Long‐term and seasonal variations in the levels of hydrogen peroxide, methylhydroperoxide, and selected compounds over the Pacific Ocean

The National Aeronautics and Space Administration's Global Tropospheric Experiment program has conducted five sampling programs examining the Pacific troposphere from 1991 to 2001. Flights on two aircraft, a P3‐B and DC‐8, sampled the Pacific troposphere at altitudes from 0.5 km to 13 km and from 130°E to 115°W and from 65°N to 70°S. Long‐term trends in the observed species mixing ratios in different geographic regions are presented. Changes in species mixing ratios are discussed with respect to changes in source, transport, and photochemical production and loss processes over the decade study period. CH 4 and CO 2 exhibited average annual mixing ratio increases of 6.39 ppbv yr −1 and 1.92 ppmv yr −1 , respectively. O 3 , CO, and NO all exhibit significantly higher mixing ratios in the north Pacific troposphere in the spring than in the fall. However, there is no clear annual trend for these species. H 2 O 2 mixing ratios were 38 ± 16% higher (Δ(fall–spring) 258 ± 34 pptv), and CH 3 OOH was 61 ± 15% higher (Δ343 ± 76 pptv) throughout the Pacific troposphere in the fall than in the spring. During 2001, SO 2 loading in the North Pacific is 2–3 times larger than during earlier years. Anthropogenic tracer gases, C 2 H 6 , C 3 H 8 , C 2 H 2 , and C 2 Cl 4 , do not exhibit comparable increases in their average mixing ratios during the decade study period. CH 3 I, a tracer for oceanic emission, also does not exhibit an increase in 2001. Sulfur loading characterized by SO 2 also does not exhibit a clear annual trend. The enhanced sulfur loading during 2001 appears to have resulted in a suppression of H 2 O 2 mixing ratios in the lower troposphere throughout the midlatitude North Pacific.