<span>Volatile organic compounds (VOCs) are key drivers of urban atmospheric chemistry, acting as precursors to ozone and secondary organic aerosol (SOA). Terpenes, typically considered biogenic, may also have important anthropogenic sources in cities, although distinguishing these contributions remains challenging.</span></p> <p><span>We measured terpenes and other VOCs (C<sub>6</sub>–C<sub>15</sub>) using in situ TD-GC-MS at a traffic supersite in Helsinki during winter 2022, when biogenic emissions were minimal. Additional offline measurements of lighter hydrocarbons (C<sub>2</sub>–C<sub>5</sub>) were conducted at residential and urban background sites. Most VOCs, including benzene, furfural, naphthalene, and p-cresol, showed higher concentrations at the residential and background sites, indicating a strong influence from wood combustion. In contrast, terpenes were elevated at the traffic site, suggesting emissions from other anthropogenic activities.</span></p> <p><span>At the traffic site, terpenes had a mean concentration of 164 ng m⁻³, contributing less than 1% of total measured VOC mass. Despite their low abundance, they played a significant role in atmospheric chemistry due to high reactivity, accounting for 7 % of hydroxyl radical reactivity, 59 % of ozone reactivity, and 65 % of nitrate radical reactivity among the measured VOCs. Combined with their strong SOA formation potential, this highlights the importance of anthropogenic terpene emissions for urban air quality.</span></p> <p><span>Intermediate-volatility VOCs (IVOCs, C<sub>11</sub>–C<sub>15</sub>) were present at low concentrations and generally had minor contributions, although undetected compounds may still be relevant. Among them, sesquiterpenes showed notable ozone reactivity (23 %) despite concentrations below 3 ng m⁻³.</span>