Distribution of hydrogen peroxide and methylhydroperoxide over the Pacific and South Atlantic Oceans

The gas phase hydrogen peroxide and methylhydroperoxide concentrations were measured in the troposphere over the tropical Pacific Ocean as a component of NASA's Global Tropospheric Experiment/Pacific Exploratory Mission-Tropics A field campaign. Flights on two aircraft covered the Pacific from 70 degrees S to 60 degrees N and 110 degrees E to 80 degrees W and South Atlantic from 40 degrees S to 15 degrees N and 45 degrees W to 70 degrees E, and extending from 76 to 13,000 m altitude. H2O2 and CH3OOH have the highest concentrations at a given altitude at the equator and decrease with increasing latitude in both the northern and southern hemispheres. Above 4 km the gradient is substantially reduced for both H2O2 and CH3OOH with latitude, and at altitudes in excess of 8 km there is no latitudinal dependence. H2O2 and CH3OOH exhibit maximum mixing ratios between 1 and 2 km at all latitudes. The mean mixing ratio of H2O2 at the equator was 1600 +/- 600 parts per trillion by volume (pptv) decreasing to 500 +/- 250 pptv at latitudes greater than 550 north and south between 1 and 2 km altitude. CH3OOH at the equator was 1400 +/- 250 pptv, decreasing to 330 +/- 200 pptv at high latitudes at altitudes between 1 and 2 km. The concentration of peroxides at high latitudes in the northern hemisphere was generally a factor of 2 higher than at corresponding latitudes in the southern hemisphere. The ratio of H2O2 to CH3OOH was between 1 and 2 from 45 degrees S to 35 degrees N at altitudes below 4 km. Between 5 degrees to 15 degrees N the ratio is less than 1, due to preferential removal of H2O2 in the Intertropical Convergence Zone.