Diode laser-based cavity ring-down instrument for NO_(3), N_(2)O_(5), NO, NO_(2) and O_(3) from aircraft

摘要

This article presents a diode laser-based, cavity ring-down spectrometer for simultaneous in situ measurements of four nitrogen oxide species, NO_(3), N_(2)O_(5), NO, NO_(2), as well as O_(3), designed for deployment on aircraft. The instrument measures NO_(3) and NO_(2) by optical extinction at 662 nm and 405 nm, respectively; N_(2)O_(5) is measured by thermal conversion to NO_(3), while NO and O_(3) are measured by chemical conversion to NO_(2). The instrument has several advantages over previous instruments developed by our group for measurement of NO_(2), NO_(3) and N_(2)O_(5) 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 NO_(2) at 405 nm is more sensitive than our previously reported 532 nm instrument, and does not have a measurable interference from O_(3). Third, the instrument includes chemical conversion of NO and O_(3) to NO_(2) to provide measurements of total NO_(x) (velence NO + NO_(2)) and O_(x) (velence NO_(2) + O_(3)) on two separate channels; mixing ratios of NO and O_(3) are determined by subtraction of NO_(2). Finally, all five species are calibrated against a single standard based on 254 nm O_(3) absorption to provide high accuracy. Disadvantages include an increased sensitivity to water vapor on the 662 nm NO_(3) and N_(2)O_(5) channels and a modest reduction in sensitivity for these species compared to the pulsed laser instrument. The in-flight detection limit for both NO_(3) and N_(2)O_(5) is 3 pptv (2sigma, 1s) and for NO, NO_(2) and O_(3) is 140, 90, and 120pptv (2sigma, 1s) respectively. Demonstrated performance of the instrument in a laboratory/ground based environment is better by approximately a factor of 2-3. The NO and NO_(2) 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.