Contribution of low-abundance terpenes to wintertime VOC reactivity in urban air in Helsinki, Finland

Recent studies suggest that terpenes in urban air may have substantial anthropogenic sources, yet distinguishing these from biogenic emissions remains challenging. In this study, we measured terpene concentrations in a street canyon in Helsinki during cold winter months (mean temperature < 0 °C), when biogenic emissions were expected to be minimal. Monoterpenes were observed at mean concentrations of ~160 ng m⁻³, more than an order of magnitude lower than the mixing ratios of aromatic hydrocarbons. Nevertheless, their high reactivity with hydroxyl radicals, nitrate radicals, and ozone led to a disproportionately large contribution to local atmospheric oxidation processes. This pronounced reactivity, combined with their high secondary organic aerosol (SOA) formation potential, indicated the important potential role of anthropogenic terpene emissions even in wintertime SOA formation.