Abstract
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.
Highlights
Organic acids play a crucial role in tropospheric chemistry, impacting secondary organic aerosol formation, air quality, and climate (Kanakidou et al, 2005; Hallquist et al, 2009)
In order to derive the pyruvic acid emission rate (Epyr) during IBAIRN, we assume that only photolysis and dry deposition contribute significantly to its overall loss rate and that pyruvic acid is in steady state
We have combined measurements of pyruvic acid in an autumn campaign in the boreal forest (IBAIRN) with theoretical calculations designed to characterize the fate of the methylhydroxy carbene radical (CH3COH, the major product of pyruvic acid photodissociation) with a box-modelling study
Summary
Organic acids play a crucial role in tropospheric chemistry, impacting secondary organic aerosol formation, air quality, and climate (Kanakidou et al, 2005; Hallquist et al, 2009). Eger et al.: Impact of pyruvic acid photolysis on CH3CHO and RO2. (Walker, 1962), is found in tropospheric air in the gas phase as well as in the aerosol phase, especially in the boundary layer over vegetated regions. Gas-phase mixing ratios ranging from a few to several hundred parts per trillion (pptv) have been reported in various locations around the world, including the tropical rainforest, boreal forest, rural areas with temperate forest, and regions influenced by urban outflow. A recent overview of existing measurements of gas-phase pyruvic acid is given by Eger et al (2020)
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