The fast-response resonance fluorescence instrument for the airborne measurement of carbon monoxide described by Gerbig et al. <1996> was modified by implementing an improved optical filter with more efficient optics and an optimized resonance lamp. Besides reductions in size and weight, the new instrument achieves a sensitivity 10 times higher, a lower background (65 ppb compared with 250 ppb), and a faster time response (<0.1s) than the original instrument. The precision is ¿1.5 ppb at an atmospheric mixing ratio of 100 ppb CO, and the detection limit is 3 ppb (2&sgr;) for an integration time of 1 s. First results from the North Atlantic Regional Aerosol Characterization Experiment (ACE-2) campaign during July 1997, when the new instrument was deployed aboard the U.K. Meteorological Office C-130 aircraft, are used to demonstrate the performance of the new instrument. ¿ 1999 American Geophysical Union