MOAS: An Absorption Laser Spectrometer for Sensitive and Local Monitoring of Tropospheric OH and Other Trace Gases

Hydroxyl radical (OH) multipass absorption spectroscopy (MOAS) is a laser optical method for the absolute and in situ monitoring of tropospheric OH with high spatial and temporal resolution. This technique is based on direct absorption measurements and it is made local by folding the absorption path inside a multiple reflection cell with an open path setup. The light source is a rapidly tuned and power-stabilized dye laser, which is frequency doubled by an intracavity BBO crystal. SO2, CH2O, and naphthalene are measured at 308 nm simultaneously with OH employing the second harmonic of the laser radiation. NO2 can be measured using the fundamental at 616 nm. The limit of detection is typically 4 X 10(5) OH cm(-3) for a 1-km absorption pathlength, a 1-min monitoring time, and a signal to noise ratio of S/N = 1. The sensitivity can change by a factor of similar to 2 in both directions dependent on the boundary conditions. The performance of the MOAS method was tested by tropospheric OH measurements achieved during four field campaigns since 1991, and a number of results interesting for the basic research of tropospheric chemistry, in particular model tests, have been obtained.