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

In an urban-impacted oak/beech/pine forest (Frankfurter Stadtwald, 50° 04′ 06″ N; 8° 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 NOx reach values of up to 25 ppb under low wind conditions and local automobile traffic. Anthropogenic NMHC dominate with up to 8.0 ppb in the air above the forest. The dominating biogenic NMHC in ambient air above the forest was isoprene with peak values of 1.5 ppb 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 ratios for HO-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 to 10,000 vehicles per hour) lead to sometimes significant emission of anthropogenic NMHC as exhaust plumes were spread in the trunk space. Isoprene fluxes 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 isoprene emission flux and ambient air mixing ratios underscore the importance of isoprene daytime and nighttime chemistry for the Frankfurt area.