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 rural area of North-Eastern Germany during August 1994. An overview of the objectives, measurements and major results of this campaign is presented. Measurements 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. Additionally, plant emissions and the exchange of trace gases between a maize field and the atmosphere were investigated. Budgets of selected trace gases were calculated to assess the relative importance of local sources, chemistry or transport. Intercomparisons between measurement techniques were a central issue of POPCORN and included measurements of OH, hydrocarbons, formaldehyde, photolysis frequencies and vertical fluxes. OH radical concentrations were measured simultaneously by LIF (Laser Induced Fluorescence) and DOAS (Differential Optical Absorption Spectroscopy). Both methods showed good agreement. Maximum OH concentrations were around 107 cm–3 and the diurnal cycles closely followed the rate of primary production via ozone photolysis. Generally, the trace gas composition during POPCORN was characterized 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). Concentrations of individual biogenic volatile organic compounds (VOC) were mostly well below 100 ppt. However, formaldehyde and acetaldehyde which partly originate from biological sources were observed at mixing ratios of some ppb.