Fast-turnaround alkyl nitrate measurements during the PROPHET 1998 summer intensive

An automated gas chromatographic system for making fast-turnaround measurem ents of alkyl nitrates was developed and tested. Every 30 min, samples were automatically acquired on a 1/16-inch OD Tenax trap made from fused silica -lined stainless steel tubing and injected by thermal desorption into the g as chromatograph. The system exhibited good chromatographic resolution, min imal breakthrough, and on-trap stability for C-3-C-5 nitrates. The detectio n limit of the instrument was <0.1 parts per trillion by volume for C-3-C-5 nitrates with an overall uncertainty of 30% for isopropyl nitrate and 50% for all other quantified compounds. Overall uncertainty of the method was l imited by calibration precision rather than by sampling method or chromatog raphy. The system was deployed in the field as part of the Program for Rese arch on Oxidants: Photochemistry, Emissions, and Transport intensive held a t the University of Michigan Biological Station in northern Michigan during the summer of 1998 from July 15 to August 22. Ambient results show a signi ficant dynamic range with absolute levels affected by air mass history and pollution sources. The sum of measured C-3-C-5 alkyl nitrates obtained duri ng this intensive ranged from 3.45 to 65.8 pptv, with a mean of 18.6 pptv a nd contributed 1% on average to total NOy, with a range of 0.5% to 3%. Diff erences in the contribution to NOy are found when the data are filtered acc ording to air mass origin. A positive relationship between n-propyl nitrate and peroxypropionyl nitrate was observed over the course of the intensive.