Biogenic and anthropogenic fluxes of non-methane hydrocarbons over an urban-impacted forest, Frankfurter Stadtwald, Germany

In an urban-impacted oak/beech/pine forest (Frankfurter Stadtwald, 50 degre es 04' 06 " N; 8 degrees 40' 17 " E) trace gas distributions and fluxes of anthropogenic and biogenic non-methane hydrocarbons (NMHC) were determined for a bright weather period in August 1995, In general, ozone peaked at 70 ppb in the early afternoon. NO and NO, reach values of up to 25 ppb under l ow wind conditions and local automobile traffic. Anthropogenic NMHC dominat e with up to 8.0 ppb in the air above the forest. The dominating biogenic N MHC in ambient air above the forest was isoprene with peak values of 1.5 pp b during daytime. The flux-gradient relationship with specific adapted and validated stability functions for this forest was used for calculating NMHC -fluxes. Transfer times of up to 100 s require a correction of the mixing r atios for MO-radical chemistry occurring along the gradient between 22 and 51 m for high reactive substances such as isoprene. The specific situation in the Frankfurter Stadtwald with high road traffic inside the forest (up t o 10,000 vehicles per hour) lead to sometimes significant emission of anthr opogenic NMHC as exhaust plumes were spread in the trunk space. Isoprene fl uxes were high and amounted to 3.5 nmol m(-2) ground area s(-1) due to the high percentage of oaks growing in the forest but were at the lower end of estimates made by current biogenic NMHC emission inventories. The high isop rene emission flux and ambient air mixing ratios underscore the importance of isoprene daytime and nighttime chemistry for the Frankfurt area. (C) 200 0 Elsevier Science Ltd. All rights reserved.