A 2-lambda laser-induced fluorescence field instrument for ground- based airborne measurements of atmospheric OH.

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-lambda LIF approach, 2 nearly identical laser systems are employed such that both 'on' line OH signal monitoring and 'off' line background levels are measured almost simultaneously (eg within 500 micro s). This approach, in effect, freezes the atmosphere for purposes of comparing 'on' versus 'off' line signal measurements. Concerning the problem of laser-generated OH, 2 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-lambda SP-LIF system displays no measurable detection bias and, under typical operating conditions, displays shot-noise-limited extraction of weak signals.-from Authors