Technical Note: Effect of varying the <i>λ</i> = 185 and 254 nm photon flux ratio on radical generation in oxidation flow reactors
oleh: J. P. Rowe, J. P. Rowe, A. T. Lambe, W. H. Brune
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2020-11-01 |
Deskripsi
<p>Oxidation flow reactors (OFRs) complement environmental smog chambers as a portable, low-cost technique for exposing atmospheric compounds to oxidants such as ozone (<span class="inline-formula">O<sub>3</sub></span>), nitrate (<span class="inline-formula">NO<sub>3</sub></span>) radicals, and hydroxyl (OH) radicals. OH is most commonly generated in OFRs via photolysis of externally added <span class="inline-formula">O<sub>3</sub></span> at <span class="inline-formula"><i>λ</i>=254</span> <span class="inline-formula">nm</span> (OFR254) or combined photolysis of <span class="inline-formula">O<sub>2</sub></span> and <span class="inline-formula">H<sub>2</sub>O</span> at <span class="inline-formula"><i>λ</i>=185</span> <span class="inline-formula">nm</span> plus photolysis of <span class="inline-formula">O<sub>3</sub></span> at <span class="inline-formula"><i>λ</i>=254</span> <span class="inline-formula">nm</span> (OFR185) using low-pressure mercury (Hg) lamps. Whereas OFR254 radical generation is influenced by [<span class="inline-formula">O<sub>3</sub></span>], [<span class="inline-formula">H<sub>2</sub>O</span>], and photon flux at <span class="inline-formula"><i>λ</i>=254</span> <span class="inline-formula">nm</span> (<span class="inline-formula"><i>I</i><sub>254</sub></span>), OFR185 radical generation is influenced by [<span class="inline-formula">O<sub>2</sub></span>], [<span class="inline-formula">H<sub>2</sub>O</span>], <span class="inline-formula"><i>I</i><sub>185</sub></span>, and <span class="inline-formula"><i>I</i><sub>254</sub></span>. Because the ratio of photon fluxes, <span class="inline-formula"><i>I</i><sub>185</sub>:<i>I</i><sub>254</sub></span>, is OFR-specific, OFR185 performance varies between different systems even when constant [<span class="inline-formula">H<sub>2</sub>O</span>] and <span class="inline-formula"><i>I</i><sub>254</sub></span> are maintained. Thus, calibrations and models developed for one OFR185 system may not be applicable to another. To investigate these issues, we conducted a series of experiments in which <span class="inline-formula"><i>I</i><sub>185</sub>:<i>I</i><sub>254</sub></span> emitted by Hg lamps installed in an OFR was systematically varied by fusing multiple segments of lamp quartz together that either transmitted or blocked <span class="inline-formula"><i>λ</i>=185</span> <span class="inline-formula">nm</span> radiation. Integrated OH exposure (OH<span class="inline-formula"><sub>exp</sub></span>) values achieved for each lamp type were obtained using the tracer decay method as a function of UV intensity, humidity, residence time, and external OH reactivity (OHR<span class="inline-formula"><sub>ext</sub></span>). Following previous related studies, a photochemical box model was used to develop a generalized OH<span class="inline-formula"><sub>exp</sub></span> estimation equation as a function of [<span class="inline-formula">H<sub>2</sub>O</span>], [<span class="inline-formula">O<sub>3</sub></span>], and OHR<span class="inline-formula"><sub>ext</sub></span> that is applicable for <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M37" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>I</mi><mn mathvariant="normal">185</mn></msub><mo>:</mo><msub><mi>I</mi><mn mathvariant="normal">254</mn></msub><mo>≈</mo><mn mathvariant="normal">0.001</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="84pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="527352cf59ee64effbab94ffad7ea3f7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-13417-2020-ie00001.svg" width="84pt" height="12pt" src="acp-20-13417-2020-ie00001.png"/></svg:svg></span></span> to 0.1.</p>