Modelling the impact of gas-phase pyruvic acid on acetaldehyde and peroxy radical formation in the boreal forest

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, CH₃O₂ and HO₂ during an autumn (IBAIRN) and summer (HUMPPA) campaign at the same site. The results are dependent on the photodissociation mechanism of pyruvic acid and we examine different scenarios in which the main photolysis products are either acetaldehyde or the CH₃C(O)O₂ radical, with different overall quantum yields. If CH₃CHO is taken to be the main product (as presently recommended by evaluation panels) we find that pyruvic acid photolysis can be a dominant source of this aldehyde in the boreal forest with a contribution of 79 % (IBAIRN) and 94 % (HUMPPA) and may help explain the high acetaldehyde levels observed during HUMPPA. On the other hand, if photolysis leads mainly to the formation of radicals, the emission of pyruvic acid has a profound impact on the rates of formation of peroxy radicals (with a contribution of ~20–50 %) and shifts the onset of radical production to earlier in the morning when actinic flux is dominated by wavelengths that are too long to initiate efficient ozone photolysis but which are absorbed by pyruvic acid.