Ambient isoprene and monoterpene concentrations in a Greek fir (Abies Borisii-regis) forest. Reconciliation with emissions measurements and effects on measured OH concentrations

Ambient biogenic hydrocarbons were monitored in a forested site in Northern Greece during the summer of 1997 as part of the AEROBIC campaign. Signific ant concentrations of isoprene were observed. consistent with enclosure mea surements presented in the previous paper (Harrison et al.. 2001, Atmospher ic Environment 35. 4687-4698.) In addition, significant concentrations of m onoterpenes were observed (nighttime maximum of 2.9 ppbv for total monoterp ene concentration). Isoprene and the monoterpenes exhibited pronounced, but different, diurnal cycles which are interpreted via enclosure and OH measu rements. The combination of relatively high emission rates at a time when O H concentrations are significantly lower leads to peak isoprene concentrati ons (similar to 3.0 ppbv) during the late evening. The temporal behaviour o f the biogenic hydrocarbons is used to interpret the radical chemistry at t he site and may indicate the presence of high (similar to 1 x 10(6) cm(-3)) nighttime concentrations of the OH radical. Biogenic hydrocarbon measureme nts were supported by a range of organic and inorganic measurements which a re also presented in this paper. (C) 2001 Elsevier Science Ltd. All rights reserved.