Seasonal cycles in short-lived hydrocarbons in baseline air masses arriving at Mace Head, Ireland

The observed seasonal cycles of a selection of reactive hydrocarbons in baseline air masses at Mace Head, Ireland are consistent with a simple picture of largely man-made sources and oxidation by hydroxyl (OH) radicals. As a result, the observed seasonal cycles become more pronounced the more reactive the species are with OH. For the pentanes, the assumption of little wintertime removal breaks down, leading to an apparent dampening of the seasonal cycles relative to ethane, propane and the butanes. A global chemistry-transport model is used to describe the seasonal cycles of the hydrocarbons at Mace Head and provided an accurate description of their observed amplitudes and phases. The model derived local OH concentrations in baseline air masses required to support the observed seasonal cycles of the hydrocarbons averaged 1.38 ± 1.1 × 106 molecule cm−3. Peak daytime levels during summertime reached 1.2 × 107 molecule cm−3.