Interference from alkenes in chemiluminescent NO_<i>x</i> measurements

• Main Authors: M. S. Alam, L. R. Crilley, J. D. Lee, L. J. Kramer, C. Pfrang, M. Vázquez-Moreno, M. Ródenas, A. Muñoz, W. J. Bloss
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-11-01
• Series: Atmospheric Measurement Techniques
• Online Access: https://amt.copernicus.org/articles/13/5977/2020/amt-13-5977-2020.pdf

<p>Nitrogen oxides (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mi>x</mi></msub><mo>=</mo><mi mathvariant="normal">NO</mi><mo>+</mo><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="80pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="003c088dc12c1422329b95deaac4c5f8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-13-5977-2020-ie00001.svg" width="80pt" height="13pt" src="amt-13-5977-2020-ie00001.png"/></svg:svg></span></span>) are critical intermediates in atmospheric chemistry and air pollution. NO<span class="inline-formula">_<i>x</i></span> levels control the cycling and hence abundance of the primary atmospheric oxidants OH and NO<span class="inline-formula">₃</span> and regulate the ozone production which results from the degradation of volatile organic compounds (VOCs) in the presence of sunlight. They are also atmospheric pollutants, and NO<span class="inline-formula">₂</span> is commonly included in air quality objectives and regulations. NO<span class="inline-formula">_<i>x</i></span> levels also affect the production of the nitrate component of secondary aerosol particles and other pollutants, such as the lachrymator peroxyacetyl nitrate (PAN). The accurate measurement of NO and NO<span class="inline-formula">₂</span> is therefore crucial for air quality monitoring and understanding atmospheric composition. The most commonly used approach for the measurement of NO is the chemiluminescent detection of electronically excited NO<span class="inline-formula">₂</span> (NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">2</mn><mo>∗</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="7pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="2dcf394ffdb05e8cf5cb05cc5106cd5b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-13-5977-2020-ie00002.svg" width="7pt" height="14pt" src="amt-13-5977-2020-ie00002.png"/></svg:svg></span></span>) formed from the NO&thinsp;<span class="inline-formula">+</span>&thinsp;O<span class="inline-formula">₃</span> reaction within the NO<span class="inline-formula">_<i>x</i></span> analyser. Alkenes, ubiquitous in the atmosphere from biogenic and anthropogenic sources, also react with ozone to produce chemiluminescence and thus may contribute to the measured NO<span class="inline-formula">_<i>x</i></span> signal. Their ozonolysis reaction may also be sufficiently rapid that their abundance in conventional instrument background cycles, which also utilises the reaction with ozone, differs from that in the measurement cycle such that the background subtraction is incomplete, and an interference effect results. This interference has been noted previously, and indeed, the effect has been used to measure both alkenes and ozone in the atmosphere. Here we report the results of a systematic investigation of the response of a selection of commercial NO<span class="inline-formula">_<i>x</i></span> monitors to a series of alkenes. These NO<span class="inline-formula">_<i>x</i></span> monitors range from systems used for routine air quality monitoring to atmospheric research instrumentation. The species-investigated range was from short-chain alkenes, such as ethene, to the biogenic monoterpenes. Experiments were performed in the European PHOtoREactor (EUPHORE) to ensure common calibration and samples for the monitors and to unequivocally confirm the alkene levels present (via Fourier transform infrared spectroscopy – FTIR). The instrument interference responses ranged from negligible levels up to 11&thinsp;%, depending upon the alkene present and conditions used (e.g. the presence of co-reactants and differing humidity). Such interferences may be of substantial importance for the interpretation of ambient NO<span class="inline-formula">_<i>x</i></span> data, particularly for high VOC, low NO<span class="inline-formula">_<i>x</i></span> environments such as forests or indoor environments where alkene abundance from personal care and cleaning products may be significant.</p>