Calibration and detection techniques for trace nitrogen compounds in the atmosphere.

摘要

Simultaneous measuremnts of NO, NOx, and NOy were made during an instrument intercomparison between the University of Maryland and the NOAA Aeronomy Laboratory. The NOy measurements used a gold catalyst at 300{dollar}spcirc{dollar}C (NOAA) or a molybdenum catalyst at 400{dollar}spcirc{dollar}C (UMD); NOx measurements involved photolysis of NO{dollar}sb2{dollar} (NOAA) or reduction of NO{dollar}sb2{dollar} by solid ferrous sulfate (FeSO{dollar}sb4){dollar}. The two instruments agreed on daytime (NO) to the limit of detection (25 pptv), and gave similar estimates of NOy concentrations. Ferrous sulfate exhibited a significant interference from PAN and apparently n-propyl nitrate.; Further tests of the UMD NOx and NOy measurement techniques were made in the laboratory. The conversion efficiencies of NO{dollar}sb2{dollar}, HNO{dollar}sb3{dollar}, and several alkyl nitrates were studied as a function of temperature on stainless steel and molybdenum. Potential interferences (NH{dollar}sb3{dollar}, HCN, and CH{dollar}sb3{dollar}CN) were also studied. NOy compounds tested on a Mo converter at 375{dollar}spcirc{dollar}C under humidified conditions exhibited the optimum conversion efficiencies with negligible interferences. The FeSO{dollar}sb4{dollar} converter reduced only NO{dollar}sb2{dollar} (and PAN) to NO with high efficiency. Nylon filters were found to selectively scrub HNO{dollar}sb3{dollar} (HONO was not tested).; A new calibration source of gaseous HNO{dollar}sb3{dollar} was developed, based on the reaction of HCl(g) with AgNO{dollar}sb3{dollar}(s). The concentration of HNO{dollar}sb3{dollar} produced by the system was determined using three independent techniques: tunable diode laser absorption spectroscopy, chemiluminescence detection, and ion chromatography. At 40{dollar}spcirc{dollar}C, the system converts HCl to nearly pure HNO{dollar}sb3{dollar} with 92 to 100% efficiency. Dynamic dilution was used to obtain HNO{dollar}sb3{dollar} concentrations of 9 ppbv.; A dual-converter (Mo) technique was used to measure NOy and (HNO{dollar}sb3{dollar} + NO{dollar}sb3sp-{dollar}) in August and September, 1989. One converter measured NOy while the second used a Nylon filter at the inlet to remove HNO{dollar}sb3{dollar} vapor and particulate nitrate. The difference between the two signals yields a measure of total nitrate. Both the instrument response and zero were determined daily using the HCl/AgNO{dollar}sb3{dollar} calibration system ((HNO{dollar}sb3{dollar}) = 9 ppbv). The average ambient concentrations were (NOy) = 2.42 {dollar}pm{dollar} 1.28 ppbv and (total nitrate) = 1.01 {dollar}pm{dollar} 0.81 ppbv (dry conditions).