An assessment of ozone photochemistry in the central/eastern North Pacific as determined from multiyear airborne field studies



[1] In this study, the photochemistry of ozone is examined for the central/eastern North Pacific (CENP), a region bounded by 180°–120°W and 0°–45°N. Measurements of ozone precursors have been made in this region during 10 previous airborne studies covering the last two decades. The two seasons for which the most extensive analysis has been possible are spring (March–May) and fall (September–November). Box model results have been displayed in the form of latitudinal/altitudinal plots for ozone formation, destruction, and net tendency. They indicate that for both seasons of the year, significant net destruction is found for altitudes in the 0–4 km range; whereas, at higher altitudes, the ozone tendency calculations lead to small values of net production. For both seasons, however, the total column integrated ozone trend is one of net destruction. The largest difference between seasons was in the value for ozone destruction. During springtime, this value was significantly larger than for fall. This trend coincides with higher average springtime ozone levels. The trends in CENP ozone were also compared to those for the western Pacific using measurements recorded during PEM-West A (fall 1991) and PEM-West B (spring 1994). While the PEM-West A results revealed a neutral ozone column tendency, those from PEM-West B showed net production at all altitudes. Both results are in sharp contrast to those reported here for the CENP region showing net destruction. This suggests that while lightning and surface emissions of NOx from the northwestern Pacific Rim have a strong influence on the ozone tendency in the surrounding region, due to the combined effects of dispersion and chemical loss most of this NOx does not reach the CENP region. Hence, ozone in the central Pacific is typically destroyed by photochemistry.