1E: Reactions of Unsaturated Aldehydes in the Atmosphere

A study has been undertaken by ACOM scientists Tyndall and Orlando in collaboration with Prof. Rainer Volkamer’s group of the University of Colorado, to study the chemistry of a series of unsaturated aldehydes. Such compounds are released from plants as a result of stress (e.g. drought, harvesting), and they have recently been identified as products of the heterogeneous photooxidation of fatty acids on the surface of marine aerosols. Upon oxidation, they are expected to produce glyoxal. Since glyoxal has been observed from satellites over the oceans, in amounts larger than what can be explained by conventional chemistry, it is important to quantify both the production rates of the aldehydes from aerosol, and their subsequent reaction rates and mechanisms.

The experiments, carried out by CU graduate student Randall Chiu, involved studying the reactions of 2-pentenal, 2-hexenal, 2-heptenal and 2-nonenal with excess ozone in the 50-L stainless steel chamber.  Samples of the aldehydes are introduced into the chamber and allowed to stabilize before the addition of an excess of ozone. The concentrations of the aldehydes and ozone are then followed over the course of roughly an hour using Fourier Transform Infrared Spectrometry and Gas Chromatography. The combined techniques enable measurement of the concentrations of the initial reactants, and products such as glyoxal and small aldehydes. Some of the experiments also included cyclohexane to scavenge any OH radicals produced, which could cause additional, unwanted loss of the aldehydes.

The ozone rate constants measured in the presence of cyclohexane were: pentenal 1.7; heptenal 1.2; nonenal 1.6 x 10‑19 cm3 molecule-1 s-1. Glyoxal was produced as a primary product of the ozonolysis with yields in the range 30-50%, in agreement with recent literature values.

The reaction rate constants appear from this systematic study to be largely independent of the length of the alkyl chain on the molecule. The results help to reduce the uncertainties in the data base associated with previous measurements. Although the rate constants are small, in the clean marine atmosphere the reactions may be significant, and the expected large yields of glyoxal have been confirmed. Future work will involve the reactions of the aldehydes with bromine atoms and hydroxyl radicals.

Figure 1 shows sequence of infrared spectra
Figure 1 shows sequence of infrared spectra obtained in the reaction of 2-heptenal and ozone (initial red, final violet).