Determination of the tropospheric hydroxyl radical by liquidphase scrubbing and HPLC: Preliminary results

A preliminary study was carried out to examine the feasibility of measuring tropospheric hydroxyl radicals (OH) by liquidphase scrubbing and high performance liquid chromatography (HPLC). The potential advantages of this approach are its simplicity, portability, and low expense. The sampling system employs glass bubblers to trap atmospheric OH into a buffered solution of salicylic acid (o-hydroxybenzoic acid, OHBA). Rapid reaction of OH with OHBA produces a stable fluorescent product, 2,5-dihydroxybenzoic acid (2,5-DHBA), which is determined by reverse-phase HPLC and fluorescence detection. Our preliminary field results indicate that this method is most suitable for OH measurements in clean tropospheric air, where interferences from other atmospheric species appear to be negligible or minor relative to polluted air. In clean air, the sampling period is about 45-90 minutes, which yields a detection limit of approximately 3-6 x 10(5) radicals cm(-3). During an OH intercomparison experiment at the Caribou sampling site in Colorado, our liquidphase scrubber method was compared with the ion-assisted mass spectrometry (MS) method. Our results were within the same range as those of the ion-assisted MS method (1-5 x 10(6) radicals cm(-)3) within our precision at that time (about +/- 30-50%). Preliminary tests in Pullman, WA indicated that the method might also function in moderately polluted air by acidifying the scrubbing solution or by adding a scavenger to suppress interferences. In Pullman, mid-day OH concentrations were usually in the range of 2-20 x 10(6) radicals cm(-3). Nighttime OH concentrations were always low, either at or slightly above the detection limit.