Abstract
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°N, 157°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 ∼1300 local solar time. Our empirical model includes in situ observations of hydroxyl [HO] and precise measurements of ethane (C2H6) mixing ratios. Ethane was ∼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) × 104 Cl cm−3, decreasing to 5.7 (±2.0) × 104 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.
Highlights
[2] Atomic chlorine (Cl), a radical species, is a powerful oxidant reacting with many volatile organic compounds at rates approaching the collisional limit
Because much of the Cl in the atmosphere originates from reactions involving sea salt aerosol [e.g., Rossi, 2003; Finlayson-Pitts, 2003], Cl is expected to be at its highest concentration in the marine boundary layer (MBL) [Singh and Kasting, 1988; Platt et al, 2004]
Oftentimes mixing must be parameterized in a Lagrangian reference frame to obtain meaningful results [Wingenter et al, 1996; Davis et al, 1999].) The difference between this dimethyl sulfide (DMS) sea-air flux estimate and that made using a micro-meteorological approach [Lenschow et al, 1999] can be resolved if loss of DMS by BrO oxidation is included in a mass balance equation
Summary
[2] Atomic chlorine (Cl), a radical species, is a powerful oxidant reacting with many volatile organic compounds at rates approaching the collisional limit. [9] Chlorine chemistry can initiate increased radical chemistry in coastal urban areas, such as southern California and Houston, Texas [Knipping and Dabdub, 2003; Tanaka et al, 2003], and causes greater oxidation of volatile organic carbon species (VOCs) This can result in an increased rate of formation of two chief urban pollutants, ozone and organic aerosol. [11] In this paper we employ time series observations of C2H6 and in situ measurements of HO concentrations made during a Lagrangian flight to estimate the [Cl] time series and DMS air-sea flux using empirical mass balance approaches. Oftentimes mixing must be parameterized in a Lagrangian reference frame to obtain meaningful results [Wingenter et al, 1996; Davis et al, 1999].) The difference between this DMS sea-air flux estimate and that made using a micro-meteorological approach [Lenschow et al, 1999] can be resolved if loss of DMS by BrO oxidation is included in a mass balance equation. The bimolecular rate constants for the reactions of Cl or HO with C2H6 [Sander et al, 2002] are referred to
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