Diode laser-based cavity ring-down instrument for NO3, N2O5, NO, NO2 and O3 from aircraft

Abstract

This article presents a diode laser-based, cavity ring-down spectrometer for simultaneous in situ measurements of four nitrogen oxide species, NO₃ , N₂ O₅ , NO, NO₂ , as well as O₃ , designed for deployment on aircraft. The instrument measures NO₃ and NO₂ by optical extinction at 662 nm and 405 nm, respectively; N₂ O₅ is measured by thermal conversion to NO₃ , while NO and O₃ are measured by chemical conversion to NO₂ . The instrument has several advantages over previous instruments developed by our group for measurement of NO₂ , NO₃ and N₂ O₅ alone, based on a pulsed Nd:YAG and dye laser. First, the use of continuous wave diode lasers reduces the requirements for power and weight and eliminates hazardous materials. Second, detection of NO₂ at 405 nm is more sensitive than our previously reported 532 nm instrument, and does not have a measurable interference from O₃ . Third, the instrument includes chemical conversion of NO and O₃ to NO₂ to provide measurements of total NO_x (= NO + NO₂ ) and O_x (= NO₂ + O₃ ) on two separate channels; mixing ratios of NO and O₃ are determined by subtraction of NO₂ . Finally, all five species are calibrated against a single standard based on 254 nm O₃ absorption to provide high accuracy. Disadvantages include an increased sensitivity to water vapor on the 662 nm NO₃ and N₂ O₅ channels and a modest reduction in sensitivity for these species compared to the pulsed laser instrument. The in-flight detection limit for both NO₃ and N₂ O₅ is 3 pptv (2 σ, 1 s) and for NO, NO₂ and O₃ is 140, 90, and 120 pptv (2 σ, 1 s) respectively. Demonstrated performance of the instrument in a laboratory/ground based environment is better by approximately a factor of 2-3. The NO and NO₂ measurements are less precise than research-grade chemiluminescence instruments. However, the combination of these five species in a single instrument, calibrated to a single analytical standard, provides a complete and accurate picture of nighttime nitrogen oxide chemistry. The instrument performance is demonstrated using data acquired during a recent field campaign in California.

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