In-situ measurements of tropospheric hydroxyl radicals by folded long-pathlaser absorption during the field campaign POPCORN

Absolutely calibrated in-situ measurements of tropospheric hydroxyl radical s, formaldehyde, sulfur dioxide, and naphthalene (C10H8) were performed by long-path laser absorption spectroscopy during the field campaign POPCORN. The absorption light path was folded into an open optical multiple reflecti on cell with a mirror separation of 38.5 m. Using a light path length of 18 48 m and an integration time of 200 s, the average 1 sigma-detection limits of OH, HCHO, SO2 and C10H8 during POPCORN were 8.710(5) cm(-3): 8.3.10(9) cm(-3), 2.4.10(9) cm(-3), 1.5.10(8) cm(-3), respectively. In total, 392 ide ntifications of OH in air spectra were made in a rural environment between August 5 and August 23, 1994. We present and discuss OH absorption spectra and diurnal OH concentration profiles of three days which are representativ e for measurements under different pollution conditions during POPCORN. The observed maximum and median OH radical concentrations are 1.3.10(7) OH/cm( 3) and 4.0.10(6) OH/cm(3), respectively. The measured diurnal variation of the OH concentration shows a good correlation with the primary formation re action of OH radicals which is the photolysis of ambient ozone. Deviations from this correlation in the morning and evening hours, when the OH concent ration is higher than expected from the ozone photolysis, demonstrate the i mportance of other photochemical HOx production pathways during POPCORN.