MEASUREMENTS OF ALKYL NITRATES IN RURAL AND POLLUTED AIR MASSES

Gas chromatographic measurements of organic nitrates (C1-C8) were made at Julich (52-degrees-N, 7-degrees-E) and at Schauinsland/Black Forest (48-degrees-N, 6-degrees-E), beginning in autumn 1988. Samples were collected cryogenically by passing 3 l 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 3 l sample. The measurements show the presence of C1-C5 nitrates at concentration levels between 1 and 230 ppt. Nitrates greater than C5 were only observed at Julich and at very low mixing ratios. Nitrates > C8 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.