C₃-C₅ alkanes in the atmosphere: concentration, seasonal cycle and contribution to the atmospheric budgets of acetone and acetaldehyde

Abstract. The atmospheric chemistry of C₃-C₅ alkanes has been incorporated in the atmospheric-chemistry general circulation model EMAC. Model output is compared with observations from the NOAA/ESRL GMD cooperative air sampling network. A new series of measurements is used to evaluate the model in representing C₃-C₅ alkanes (i.e. propane, isobutane, butane, isopentane and pentane). While the representation of propane is within the measurement standard deviation, some deviations are found for the other tracers. The model is able to reproduce the main features of the C₃-C₅ alkanes (e.g., seasonality). However, in the Northern Hemisphere during winter the mixing ratios of these alkanes are generally overestimated. Conversely, the model shows an underestimation in the Southern Hemisphere. Moreover, only for iso-pentane there is a net overestimation of the mixing ratios, while for the other alkanes, the results are at the higher end of the measurement range.

The effects of the C₃-C₅ alkanes to atmospheric acetone and acetaldehyde are quantified. The total amount of acetone produced by propane, isobutane and isopentane oxidation is 11.6 Tg/yr, 4.2 Tg/yr and 5.8 Tg/yr, respectively. These chemical sources are largely controlled by the reaction with OH, while the reactions with NO₃ and Cl contribute only to a little extent.

Moreover, 3.1, 4.5, 1.9 and 6.7 Tg/yr of acetaldehyde are formed from the oxidation of propane, butane, pentane and isopentane, respectively. Also for acetaldehyde, the formation is controlled by the reaction of alkanes with OH. However, since isopentane is generally overestimated, its contribution to the production of these oxygenated compounds represents an upper limit.