Assessing the bias of molybdenum catalytic conversion in the measurement of NO2 in rural air quality networks

Cowan, Nicholas ORCID:; Twigg, Marsailidh M. ORCID:; Leeson, Sarah R. ORCID:; Jones, Matthew R.; Harvey, Duncan ORCID:; Simmons, Ivan; Coyle, Mhairi ORCID:; Kentisbeer, John; Walker, Hannah ORCID:; Braban, Christine F. ORCID: 2024 Assessing the bias of molybdenum catalytic conversion in the measurement of NO2 in rural air quality networks. Atmospheric Environment, 322, 120375. 10, pp.

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The measurement method of NO2 with continuous analysers is specified for EU Ambient Air Quality Directive compliance reporting, which provides a consistent methodology and concurrent NO measurements (85/203/EEC-NO2). While the established method of measurement of NO2, following conversion of NO2 to NO using a molybdenum-conversion process, has known interference uncertainties (due to conversion of other oxidised nitrogen (NOy) chemicals, the consistency and traceability of compliance measurement is important. This study compared three continuous NO2 analyser instruments: a Thermo-NOx molybdenum convertor chemiluminescence analyser (Model 42C, ThermoFisher Scientific Inc., MA, USA), a photolytic chemiluminescence analyser (T200UP, Teledyne Technologies Inc., San Diego, USA) and a Cavity Attenuated Phase Shift (CAPS) analyser (T500U, Teledyne Technologies Inc., CA, USA). The instruments were run for over a year at the Auchencorth Moss long-term peatland monitoring site (Southeast Scotland) which is a low NOx atmosphere away from sources. NOy and NHx chemicals were also measured concurrently. This study concludes that there is a strong artefact in molybdenum catalyst chemiluminescent instruments as a result of unselective catalysis of airborne NOy compounds that causes an overestimate of NO2 measured in the atmosphere. The observed artefact in concentration measurements is likely to be observed at the entire UK scale as almost the entirety of the rural air network relies on molybdenum catalyst instruments. We therefore recommend that molybdenum catalyst instruments should be phased out and replaced in air quality monitoring networks with molecule specific (spectroscopy) instrumentation (equivalent in cost, such as those described in this study) that do not suffer from the same measurement artefacts.

Item Type: Publication - Article
Digital Object Identifier (DOI):
UKCEH and CEH Sections/Science Areas: Atmospheric Chemistry and Effects (Science Area 2017-)
ISSN: 1352-2310
Additional Information. Not used in RCUK Gateway to Research.: Open Access paper - full text available via Official URL link.
Additional Keywords: nitrogen oxide, air pollution, particulate matter, air chemistry, instrument comparison
NORA Subject Terms: Atmospheric Sciences
Date made live: 09 Feb 2024 13:54 +0 (UTC)

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