Hydrogen peroxide and methylhydroperoxide were measured in the troposphere over the western North Pacific as part of the airborne portion of NASA's Global Tropospheric Experiment/Pacific Exploratory Mission-West A field mission. The flights circled the North Pacific, focusing on the western Pacific, and extended from 300 to 13,000 rn altitude. The hydroperoxides were uniquely separated and quantified using a high-pressure liquid chromatography system in conjunction with a continuous enzyme fluorometric instrument. Results show a latitudinal gradient in both peroxides at all altitudes; for example, between 3 and 5 km, H20 2 median values decrease from 1700 to 500 parts per trillion by volume (pptv) in going from 0ø-15øN to 45ø-60øN, and the corresponding decrease in CH3OOH was 1100 to 200 pptv. Concentration maxima are observed in both species at altitudes of 2 to 3 km with H20 2 concentrations below 1 km lower by 30%, 10% for CH3OOH, and even lower, by a factor of 10, for both above 9 km. The H20 2 to CH3OOH ratio increased with altitude and latitude with ratios <1 in the tropical surface layer and >2 at midlatitude high altitude. Highest peroxide concentrations were encountered over the Celebes Sea in air which was impacted by aged biomass fire and urban pollutants. CH3OOH was below the level of detection in stratospheric air. H20 2 exceeded SO2 95% of the time, with the exceptions generally above 9 km. Above 3 km, 03 increases with decreasing H20 2 and CH3OOH. Below 3 km the O3-CH3OOH trend is the same but 03 increases with increasing H20 2. The measurements are compared with predictions based upon a photochemical steady state zero-dimensional model and a three-dimensional mesoscale time-dependent model. These models capture observed trends in H20 2 and CH3OOH, with the possible exception of H20 2 below 2 km where surface removal is important. A surface removal lifetime of 3.5 days brings the observed and zero-dimensional model-estimated H20 2 into agreement. The steady state model suggests a strong correlation between the ratios of NO/CO or HO2/HO and the ratio of H202/CH3OOH. The observed hydroperoxide ratios bracket the modeled relationship with occasionally much lower H20 2 than expected. , which was a component of the IGAC-APARE program. The NASA GTE scientific program and the PEM-West A experiment are described in detail by Hoell et al. [this issue]. In summary, the goals of the mission were twofold: (1) to improve our knowledge of tropospheric ozone over the western Pacific Ocean through an exploration of atmospheric composition and dynamics and (2) to elucidate the chemistry of sulfur gases in this region. The meteorological overview for the experiment is presented by Bachmeier et al. [this issue] and the encountered synoptic conditions and transport paths were considered typical for this region in this season [Merrill, this issue]. A total of 18 research flights circling the North Pacific were conducted, spanning 0 ø to 65øN latitude, 115øE to 125øW longitude, and 0 to 13 km altitude. The flights were ...

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