LONG-TERM MEASUREMENTS OF ALKYL NITRATES IN SOUTHERN GERMANY 1 - GENERAL BEHAVIOR AND SEASONAL AND DIURNAL-VARIATION

Continuous measurements of alkyl nitrates were made during the Troposp heric Ozone Research subproject of the European Experiment on Transpor t and Transformation of Environmentally Relevant Trace Constituents in the Troposphere Over Europe at the Schauinsland station in the Black Forest between June 1990 and May 1991, using an automated gas chromato graph with a NOy detector. Mor-than 2000 samples were analyzed with a sampling frequency of six to nine samples per day. The mixing ratio of the sum of alkyl nitrates (C-1-C-8) ranged between 30 and 630 parts p er trillion (ppt) and averaged 120 ppt over the whole measurement peri od. The average contribution of alkyl nitrates to total odd nitrogen w as about 3%, with little seasonal variation. The most abundant individ ual nitrates were 2-propyl, 2-butyl, and methyl nitrate. When lumped b y carbon number, the sum of C-5 nitrates represented the largest ;Frac tion after methyl nitrate. The mixing ratios of nitrates greater than or equal to C-7 were very small. In polluted air masses, the mixing ra tios of alkyl nitrates exhibited summer maxima and pronounced daily va riations, with maxima in the early evening. The abundance increased fr om C-3 to C-5. In clean, photochemically aged air masses, alkyl nitrat e mixing ratios showed a summer minimum and no pronounced diurnal vari ations. A decrease of abundance from C-3 toward higher carbon chain le ngths was observed, as is expected for air that is characterized by ph otochemical destruction. The measured concentration ratios between dif ferent alkyl nitrate isomers are utilized to resolve discrepancies in the existing data on branching ratios for the formation of alkyl nitra tes from the reaction of peroxy radicals with NO and to classify air m asses with respect to photochemical age. The observed behavior and mix ing ratios of methyl and ethyl nitrate cannot be explained by formatio n solely from the reaction of peroxy radicals and NO.