Yearlong measurements of monoterpenes and isoprene in a Mediterranean city (Athens): Natural vs anthropogenic origin

Monoterpenes and isoprene are important constituents of the volatile organic compounds (VOCs) due to their high reactivity and participation in ozone and secondary aerosol formation. The current work focuses on the results of a 13-month intensive campaign of high resolution time-resolved measurements of these compounds, at an urban background site in Athens, Greece. On an annual basis, monoterpenes (alpha-pinene and limonene) surpass the isoprene levels presenting mean values of 0.70 +/- 0.83 mu g m(-3), 0.33 +/- 0.78 mu g m(-3) and 0.19 +/- 0.36 mu g m(-3), respectively. The large standard deviation highlights the significant diurnal and day-to-day variability. Isoprene presents a typical seasonal cycle, with a photochemically induced summer-time maximum. Enhanced noon levels are observed during summer, whereas a morning peak in the autumn and winter profiles occurs, depite the generally low levels encountered during these seasons. The monoterpenes deviate from the expected biogenic pattern, presenting higher mean levels during the cold period and a night-to-early morning enhancement strongly related to local anthropogenic tracers such as BC, CO, NO or toluene, as well as increased levels under wind speeds lower than 3 m s(-1). Estimations of the anthropogenic and biogenic fractions based on the enhancement ratios of a-pinene versus a variety of anthropogenic tracers, demonstrate a clear dominance of the anthropogenic sources in all studied seasons. Simultaneously, the biogenic fraction increased during summer relative to winter by more than 10 times. Both a-pinene and limonene significantly contribute to locally formed secondary organic aerosol (SOA), determined by means of an ACSM, accounting for at least 22% and 13% of their levels in summer and winter respectively. Additionally, monoterpenes and isoprene contribute 6% to the observed oxidants levels (O-3 + NOx) during summer.