Importance of monoterpenes and isoprene in urban air in northern Europe

The sources, concentration levels and potential effects of isoprene and the monoterpenes on local atmospheric chemistry were studied in urban background air in Helsinki, Finland. Ambient air concentration measurements were conducted using an in-situ gas chromatograph with a mass spectrometer at an urban background station during different seasons in 2011. ghest concentrations of isoprene and monoterpenes were measured in summer (990 ng m−3), but concentrations were also clearly above the detection limit in winter (230 ng m−3). The concentrations of aromatic hydrocarbons were higher during all seasons, but when viewed in terms of reactivity-scaled concentrations the data showed that also isoprene and monoterpenes have a strong influence on local atmospheric chemistry in urban air. While they explain 71% of the potential hydroxyl radical reactivity of air masses in summer, they still explain 32% of the reactivity also in winter. ter and spring the urban background concentrations were higher than at a forested site in Finland, indicating anthropogenic sources of isoprene and monoterpenes. Source estimates obtained with the Unmix multivariate receptor model showed that traffic and wood combustion are the main local contributors to the measured concentration levels in winter, spring and November, but in July and October biogenic sources are dominant.