MEASURING INORGANIC NITRATE SPECIES WITH SHORT-TIME RESOLUTION FROM AN AIRCRAFT PLATFORM BY DUAL-CHANNEL OZONE CHEMILUMINESCENCE

A measurement technique for determining nitrate (the sum of nitric aci d and particulate nitrate) with a few seconds time resolution in plume s is needed to resolve within-plume features. A technique using dual o zone-chemiluminescent NO detectors with a selective nitrate scrubber i n one sampling train is promising if used with an appropriate sampling inlet, and if nitrate is the desired analyte. We report the design of , and preliminary results from a dual channel ozone-chemiluminescent s ystem, each channel containing a gold-CO catalyzed converter which red uces all odd nitrogen species (NOy) quantitatively to NO; one channel also contains a nylon filter to remove nitrate from the air stream pri or to the converter (this signal is termed NOy.). This system was depl oyed successfully in a Bell 205 helicopter during the 1995 Southern Ox idants Study Nashville Ozone Study. The converters were mounted forwar d near the air intake, and zero air and calibration gases admitted sim ultaneously to both channels during flight operations. The difference signal between the two channels (NOy-NOy.) indicated apparent nitrate levels in the sampled air with a time resolution of <5 s and a limit o f detection of about 1 ppbv. Nitrate levels observed with this system in plumes and background air during the Nashville Ozone Study were hig hly correlated with ozone and varied from below detection limits to ap proximate to 20 ppbv. Nitrate levels were also highly correlated with the calculated difference between NOy and the sum of NO and NO2 (NOz). Higher nitrate levels as a fraction of NOz were found in power plant plumes (greater than or equal to 60%) compared with urban plumes (appr oximate to 50%) and background air, consistent with apparently lower o zone production efficiencies in power plant plumes vis-a-vis urban plu mes.