Absorption technique for OH measurements and calibration

An absorption technique is described which utilizes a stabilized frequency-doubled tunable dye laser and a long-path White cell with high mirror reflectivities both in the red and UV. In laboratory conditions we have been able to obtain routinely a detection sensitivity of 3 parts in 10(6) over absorption paths <1 m in length and a detection sensitivity of ~6 parts in 10(5) over an absorption path of the order of 1 km. The latter number corresponds to 3 x 10(6) OH molecules/cm(3), and therefore the technique should be particularly useful for calibration of our fluorescence instrument for OH measurements. However, the presence of atmospheric fluctuations coupled with intensity variation accompanying frequency scanning appears to degrade the detection sensitivity in outdoor ambient conditions, thus making it unlikely that this technique can be employed for direct OH monitoring.