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 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 m 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, H 2 O 2 median values decrease from 1700 to 500 parts per trillion by volume (pptv) in going from 0°–15°N to 45°–60°N, and the corresponding decrease in CH 3 OOH was 1100 to 200 pptv. Concentration maxima are observed in both species at altitudes of 2 to 3 km with H 2 O 2 concentrations below 1 km lower by 30%, 10% for CH 3 OOH, and even lower, by a factor of 10, for both above 9 km. The H 2 O 2 to CH 3 OOH 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. CH 3 OOH was below the level of detection in stratospheric air. H 2 O 2 exceeded SO 2 95% of the time, with the exceptions generally above 9 km. Above 3 km, O 3 increases with decreasing H 2 O 2 and CH 3 OOH. Below 3 km the O 3 ‐CH 3 OOH trend is the same but O 3 increases with increasing H 2 O 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 H 2 O 2 and CH 3 OOH, with the possible exception of H 2 O 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 H 2 O 2 into agreement. The steady state model suggests a strong correlation between the ratios of NO/CO or HO 2 /HO and the ratio of H 2 O 2 /CH 3 OOH. The observed hydroperoxide ratios bracket the modeled relationship with occasionally much lower H 2 O 2 than expected.