Based on the first measurements of gas-phase pyruvic acid (CH.sub.3 C(O)C(O)OH) 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 (CH.sub.3 CHO) and the peroxy radicals CH.sub.3 C(O)O.sub.2 and HO.sub.2 during an autumn campaign in the boreal forest. The results are dependent on the quantum yield (Ï) and mechanism of the photodissociation of pyruvic acid and the fate of a likely major product, methylhydroxy carbene (CH.sub.3 COH). 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 Ï = 0.2 (at 1 bar of air), and the main photolysis products (60 %) are acetaldehyde + CO.sub.2 with 35 % C-C bond fission to form HOCO and CH.sub.3 CO (scenario A). In the second scenario (B), the formation of vibrationally hot CH.sub.3 COH (and CO.sub.2) represents the main dissociation pathway at longer wavelengths (â¼ 75 %) with a â¼ 25 % contribution from C-C bond fission to form HOCO and CH.sub.3 CO (at shorter wavelengths). In scenario 2 we vary Ï between 0.2 and 1 and, based on the results of our theoretical calculations, allow the thermalized CH.sub.3 COH to react with O.sub.2 (forming peroxy radicals) and to undergo acid-catalysed isomerization to CH.sub.3 CHO. 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 CH.sub.3 CHO with a contribution between â¼ 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 â¼ 20 % and 60 % dependent on the choice of Ï and the products formed. HO.sub.2 production rates are also enhanced, mainly via the formation of CH.sub.3 O.sub.2 . The elevated production rates of CH.sub.3 C(O)O.sub.2 and HO.sub.2 and concentration of CH.sub.3 CHO result in significant increases in the modelled mixing ratios of CH.sub.3 C(O)OOH, CH.sub.3 OOH, HCHO, and H.sub.2 O.sub.2.