Seasonal differences in the photochemistry of the South Pacific: A comparison of observations and model results from PEM‐Tropics A and B

A time‐dependent photochemical box model is used to examine the photochemistry of the equatorial and southern subtropical Pacific troposphere with aircraft data obtained during two distinct seasons: the Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A) field campaign in September and October of 1996 and the Pacific Exploratory Mission‐Tropics B (PEM‐Tropics B) campaign in March and April of 1999. Model‐predicted values were compared to observations for selected species (e.g., NO 2 , OH, HO 2 ) with generally good agreement. Predicted values of HO 2 were larger than those observed in the upper troposphere, in contrast to previous studies which show a general underprediction Of HO 2 at upper altitudes. Some characteristics of the budgets Of HO x , NO x , and peroxides are discussed. The integrated net tendency for O 3 is negative over the remote Pacific during both seasons, with gross formation equal to no more than half of the gross destruction. This suggests that a continual supply of O 3 into the Pacific region throughout the year must exist in order to maintain O 3 levels. Integrated net tendencies for equatorial O 3 showed a seasonality, with a net loss of 1.06×10 11 molecules cm −2 s −1 during PEM‐Tropics B (March) increasing by 50% to 1.60×10 11 molecules cm −2 s −1 during PEM‐Tropics A (September). The seasonality over the southern subtropical Pacific was somewhat lower, with losses of 1.21×10 11 molecules cm −2 s −1 during PEM‐Tropics B (March) increasing by 25% to 1.51×10 11 molecules cm −2 s −1 during PEM‐Tropics A (September). While the larger net losses during PEM‐Tropics A were primarily driven by higher concentrations of O 3 , the ability of the subtropical atmosphere to destroy O 3 was ∼30% less effective during the PEM‐Tropics A (September) campaign due to a drier atmosphere and higher overhead O 3 column amounts.