HYDROGEN-PEROXIDE AND METHYLHYDROPEROXIDE DISTRIBUTIONS RELATED TO OZONE AND ODD HYDROGEN OVER THE NORTH PACIFIC IN THE FALL OF 1991

Hydrogen peroxide and methylhydroperoxide were measured in the troposp here over the western North Pacific as part of the airborne portion of NASA's Global Tropospheric Experiment/Pacific Exploratory Mission-Wes t A field mission. The flights circled the North Pacific, focusing on the western Pacific, and extended from 300 to 13,000 m altitude. The h ydroperoxides were uniquely separated and quantified using a high-pres sure liquid chromatography system in conjunction with a continuous enz yme fluorometric instrument. Results show a latitudinal gradient in bo th peroxides at all altitudes; for example, between 3 and 5 km, H2O2 m edian values decrease from 1700 to 500 parts per trillion by volume (p ptv) in going from 0 degrees-15 degrees N to 45 degrees-60 degrees N, and the corresponding decrease in CH3OOH was 1100 to 200 pptv. Concent ration maxima are observed in both species at altitudes of 2 to 3 km w ith H2O2 concentrations below 1 km lower by 30%, 10% for CH3OOH, and e ven lower, by a factor of 10, for both above 9 km. The H2O2 to CH3OOH ratio increased with altitude and latitude with ratios (1 in the tropi cal surface layer and >2 at midlatitude high altitude. Highest peroxid e concentrations were encountered over the Celebes Sea in air which wa s impacted by aged biomass fire and urban pollutants. CH3OOH was below the level of detection in stratospheric air, H2O2 exceeded SO2 95% of the time, with the exceptions generally above 9 km. Above 3 km, O-3 i ncreases with decreasing H2O2 and CH3OOH. Below 3 km the O-3-CH3OOH tr end is the same but O-3 increases with increasing H2O2. The measuremen ts are compared with predictions based upon a photochemical steady sta te zero-dimensional model and a three-dimensional mesoscale time-depen dent model. These models capture observed trends in H2O2 and CH3OOH, w ith the possible exception of H2O2 below 2 km where surface removal is important. A surface removal lifetime of 3.5 days brings the observed and zero-dimensional model-estimated H2O2 into agreement. The steady state model suggests a strong correlation between the ratios of NO/CO or HO2/HO and the ratio of H2O2/CH3OOH. The observed hydroperoxide rat ios bracket the modeled relationship with occasionally much lower H2O2 than expected.