IN-SITU DETECTION OF TROPOSPHERIC OH RADICALS BY FOLDED LONG-PATH LASER-ABSORPTION - RESULTS FROM THE POPCORN FIELD CAMPAIGN IN AUGUST 1994

Ground based in-situ measurements of tropospheric hydroxyl radicals we re conducted by folded long-path laser absorption as part of the field campaign POPCORN in August 1994. The OH instrument used an open optic al multiple-reflection cell of 38.5 m base length through which the la ser beam was passed up to 80 times. The broadband emission of a short- pulse UV laser together with a multichannel detection system allowed t he simultaneous observation of six OH absorption lines in a spectral i nterval of Delta lambda similar or equal to 0.24 nm at 308.1 nm (A(2) Sigma(+), v'=0 <-- X(2) Pi, v ''=0 transition). Along with the OH radi cals; the trace gases SO2, HCHO, and naphthalene were measured by this technique. The large spectral detection range covered a multitude of rotational absorption lines of these trace gases which were all used f or multicomponent analysis, thus allowing a specific and sensitive det ection of tropospheric OH radicals. An average 2 sigma detection limit of 1.5x10(6) OH/cm(3) for an integration time of 200 seconds and an a bsorption light path length of 1848m was determined from the field mea surements. In total, 392 OH data were obtained by long-path absorption during 16 days of field measurements. The observed OH concentrations reached peak values of 13x10(6) cm(-3) at noon.