Influence of Biogenic VOCs on the Budget of Hydroxyl Radicals in the Plume of Freiburg on its Way to Schauinsland A contribution to subproject LOOP

During the Schauins/and Ozone Precursor Experiment (SLOPE) in June 1996 measurements of ozone and ozone precursors were made at Schauinsland and in Kappel, a suburb of the city Freiburg, at the entrance of a valley, which channels the flow of polluted air from Freiburg to Schauinsland. The aim was to study the photochemical ozone formation in the plume of Freiburg (see VolzThomas et al., this volume). An important aspect of the study was to estimate the average OH radical concentration from the decay of hydrocarbons and NOx during transport between the two measuring sites. When including mixing of the plume with the background airmasses, the average OH concentration was determined to 1 x 10^ cnT\ The influence of mixing of the plume with the VOCs and NOx in the background air, as measured aboard a small aircraft, on the estimated OH concentration was less than 10 %. A budget analysis for OH and HOx radicals shows that the relatively high OH concentrations can only be sustained by recycling of each consumed OH by reaction with an anthropogenic hydrocarbon, via 1.7 HO:. Including the biogenic VOCs in the budget, the amplification factor needed to close the budget is 1.4. Proceedings ofEUROTRAC Symposium '98 Editors: P.M. Borrell and P. Borrell © 1999: WITPRESS, Southampton Transactions on Ecology and the Environment vol 28, © 1999 WIT Press, www.witpress.com, ISSN 1743-3541