Ambient biogenic hydrocarbons were monitored in a forested site in Northern Greece during the summer of 1997 as part of the AEROBIC campaign. Significant concentrations of isoprene were observed, consistent with enclosure measurements presented in the previous paper (Harrison et al., 2001, Atmospheric Environment 35, 4687-4698). In addition, significant concentrations of monoterpenes were observed (nighttime maximum of 2.9 ppbv for total monoterpene concentration). Isoprene and the monoterpenes exhibited pronounced, but different, diurnal cycles which are interpreted via enclosure and OH measurements. The combination of relatively high emission rates at a time when OH concentrations are significantly lower leads to peak isoprene concentrations (∼ 3.0 ppbv) during the late evening. The temporal behaviour of the biogenic hydrocarbons is used to interpret the radical chemistry at the site and may indicate the presence of high (∼ 1 × 106 cm-3) nighttime concentrations of the OH radical. Biogenic hydrocarbon measurements were supported by a range of organic and inorganic measurements which are also presented in this paper. © 2001 Elsevier Science Ltd. All rights reserved.