OH radicals in the boundary layer of the Atlantic Ocean 1. Measurements bylong-path laser absorption spectroscopy

Knowing the concentration of hydroxyl (OH) radicals is most important for t he understanding of the chemical processes in the troposphere. This paper d escribes the first direct measurements of OH in the boundary layer of the t ropical Atlantic Ocean. The use of Differential Optical Absorption Spectros copy; provided direct measurements of OH with a calibration uncertainty of 6%. The 1-sigma precision of the OH data was in the range of (1-4) x 10(6) cm(-3) because of the exceptional experimental conditions encountered on th e ship. On 10 measurement days we collected a total of 483 OH concentration data between 5 degreesN and 40 degreesS. Careful analysis was applied to s elect data not affected by the ship and its exhaust. The selected data (N = 238) exhibit diurnal profiles with maxima around 7 x 10(6) cm(-3) for over head Sun and clean air conditions. On average the measured OK concentration s are 16% higher than corresponding box model calculations based on simulta neously measured trace gas concentrations and photolysis frequencies. The d eviation from the 1:1 relation, however, is covered by the combined calibra tion errors of OH, CO, and the photolysis frequencies.