Measurement of NO x and NO y with a thermal dissociation cavity ring-down spectrometer (TD-CRDS): instrument characterisation and first deployment

<p>We present a newly constructed, two-channel thermal dissociation cavity ring-down spectrometer (TD-CRDS) for the measurement of <span class="inline-formula">NO_<i>x</i></span> (<span class="inline-formula">NO+NO₂</span>), <span class="inline-formula">NO_<i>y</i></span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mi>x</mi></msub><mo>+</mo><msub><mi mathvariant="normal">HNO</mi><mn mathvariant="normal">3</mn></msub><mo>+</mo><msub><mi mathvariant="normal">RO</mi><mn mathvariant="normal">2</mn></msub><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub><mo>+</mo><mn mathvariant="normal">2</mn><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">5</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="161pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="11678d3d4ff61e4be37d70b8eec0e724"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-13-5739-2020-ie00001.svg" width="161pt" height="13pt" src="amt-13-5739-2020-ie00001.png"/></svg:svg></span></span> etc.), <span class="inline-formula">NO_<i>z</i></span> (<span class="inline-formula">NO_<i>y</i>−NO_<i>x</i></span>) and particulate nitrate (pNit). <span class="inline-formula">NO_<i>y</i></span>-containing trace gases are detected as <span class="inline-formula">NO₂</span> by the CRDS at 405&thinsp;nm following sampling through inlets at ambient temperature (<span class="inline-formula">NO_<i>x</i></span>) or at 850&thinsp;<span class="inline-formula">^∘</span>C (<span class="inline-formula">NO_<i>y</i></span>). In both cases, <span class="inline-formula">O₃</span> was added to the air sample directly upstream of the cavities to convert NO (either ambient or formed in the 850&thinsp;<span class="inline-formula">^∘</span>C oven) to <span class="inline-formula">NO₂</span>. An activated carbon denuder was used to remove gas-phase components of <span class="inline-formula">NO_<i>y</i></span> when sampling pNit. Detection limits, defined as the 2<span class="inline-formula"><i>σ</i></span> precision for 1&thinsp;min averaging, are 40&thinsp;pptv for both <span class="inline-formula">NO_<i>x</i></span> and <span class="inline-formula">NO_<i>y</i></span>. The total measurement uncertainties (at 50&thinsp;% relative humidity, RH) in the <span class="inline-formula">NO_<i>x</i></span> and <span class="inline-formula">NO_<i>y</i></span> channels are <span class="inline-formula">11 <i>%</i>+10</span>&thinsp;pptv and <span class="inline-formula">16 <i>%</i>+14</span>&thinsp;pptv for <span class="inline-formula">NO_<i>z</i></span> respectively. Thermograms of various trace gases of the <span class="inline-formula">NO_<i>z</i></span> family confirm stoichiometric conversion to <span class="inline-formula">NO₂</span> (and/or NO) at the oven temperature and rule out significant interferences from <span class="inline-formula">NH₃</span> detection (<span class="inline-formula">&lt;2</span>&thinsp;%) or radical recombination reactions under ambient conditions. While fulfilling the requirement of high particle transmission (<span class="inline-formula">&gt;80</span>&thinsp;% between 30 and 400&thinsp;nm) and essentially complete removal of reactive nitrogen under dry conditions (<span class="inline-formula">&gt;99</span>&thinsp;%), the denuder suffered from <span class="inline-formula">NO_<i>x</i></span> breakthrough and memory effects (i.e. release of stored <span class="inline-formula">NO_<i>y</i></span>) under humid conditions, which may potentially bias measurements of particle nitrate.</p> <p>Summertime <span class="inline-formula">NO_<i>x</i></span> measurements obtained from a ship sailing through the Red Sea, Indian Ocean and Arabian Gulf (<span class="inline-formula">NO_<i>x</i></span> levels from <span class="inline-formula">&lt;20</span>&thinsp;pptv to 25&thinsp;ppbv) were in excellent agreement with those taken by a chemiluminescence detector of NO and <span class="inline-formula">NO₂</span>. A data set obtained locally under vastly different conditions (urban location in winter) revealed large diel variations in the <span class="inline-formula">NO_<i>z</i></span> to <span class="inline-formula">NO_<i>y</i></span> ratio which could be attributed to the impact of local emissions by road traffic.</p>

Abstract licence: CC BY

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