On the budget of OH radicals and ozone in an urban plume from the decay of C-5-C-8 hydrocarbons and NOx

Simultaneous measurements of ozone and ozone precursors were made during a field campaign at Schauinsland in the Black Forest and in the valley north of Schauinsland that channels the flow of polluted air from the city of Freiburg to the site. From the decay of hydrocarbons and NOx between the two measuring sites and the known rate coefficients, the concentration of OH radicals was calculated. From a budget analysis of OH and HOx it is concluded that the relatively high OH concentrations (5-8 x 10(6) cm(-3)) in the presence of high NO2 concentrations cannot be explained by the known primary sources. The budget can be closed if efficient recycling of OH via HO2 is assumed to occur and that, based on the measured hydrocarbons, 2 HO2 molecules are formed for each OH radical that reacts with a hydrocarbon molecule. This assumption is in accordance with the budget of O-x obtained from our measurements and with results from earlier measurements of alkylnitrates and peroxy radicals at Schauinsland. A possible conclusion is that the decay of precursors and production of photooxidants in urban plumes proceeds at a faster rate than is currently assumed. The potential role of biogenic hydrocarbons for the radical budget is also discussed.