Design and initial characterization of an inlet for gas‐phase NO y measurements from aircraft

An understanding of gas‐phase HNO 3 transmission through an inlet is necessary to evaluate the quality of NO y measurements from an aircraft platform. A simple, inexpensive, low‐volume Teflon inlet is described and its suitability as an aircraft inlet for gas‐phase NO y is assessed. Aerosol transmission is not characterized, but inlet design and orientation probably discriminates against the majority of aerosol by mass. Laboratory data, in‐flight HNO 3 standard addition calibrations, and ambient NO y measurements from the 1997 North Atlantic Regional Experiment aircraft mission are used to characterize inlet transmission efficiencies and time constants. Laboratory tests show high transmission efficiencies for HNO 3 which are relatively independent of ambient temperature and humidity. In‐flight standard addition calibrations were carried out at ambient temperatures ranging from −20° to +8°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 ∼1.5 s. Ambient data are presented to demonstrate negligible hysteresis in 1‐Hz NO y measurements, relative to variability observed in ozone data, from an air mass where HNO 3 is expected to be a large fraction of the total NO y . Power spectra of ambient NO y (at temperatures from −35° to +35°C and relative humidities from 3% to 100%) and ozone measurements suggest an effective NO y instrument time constant of ∼2 s.