Development and characterization of an airborne‐based instrument used to measure nitric acid during the NASA Transport and Chemical Evolution over the Pacific field experiment

A new inlet and instrument have been developed for the rapid measurement of gas phase nitric acid (HNO3) from an airborne platform. The inlet was kept near ambient temperatures with a very short sampling time (100 ms) to minimize desorption of particle nitrates. In addition, inlet surface adsorption problems were minimized by the use of extruded perfluoroalkoxy as a sampling material. Nitric acid was detected by selected ion chemical ionization mass spectrometry using deprontonated methanesulfonic acid as a reagent ion. Laboratory tests showed no interferences from NO, NO2, NO3, and N2O5 under wet (relative humidity (RH) = 100%) or dry (RH = 0%) conditions at levels exceeding those found in the troposphere. The inlet and instrument were flown on the NASA P‐3B aircraft as part of the NASA Transport and Chemical Evolution over the Pacific (TRACE‐P) field campaign off the coast of Asia during February–April 2001. Nitric acid was measured every 5 s for a 3 s integration period with a limit of detection of ∼10 parts per trillion by volume (pptv). The instrument was calibrated by the addition of isotopically labeled H15NO3 near the front of the ion source on a continual basis. Absolute uncertainties including systematic errors are the limit of detection (10 pptv) plus ±20% for HNO3 > 200 pptv, ±25% for HNO3 100–200 pptv, and ±30% for HNO3 < 100 pptv (±2 σ). Rapid changes in ambient HNO3 were resolved, suggesting minimal influences from instrument surfaces. Finally, the measurements compared favorably with the University of New Hampshire’s mist chamber/ion chromatography instrument flown on board the NASA DC‐8 aircraft during two intercomparison flights. The in‐flight performance of the instrument is demonstrated under the wide range of conditions observed in TRACE‐P.