burden of detrimental health outcomes from NO2 exposure. To enable this requires greater confidence in measured trends in emissions and ambient air leading to better evidence based policy and more effective mitigation policies, which are strongly dependent on measurement accuracy. To achieve the necessary improvements in measurement accuracy requires the:

• Direct measurement of NO2, because previous methodologies to measure it indirectly as the difference between NO and total NOx are no longer fit for purpose because of their high uncertainties, e.g., > 10 %.
• Calibration of instruments with highly accurate NO2 calibration gases at atmospherically relevant concentrations, e.g., 10 nmol/mol – 500 nmol/mol. Current NO2 reference standards are not sufficiently accurate or stable enough to fulfil the requirements of the monitoring community.
• Full characterisation and minimisation of impurities in reference standards such as water vapour and reactive nitrogen compounds (e.g., nitric acid), which increase uncertainty and decrease long-term stability.

This is a joint research project carried out in the framework of the European Metrology Programme for Innovation and Research (EMPIR) (see:http://www.euramet.org/research-innovation/empir/). The EMPIR initiative is co-funded by the European Unions's Horizon 2020 research and innovation programme and the participating states. METAS is one of the project partners in the Project.

Projektziele (Englisch)

The overall objective of this project is to deliver a highly accurate SI traceable measurement infrastructure to underpin direct measurements of NO2 concentrations in the atmosphere that meet the Data Quality Objectives (DQOs) established by the World Meteorological Organisation Global Atmospheric Watch (WMO-GAW) Scientific Advisory expert group. The project will increase the accuracy and stability of NO2 reference standards, which is challenging due to the highly reactive nature of NO2. This will require a more comprehensive characterisation of the impurities formed unintentionally during the preparation and development of new methods that are designed to suppress the formation of impurities. New selective spectroscopic NO2 measurement techniques will be developed, characterised and validated with reference to the standard chemiluminescence method.
The project has been structured into the following objectives:

1. To develop high concentration traceable static reference standards for NO2 (1 μmol/mol - 10 μmol/mol) with a target uncertainty of ≤ 0.5 % and stability of ≥ 2 years. To achieve these challengingly low uncertainties will require the full characterisation of critical impurities and the development of new methods to minimise their formation during static standard preparation.
2. To develop high accuracy traceable dynamic reference standards for low amount fractions of NO2 (10 nmol/mol – 500 nmol/mol) with a target uncertainty of ≤ 1 %. To achieve these challengingly low uncertainties will require the full characterisation of critical impurities and the development of new methods to minimise their formation during dynamic standard preparation.
3. To develop analytical methods to quantify the main impurities formed unintentionally during the preparation of static and dynamic NO2 reference standards. To validate selective spectroscopic methods for directly measuring NO2 and compare them with the standard reference method as described in EN 14211:2012, using field trials at an atmospheric simulation chamber.
4. To engage with stakeholders to ensure the uptake of the reference standards, calibration methods and devices developed in this project by standards development organisations, atmospheric monitoring networks, speciality gas companies, instrument manufacturers and other measurement infrastructures.

Abstract (Englisch)

METAS adapted and characterized their dynamic generation systems to produce NO₂ reference gas mixtures at low amount fractions (10 – 500 nmol/mol). By modifying the protocol for the generation and extending the calibration period to 6 days instead of 3, the NO₂ reference gas mixtures were able to be produced with a relative expanded uncertainty (k = 2) of 0.9 % (1-step dilution) and 1.0 % (2-step dilution). The NO₂ reference gas mixtures generated using ReGaS1 had a relative expanded uncertainty (k = 2) between 2.5 % and 2.7 % for 1- and 2-step dilutions.

In addition, the dynamic generation systems were used to calibrate and generate HNO₃ reference gas mixtures that were used in activities A2.2.2, A3.3.1, and A3.3.3. For the first time, stable and SI-traceable HNO₃ reference gas mixtures were generated, although with associated uncertainties (> 30%) above the target uncertainty (< 5%). The main contribution to the uncertainty (99.9 %) was the purity of the HNO₃ permeation tubes.

5 different permeators (3 NO₂ permeators and 3 HNO₃ permeators) were tested for temporal stability and the effects of temperature, pressure, and preconditioning.

• Temporal stability: For NO₂ the permeation rate remained stable in the first 20 months, but increased in month 25, by > 3%, and by up to 7% by month 30. HNO₃ permeators were evaluated over 24 months, with no clear trend.
• Temperature: An increase of 1 ^oC resulted in a 4% and 8% increase in permeation rates for NO₂ and HNO₃, respectively.
• Pressure: Permeation rates decreased linearly with increasing pressure, however, pressure variations during the calibration of a permeator does not affect the permeation rate. For pressures varying between 800 mbar and 2600 mbar, the permeation rate remained within the uncertainty.
• Pre-conditioning: The time needed for the membrane to reach stability was 3000 minutes for NO₂ and 1200 minutes for HNO₃, which has a thicker membrane. This period was reduced by keeping the permeator at calibration conditions (temperature and pressure) for 3 – 5 days before placing the permeator in MSB.

Further optimization of the FTIR-spectrometer analytical system is needed in order to detect and measure HNO₃ impurities in NO₂ permeators. HNO₃ coming out of the permeator were lower than the detection limit of the FTIR.

A field-based side-by-side comparison organized by Deutsche Wetter Dienst in FZ-Jülich included calibrated NO₂ and HNO₃ with the portable generator ReGaS1 to calibrate the measurement instruments. Results from the comparison were included in deliverable D7: "Report on the field-based side-by-side comparison of selective NO₂ instrumentation", which will be submitted as a manuscript by FZ-Jülich (April 2021), and co-authored by METAS.

Umsetzung und Anwendungen (Englisch)

Results from the permeator stability tests were used to write guidelines on the use of dynamic gas mixtures for the calibration of field instrumentation, which are part of deliverable D5 (A2.3.5 – A2.3.6): "Best practice guide for the use of static and dynamic gas standards at monitoring stations including the evaluation of measurement uncertainty". These guidelines are expected to be overtaken by air quality monitoring sites in order to improve their calibration procedure. Moreover, the improved portable generator developed by METAS (ReGaS1), which were adapted to generate SI-traceable and accurate NO2 reference gas mixtures, may become a good alternative as working standard to calibrate new selective NO2 analysers at air quality monitoring networks. During the project, several requests from partners (e.g. Empa, DWD) have been sent to METAS regarding the use/the purchase of ReGaS1. In addition, the results of the comparison (A2.1.4) and the CCQM-K74.2018 intercomparison will enable METAS to become a reference for NO2 calibration services at low amount fractions (10 - 500 nmol/mol). 

Publikationen / Ergebnisse (Englisch)

Within the framework of this project, it is planned to submit the following publications, with METAS as co-author, in the months following the end of the project:
1. Manuscript on different calibration methods for NO2 analyzers (A2.3.1) (lead author: Empa)
2. Manuscript on side-by-side comparison with the SAPHIR chamber (A3.3.3) (lead author: FZ-Jülich)