POPCORN: A field study of photochemistry in North-Eastern Germany

The intensive field study POPCORN (Photo-Oxidant Formation by Plant Emitted Compounds and OH Radicals in North-Eastern Germany) was carried out in a r ural area of North-Eastern Germany during August 1994. An overview of the o bjectives, measurements and major results of this campaign is presented. Me asurements of a set of relevant atmospheric trace compounds, including the hydroxyl radical, along with meteorological data were performed to increase the understanding. of OH radical chemistry and photo-oxidant formation. Ad ditionally, plant emissions and the exchange of trace gases between a maize field and the atmosphere were investigated. Budgets of selected trace gase s were calculated to assess the relative importance of local sources, chemi stry or transport. Intercomparisons between measurement techniques were a c entral issue of POPCORN and included measurements of OH, hydrocarbons, form aldehyde, photolysis frequencies and vertical fluxes. OH radical concentrat ions were measured simultaneously by LIF (Laser Induced Fluorescence) and D OAS (Differential Optical Absorption Spectroscopy). Both methods showed goo d agreement. Maximum OH concentrations were around 10(7) cm(-3) and the diu rnal cycles closely followed the rate of primary production via ozone photo lysis. Generally, the trace gas composition during POPCORN was characterize d by relatively low concentrations of most compounds, e.g. CO: 85-200 ppb, ethane: 0.6-2 ppb, and moderate NOx levels: 0.5-5 ppb (at noontime). Concen trations of individual biogenic volatile organic compounds (VOC) were mostl y well below 100 ppt. However, formaldehyde and acetaldehyde which partly o riginate from biological sources were observed at mixing ratios of some ppb .