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<p>Hydroxyl (OH) and hydroperoxy (<span class="inline-formula">HO₂</span>) radicals are central to the understanding of atmospheric chemistry. Owing to their short lifetimes, these species are frequently used to test the accuracy of model predictions and their underlying chemical mechanisms. In forested environments, laser-induced fluorescence–fluorescence assay by gas expansion (LIF–FAGE) measurements of OH have often shown substantial disagreement with model predictions, suggesting the presence of unknown OH sources in such environments. However, it is also possible that the measurements have been affected by instrumental artefacts, due to the presence of interfering species that cannot be discriminated using the traditional method of obtaining background signals via modulation of the laser excitation wavelength (“<span class="inline-formula">OH</span><span class="inline-formula">_wave</span>”). The interference hypothesis can be tested by using an alternative method to determine the OH background signal, via the addition of a chemical scavenger prior to sampling of ambient air (“<span class="inline-formula">OH</span><span class="inline-formula">_chem</span>”). In this work, the Leeds FAGE instrument was modified to include such a system to facilitate measurements of <span class="inline-formula">OH</span><span class="inline-formula">_chem</span>, in which propane was used to selectively remove OH from ambient air using an inlet pre-injector (IPI). The IPI system was characterised in detail, and it was found that the system did not reduce the instrument sensitivity towards OH (&lt;&thinsp;5&thinsp;% difference to conventional sampling) and was able to efficiently scavenge external OH (&gt;&thinsp;99&thinsp;%) without the removal of OH formed inside the fluorescence cell (&lt;&thinsp;5&thinsp;%). Tests of the photolytic interference from ozone in the presence of water vapour revealed a small but potentially significant interference, equivalent to an OH concentration of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mn mathvariant="normal">4</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mn mathvariant="normal">5</mn></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="8a6b5d861df43b007e8b4e105b08f10a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-13-3119-2020-ie00001.svg" width="46pt" height="13pt" src="amt-13-3119-2020-ie00001.png"/></svg:svg></span></span>&thinsp;molec.&thinsp;cm<span class="inline-formula">⁻³</span> under typical atmospheric conditions of [<span class="inline-formula">O₃</span>] <span class="inline-formula">=5</span>0&thinsp;ppbv and [<span class="inline-formula">H₂O</span>] <span class="inline-formula">=1</span>&thinsp;%. Laboratory experiments to investigate potential interferences from products of isoprene ozonolysis did result in interference signals, but these were negligible when extrapolated down to ambient ozone and isoprene levels. The interference from <span class="inline-formula">NO₃</span> radicals was also tested but was found to be insignificant in our system. The Leeds IPI module was deployed during three separate field intensives that took place in summer at a coastal site in the UK and both in summer and winter in the megacity of Beijing, China, allowing for investigations of ambient OH interferences under a wide range of chemical and meteorological conditions. Comparisons of ambient <span class="inline-formula">OH</span><span class="inline-formula">_chem</span> measurements to the traditional <span class="inline-formula">OH</span><span class="inline-formula">_wave</span> method showed excellent agreement, with <span class="inline-formula">OH</span><span class="inline-formula">_wave</span> vs <span class="inline-formula">OH</span><span class="inline-formula">_chem</span> slopes of 1.05–1.16 and identical behaviour on a diel basis, consistent with laboratory interference tests. The difference between <span class="inline-formula">OH</span><span class="inline-formula">_wave</span> and <span class="inline-formula">OH</span><span class="inline-formula">_chem</span> (“<span class="inline-formula">OH</span><span class="inline-formula">_int</span>”) was found to scale non-linearly with <span class="inline-formula">OH</span><span class="inline-formula">_chem</span>, resulting in an upper limit interference of (<span class="inline-formula">5.0±1.4</span>) <span class="inline-formula">×10⁶</span>&thinsp;molec.&thinsp;cm<span class="inline-formula">⁻³</span> at the very highest <span class="inline-formula">OH</span><span class="inline-formula">_chem</span> concentrations measured (<span class="inline-formula">23×10⁶</span>&thinsp;molec.&thinsp;cm<span class="inline-formula">⁻³</span>), accounting for <span class="inline-formula">∼14</span>&thinsp;%–21&thinsp;% of the total <span class="inline-formula">OH</span><span class="inline-formula">_wave</span> signal.</p>