An Assessment of HOx Chemistry in the Tropical Pacific Boundary Layer: Comparison of Model Simulations with Observations Recorded during PEM Tropics A

Abstract

Reported are the results from a comparison of OH,H 2 O 2 CH 3 OOH, and O 3 observationswithmodel predictions based on current HO x –CH 4 reaction mechanisms. The field observations are thoserecorded during the NASA GTE field program, PEM-Tropics A. The major focus ofthis paper is on thosedata generated on the NASA P-3B aircraft during a mission flown in the marineboundary layer (MBL) nearChristmas Island, a site located in the central equatorial Pacific (i.e.,2° N, 157° W). Taking advantage of thestability of the southeastern trade-winds, an air parcel was sampled in aLagrangian mode over a significantfraction of a solar day. Analyses of these data revealed excellent agreementbetween model simulated andobserved OH. In addition, the model simulations reproduced the major featuresin the observed diurnalprofiles of H 2 O 2 and CH 3 OOH. In the case ofO 3 , the model captured the key observational feature whichinvolved an early morning maximum. An examination of the MBL HO x budget indicated that the O( 1 D) + H 2 Oreaction is the major source of HO x while the major sinks involveboth physical and chemical processes involving the peroxide species,H 2 O 2 and CH 3 OOH. Overall, the generally goodagreement between modeland observations suggests that our current understanding ofHO x –CH 4 chemistry in the tropical MBL isquite good; however, there remains a need to critically examine this chemistrywhen both CH 2 O and HO 2 are added to the species measured.