An assessment of HOx chemistry in the tropical Pacific boundary layer:: Comparison of model simulations with observations recorded during PEM tropics A

Reported are the results from a comparison of OH, H2O2CH3OOH, and O-3 observations with model predictions based on current HOx-CH4 reaction mechanisms. The field observations are those recorded during the NASA GTE field program, PEM-Tropics A. The major focus of this paper is on those data generated on the NASA P-3B aircraft during a mission flown in the marine boundary layer (MBL) near Christmas Island, a site located in the central equatorial Pacific (i.e., 2 degrees N, 157 degrees W). Taking advantage of the stability of the southeastern trade-winds, an air parcel was sampled in a Lagrangian mode over a significant fraction of a solar day. Analyses of these data revealed excellent agreement between model simulated and observed OH. In addition, the model simulations reproduced the major features in the observed diurnal profiles of H2O2 and CH3OOH. In the case of O-3, the model captured the key observational feature which involved an early morning maximum. An examination of the MBL HOx budget indicated that the O(D-1) + H2O reaction is the major source of HOx while the major sinks involve both physical and chemical processes involving the peroxide species, H2O2 and CH3OOH. Overall, the generally good agreement between model and observations suggests that our current understanding of HOx-CH4 chemistry in the tropical MBL is quite good; however, there remains a need to critically examine this chemistry when both CH2O and HO2 are added to the species measured.