A modified laser-induced fluorescence (LIF) technique for measuring tropospheric levels of OH is discussed. Although this system is still of the single-photon laser-induced fluorescence type (SP-LIF), it has undergone major design changes. These changes have overcome several of the major problems encountered in first generation SP-LIF sensors. Two of the more important of these are (1) the generation of high artificial levels of OH from the laser photolysis of atmospheric O3 and (2) the degradation in detection sensitivity resulting from temporal fluctuations in the nonresonant fluorescence background. In the 2-λ LIF approach, two nearly identical laser systems are employed such that both ''on'' line OH signal monitoring and ''off'' line background levels are measured almost simultaneously (e.g., within 500 μs). This approach, in effect, freezes the atmosphere for purposes of comparing ''on'' versus ''off'' line signal measurements. Concerning the problem of laser-generated OH, two approaches have been explored: the use of very short laser pulses and the use of reduced laser energies. The OH field measuring system reported on in this work used only the reduced energy scheme. Numerous tests have shown that the 2-λ SP-LIF system displays no measurable detection bias and, under typical operating conditions, displays shot-noise-limited extraction of weak signals. In addition, this in situ sampling system has lent itself to direct in-flight OH calibrations. Tests also have been developed that quantitatively establish the magnitude of the O3/H2O/OH interference signal (which in nearly all cases was ≤20% of the total OH signal). Finally, the question of OH losses in the sampling manifold has been addressed, and the evidence strongly suggests that this loss is negligibly small. Null experiments, laboratory and in-flight calibration exercises, and O3/H2O interference tests, as well as ground-based OH measurements, are reported on.

Collectively, these results indicate that the 2-λ LIF OH system can provide reliable OH measurements down to the fundamental limits of its sensitivity, e.g., typically 1¿106 molecules/cm3 for a 30-min integration time at mid free-tropospheric altitudes. The detection limit of this system at ground level as well as at mid-latitudes should be adequate to address many interesting questions in tropospheric chemistry; however, further improvements will be required for routine high-resolution OH measurements on aircraft sampling platforms.