Measurements of alkyl nitrates in rural and polluted air masses

Gas chromatographic measurements of organic nitrates (C₁C₈) were made at Jülich (52°N, 7°E) and at Schauinsland/Black Forest (48°N, 6°E), beginning in autumn 1988. Samples were collected cryogenically by passing 3ℓ of ambient air through a sampling loop, filled with glass beads, at 77 K. Separation was performed by capillary gas chromatography. An optimized chemiluminescence NO-analyser operating at a pressure of 1 mbar in combination with a catalytic converter was used as a specific detector for NO-containing molecules with a detection limit on the order of 1 ppt for a 3ℓ sample. The measurements show the presence of C₁C₅ nitrates at concentration levels between 1 and 230 ppt. Nitrates greater than C₅ were only observed at Jülich and at very low mixing ratios. Nitrates > C ₈ were not observed. The combined concentrations of organic nitrates account, on the average, for 1-2% of the NO_y. On a few occasions, the fraction could be as high as 7%. Therefore, organic nitrates are not likely to explain the missing fraction of NO_y, first reported by Fahey et al. (1986). From the correlation between the concentrations of organic nitrates and ozone and from the absence of nitrates > C₅ at Schauinsland it is concluded that ozone formation proceeds mainly via the smaller alkylperoxy radicals, HO₂ and CH₃C(O)O₂.