Find in Library

<p>The photooxidation of pinonaldehyde, one product of the <span class="inline-formula"><i>α</i></span>-pinene degradation, was investigated in the atmospheric simulation chamber SAPHIR under natural sunlight at low NO concentrations (<span class="inline-formula">&lt;0.2</span>&thinsp;ppbv) with and without an added hydroxyl radical (<span class="inline-formula">OH</span>) scavenger. With a scavenger, pinonaldehyde was exclusively removed by photolysis, whereas without a scavenger, the degradation was dominated by reaction with OH. In both cases, the observed rate of pinonaldehyde consumption was faster than predicted by an explicit chemical model, the Master Chemical Mechanism (MCM, version 3.3.1). In the case with an OH scavenger, the observed photolytic decay can be reproduced by the model if an experimentally determined photolysis frequency is used instead of the parameterization in the MCM. A good fit is obtained when the photolysis frequency is calculated from the measured solar actinic flux spectrum, absorption cross sections published by <span class="cit" id="xref_text.1"><a href="#bib1.bibx30">Hallquist et al.</a> (<a href="#bib1.bibx30">1997</a>)</span>, and an effective quantum yield of 0.9. The resulting photolysis frequency is 3.5 times faster than the parameterization in the MCM. When pinonaldehyde is mainly removed by reaction with OH, the observed OH and hydroperoxy radical (<span class="inline-formula">HO₂</span>) concentrations are underestimated in the model by a factor of 2. Using measured <span class="inline-formula">HO₂</span> as a model constraint brings modeled and measured OH concentrations into agreement. This suggests that the chemical mechanism includes all relevant OH-producing reactions but is missing a source for <span class="inline-formula">HO₂</span>. The missing <span class="inline-formula">HO₂</span> source strength of (0.8&thinsp;to&thinsp;1.5)&thinsp;<span class="inline-formula">ppbv h⁻¹</span> is similar to the rate of the pinonaldehyde consumption of up to 2.5&thinsp;<span class="inline-formula">ppbv h⁻¹</span>. When the model is constrained by <span class="inline-formula">HO₂</span> concentrations and the experimentally derived photolysis frequency, the pinonaldehyde decay is well represented. The photolysis of pinonaldehyde yields 0.18&thinsp;<span class="inline-formula">±</span>&thinsp;0.20 formaldehyde molecules at NO concentrations of less than 200&thinsp;<span class="inline-formula">pptv</span>, but no significant acetone formation is observed. When pinonaldehyde is also oxidized by OH under low NO conditions (maximum 80&thinsp;<span class="inline-formula">pptv</span>), yields of acetone and formaldehyde increase over the course of the experiment from 0.2 to 0.3 and from 0.15 to 0.45, respectively. <span class="cit" id="xref_text.2"><a href="#bib1.bibx18">Fantechi et al.</a> (<a href="#bib1.bibx18">2002</a>)</span> proposed a degradation mechanism based on quantum-chemical calculations, which is considerably more complex than the MCM scheme and contains additional reaction pathways and products. Implementing these modifications results in a closure of the model–measurement discrepancy for the products acetone and formaldehyde, when pinonaldehyde is degraded only by photolysis. In contrast, the underprediction of formed acetone and formaldehyde is worsened compared to model results by the MCM, when pinonaldehyde is mainly degraded in the reaction with <span class="inline-formula">OH</span>. This shows that the current mechanisms lack acetone and formaldehyde sources for low NO conditions like in these experiments. Implementing the modifications suggested by <span class="cit" id="xref_text.3"><a href="#bib1.bibx18">Fantechi et al.</a> (<a href="#bib1.bibx18">2002</a>)</span> does not improve the model–measurement agreement of OH and <span class="inline-formula">HO₂</span>.</p>