HYDROPEROXIDES IN THE MARINE TROPOSPHERE OVER THE ATLANTIC-OCEAN

Hydrogen peroxide (H2O2) and organic hydroperoxides (ROOH) were measur ed on board of the RV Polarstern during its cruise across the Atlantic Ocean from 20 October to 12 November 1990 (54 degrees N to 51 degrees S latitude) by the enzyme fluorometric method. The H2O2 mixing ratio varied from below the detection limit of about 0.12 ppbv up to 3.89 pp bv, showing a latitudinal dependence with generally higher values arou nd the equator and decreasing values poleward. The shape of the latitu dinal H2O2 distribution agrees well with an analytical steady state ex pression for H2O2 using the measured H2O and O-3 distribution and a wi nd dependent H2O2 deposition rate. The ROOH mixing ratio varied from b elow the detection limit of about 0.08 ppbv up to 1.25 ppbv with quali tatively the same latitudinal dependence as H2O2. The observed ratio R OOH/(ROOH + H2O2) varied between 0.17 and 0.98 showing higher values a t the lowest H2O2 mixing ratios at high latitudes. The measured H2O2 m ixing ratio shows a significant diurnal variation with a maximum aroun d 14:00 local time, explicable by a superposition of the photochemical H2O2 production with a constant H2O2 deposition rate. Four independen t estimations of the average effective H2O2 deposition rate inferred f rom the H2O2 decrease in the night, from the midday H2O2 production de ficit (as derived from comparison with a photochemical model and from the daily ozone loss), and from the offset in the latitudinal H2O2 dis tribution, were consistent. An episode of maximum H2O2 concentration s uggests the possibility of its formation in clouds.