Airborne intercomparison of vacuum ultraviolet fluorescence and tunable diode laser absorption measurements of tropospheric carbon monoxide

During the fall 1997 North Atlantic Regional Experiment (NARE 97), two sepa rate intercomparisons of aircraft-based carbon monoxide measurement instrum entation were conducted. On September 2, CO measurements were simultaneousl y made aboard the National Oceanic and Atmospheric Administration (NOAA) WP -3 by vacuum ultraviolet (VUV) fluorescence and by tunable diode laser abso rption spectroscopy (TDLAS), On September 18, an intercomparison flight was conducted between two separate instruments, both employing the VUV fluores cence method, on the NOAA WP-3 and the U,K. Meteorological Office C-130 Her cules. The results indicate that both of the VUV fluorescence instruments a nd the TDLAS system are capable of measuring ambient CO accurately and prec isely with no apparent interferences in 5 s. The accuracy of the measuremen ts, based upon three independent calibration systems, is indicated by the a greement to within 11% with systematic offsets of less than 1 ppbv. In addi tion, one of the groups participated in the Measurement of Air Pollution Fr om Satellite (MAPS) intercomparison [Novelli ef at., 1998] with a different measurement technique but very similar calibration system, and agreed with the accepted analysis to within 5%. The precision of the measurements is i ndicated by the variability of the ratio of simultaneous measurements from the separate instruments, This variability is consistent with the estimated precisions of 1.5 ppbv and 2.2 ppbv for the 5 s average results of the C-1 30 and the WP-3 instruments, respectively, and indicates a precision of app roximately 3.6% for the TDLAS instrument. The excellent agreement of the in struments in both intercomparisons demonstrates that significant interferen ces in the measurements are absent in air masses that ranged from 7 km in t he midtroposphere to boundary layer conditions including subtropical marine air and continental outflow with embedded urban plumes. The intercompariso n of the two VUV instruments that differed widely in their design indicates that the VUV fluorescence technique for CO measurements is not particularl y sensitive to the details of its implementation. These intercomparisons he lp to establish the reliability of ambient CO measurements by the VUV fluor escence technique.