Impact of pyruvic acid photolysis on acetaldehyde and peroxy radical formation in the boreal forest: theoretical calculations and model results

Abstract

Based on the first measurements of gas-phase pyruvic acid (CH3C(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 (CH3CHO) and the peroxy radicals CH3C(O)O2 and HO2 during an autumn campaign in the boreal forest. The results are dependent on the quantum yield (φ) and mechanism of the photodissociation of pyruvic acid and the fate of a likely major product, methylhydroxy carbene (CH3COH). 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 φ = 0.2 (at 1 bar of air), and the main photolysis products (60 %) are acetaldehyde + CO2 with 35 % C–C bond fission to form HOCO and CH3CO (scenario A). In the second scenario (B), the formation of vibrationally hot CH3COH (and CO2) represents the main dissociation pathway at longer wavelengths (∼ 75 %) with a ∼ 25 % contribution from C–C bond fission to form HOCO and CH3CO (at shorter wavelengths). In scenario 2 we vary φ between 0.2 and 1 and, based on the results of our theoretical calculations, allow the thermalized CH3COH to react with O2 (forming peroxy radicals) and to undergo acid-catalysed isomerization to CH3CHO. 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 CH3CHO with a contribution between ∼ 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 ∼ 20 % and 60 % dependent on the choice of φ and the products formed. HO2 production rates are also enhanced, mainly via the formation of CH3O2. The elevated production rates of CH3C(O)O2 and HO2 and concentration of CH3CHO result in significant increases in the modelled mixing ratios of CH3C(O)OOH, CH3OOH, HCHO, and H2O2.