Diode laser-based cavity ring-down instrument for NO 3, N 2O 5, NO, NO 2 and O 3 from aircraft

Abstract

This article presents a diode laser-based, cavity ring-down spectrometer for simultaneous in situ measurements of four nitrogen oxide species, NO 3, N 2O 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 2O 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 2O 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 (= NO + NO 2) and O x (= 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 2O 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 2O 5 is 3 pptv (2σ, 1 s) and for NO, NO 2 and O 3 is 140, 90, and 120 pptv (2σ, 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 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. © 2011 Author(s).