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

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, 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.

Measuring Photodissociation Product Quantum Yields Using Chemical Ionization Mass Spectrometry: A Case Study with Ketones

“…Therefore, one could utilize light sources that are not very intense. The selectivity of the reagent ion (in this case, iodide ion) allows for low background and interference from unwanted impurities or reaction products, as demonstrated by low background signals in our photolysis experiments. Multiple similar radicals can be measured simultaneously, which allows a more straightforward determination of branching ratios and total photodissociation quantum yields. Evaluating the possible differences in the detection sensitivities would enhance this method. In addition to highlighting the advantages of ToF-CIMS in this current laboratory-based application, we note some of the potential future applications: Replacing the UV pen-lamp used in this study with a light source capable of producing wavelengths relevant for ketone photolysis (<400 nm) would help quantify important knowledge gaps in photolysis quantum yields. This experimental design could measure atmospheric ketone photolysis under a field setting because of the portability of the instrument. Rapid data acquisition (<1 Hz) of RC­(O)­O 2 signals allows for in situ observations of photolysis processes in ambient environments Use of a light source where the photolysis volume is small allows for better control of reaction times and simultaneous measurements of the kinetics of the radicals involved. Equipping the mass spectrometer with reagent-switching capabilities , would allow for measurement of multiple photolysis products and the photolytic loss of carbonyl photolyte …”

“…Rapid data acquisition (<1 Hz) of RC­(O)­O 2 signals allows for in situ observations of photolysis processes in ambient environments …”

“…• Rapid data acquisition (<1 Hz) of RC(O)O 2 signals allows for in situ observations of photolysis processes in ambient environments. 26 • Use of a light source where the photolysis volume is small allows for better control of reaction times and simultaneous measurements of the kinetics of the radicals involved. • Equipping the mass spectrometer with reagent-switching capabilities 27,28 would allow for measurement of multiple photolysis products 29 and the photolytic loss of carbonyl photolyte.…”