Design and initial characterization of an inlet for gas-phase NOy measurements from aircraft

An understanding of gas-phase HNO3 transmission through an inlet is necessa ry to evaluate the quality of NOy measurements from an aircraft platform. A simple, inexpensive, low-volume Tenon inlet Is described and its suitabili ty as an aircraft inlet for gas-phase NOy is assessed. Aerosol transmission is not characterized, but inlet design and orientation probably discrimina tes against the majority of aerosol by mass. Laboratory data, in-flight HNO 3 standard addition calibrations, and ambient NOy measurements from the 199 7 North Atlantic Regional Experiment aircraft mission are used to character ize inlet transmission efficiencies and time constants. Laboratory tests sh ow high transmission efficiencies for HNO3 which are relatively independent of ambient temperature and humidity. In-flight standard addition calibrati ons were carried out at ambient temperatures ranging from -20 degrees to +8 degrees C and relative humidities from 3% to 71%. These data suggest that nearly all the sampled air contacts an inlet surface, with 90% of added HNO 3 being transmitted in similar to 1.5 s. Ambient data are presented to demo nstrate negligible hysteresis in l-Hz NOy measurements, relative to variabi lity observed in ozone data, from an air mass where HNO3 is expected to be a large fraction of the total NOy. Power spectra of ambient NOy (at tempera tures from -35 degrees to +35 degrees C and relative humidities from 3% to 100%) and ozone measurements suggest an effective NOy instrument time const ant of similar to 2 s.