Atomic chlorine concentrations derived from ethane and hydroxyl measurements over the equatorial Pacific Ocean: Implication for dimethyl sulfide and bromine monoxide

Atomic chlorine and bromine monoxide concentrations ([Cl] and [BrO]) and dimethyl sulfide (DMS) sea-air fluxes are estimated from data collected during a Lagrangian flight made near Christmas Island (2 degrees N, 157 degrees W) during August 1996 aboard the NASA P3-B aircraft. Intensive hydrocarbon sampling began in the surface layer (SL) one-half hour after sunrise and continued until similar to 1300 local solar time. Our empirical model includes in situ observations of hydroxyl [HO] and precise measurements of ethane (C2H6) mixing ratios. Ethane was similar to 40 pptv higher in the buffer layer (BuL) than in the SL, thus vertical exchange tended to replace any C2H6 that was photochemically removed in the SL. In spite of this, SL C2H6 mixing ratios decreased significantly during the flight. Using only the measured [HO] and estimated vertical mixing, our mass balance equation cannot explain all of the observed SL C2H6 loss. However, when an initial [Cl] of 8.4 (+/-2.0) x 10(4) Cl cm(-3), decreasing to 5.7 (+/-2.0) x 10(4) Cl cm(-3) at midday, is included, the observed and estimated C2H6 values are in excellent agreement. Using our [Cl], we estimate a DMS flux a factor of 2 higher than when HO is the only oxidant considered. This flux estimate, when compared to that derived by Lenschow et al. (1999), suggests that if the differences are real, we may be missing a loss term. Best agreement occurs when an average BrO mixing ratio of 1.3 (+/-1.3) pptv is included in our mass balance equation.