OH in the coastal boundary layer of Crete during MINOS: Measurements and relationship with ozone photolysis
1German Weather Service, Meteorological Observatory Hohenpeissenberg, Germany
2University of Crete, Environmental Chemical Processes Laboratory, Heraklion, Greece
3Research Center Jülich, Institute for Chemistry and Dynamics of the Geosphere II, Jülich, Germany
Abstract. Hydroxyl radical (OH) concentrations were measured in August 2001 at Finokalia Station on the northeastern coast of Crete during the Mediterranean Intensive Oxidant Study (MINOS). OH was measured based on selected ion chemical ionization mass spectrometry (SI/CIMS) with a time resolution of 30 s and signal integration of 5 min. The corresponding accuracy, precision, and detection limit were 20% (1s), 11% (1s), and 2.4×105 molecules cm−3 (2 s), respectively. OH levels showed a strong diurnal variability with high maxima (approximately 2×107 molecules cm−3) occurring around 13:30 LT (10:30 UTC) and nighttime values below the detection limit. Daily 24-hour average concentrations varied between 3.6–6.7×1016 cm−3. For the total measurement period (6–21 August) the mean and standard deviation were 4.5±1.1×106 cm−3. The OH data set is analyzed based on a classification into three periods: I: 6–8 August, II: 9–11 August, III: 13–18 August. For each of the three periods the measured OH concentrations are described by the empirical function [OH] = a J(O1D)0.68, with J(O1D) being the ozone photolysis frequency and a = 1.4×1010 s cm−3, 1.7×1010 s cm−3, and 2.2×1010 s cm−3, respectively. It is shown that this relationship is consistent with a CH4-CO box model yielding a corresponding exponent of 0.70. Taking into account the estimated precision of the OH measurements this empirical function explains 99% of the observed variance of OH.