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<p>The photooxidation of the most abundant monoterpene, <span class="inline-formula"><i>α</i></span>-pinene, by the hydroxyl radical (<span class="inline-formula">OH</span>) was investigated at atmospheric concentrations in the atmospheric simulation chamber SAPHIR. Concentrations of nitric oxide (<span class="inline-formula">NO</span>) were below 120&thinsp;<span class="inline-formula">pptv</span>. Yields of organic oxidation products are determined from measured time series giving values of <span class="inline-formula">0.11±0.05</span>, <span class="inline-formula">0.19±0.06</span>, and <span class="inline-formula">0.05±0.03</span> for formaldehyde, acetone, and pinonaldehyde, respectively. The pinonaldehyde yield is at the low side of yields measured in previous laboratory studies, ranging from 0.06 to 0.87. These studies were mostly performed at reactant concentrations much higher than observed in the atmosphere. Time series of measured radical and trace-gas concentrations are compared to results from model calculations applying the Master Chemical Mechanism (MCM) 3.3.1. The model predicts pinonaldehyde mixing ratios that are at least a factor of 4 higher than measured values. At the same time, modeled hydroxyl and hydroperoxy (<span class="inline-formula">HO₂</span>) radical concentrations are approximately 25&thinsp;% lower than measured values. <span class="cit" id="xref_text.1"><a href="#bib1.bibx52">Vereecken et al.</a> (<a href="#bib1.bibx52">2007</a>)</span> suggested a shift of the initial organic peroxy radical (<span class="inline-formula">RO₂</span>) distribution towards <span class="inline-formula">RO₂</span> species that do not yield pinonaldehyde but produce other organic products. Implementing these modifications reduces the model–measurement gap of pinonaldehyde by 20&thinsp;% and also improves the agreement in modeled and measured radical concentrations by 10&thinsp;%. However, the chemical oxidation mechanism needs further adjustment to explain observed radical and pinonaldehyde concentrations. This could be achieved by adjusting the initial <span class="inline-formula">RO₂</span> distribution, but could also be done by implementing alternative reaction channels of <span class="inline-formula">RO₂</span> species that currently lead to the formation of pinonaldehyde in the model.</p>