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<p>Based on the first measurements of gas-phase pyruvic acid (<span class="inline-formula">CH₃C(O)C(O)OH</span>) in the boreal forest, we derive effective emission rates of pyruvic acid and compare them with monoterpene emission rates over the diel cycle. Using a data-constrained box model, we determine the impact of pyruvic acid photolysis on the formation of acetaldehyde (<span class="inline-formula">CH₃CHO</span>) and the peroxy radicals <span class="inline-formula">CH₃C(O)O₂</span> and <span class="inline-formula">HO₂</span> during an autumn campaign in the boreal forest.</p> <p>The results are dependent on the quantum yield (<span class="inline-formula"><i>φ</i></span>) and mechanism of the photodissociation of pyruvic acid and the fate of a likely major product, methylhydroxy carbene (<span class="inline-formula">CH₃COH</span>). With the box model, we investigate two different scenarios in which we follow the present IUPAC (IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation, 2021) recommendations with <span class="inline-formula"><i>φ</i></span> <span class="inline-formula">=</span> 0.2 (at 1 <span class="inline-formula">bar</span> of air), and the main photolysis products (60 %) are acetaldehyde <span class="inline-formula">+</span> <span class="inline-formula">CO₂</span> with 35 % <span class="inline-formula">C–C</span> bond fission to form <span class="inline-formula">HOCO</span> and <span class="inline-formula">CH₃CO</span> (scenario A). In the second scenario (B), the formation of vibrationally hot <span class="inline-formula">CH₃COH</span> (and <span class="inline-formula">CO₂</span>) represents the main dissociation pathway at longer wavelengths (<span class="inline-formula">∼</span> 75 %) with a <span class="inline-formula">∼</span> 25 % contribution from <span class="inline-formula">C–C</span> bond fission to form <span class="inline-formula">HOCO</span> and <span class="inline-formula">CH₃CO</span> (at shorter wavelengths). In scenario 2 we vary <span class="inline-formula"><i>φ</i></span> between 0.2 and 1 and, based on the results of our theoretical calculations, allow the thermalized <span class="inline-formula">CH₃COH</span> to react with <span class="inline-formula">O₂</span> (forming peroxy radicals) and to undergo acid-catalysed isomerization to <span class="inline-formula">CH₃CHO</span>.</p> <p>When constraining the pyruvic acid to measured mixing ratios and independent of the model scenario, we find that the photolysis of pyruvic acid is the dominant source of <span class="inline-formula">CH₃CHO</span> with a contribution between <span class="inline-formula">∼</span> 70 % and 90 % to the total production rate. We find that the photolysis of pyruvic acid is also a major source of the acetylperoxy radical, with contributions varying between <span class="inline-formula">∼</span> 20 % and 60 % dependent on the choice of <span class="inline-formula"><i>φ</i></span> and the products formed. <span class="inline-formula">HO₂</span> production rates are also enhanced, mainly via the formation of <span class="inline-formula">CH₃O₂</span>. The elevated production rates of <span class="inline-formula">CH₃C(O)O₂</span> and <span class="inline-formula">HO₂</span> and concentration of <span class="inline-formula">CH₃CHO</span> result in significant increases in the modelled mixing ratios of <span class="inline-formula">CH₃C(O)OOH</span>, <span class="inline-formula">CH₃OOH</span>, HCHO, and <span class="inline-formula">H₂O₂</span>.</p>