title: Competition between organics and bromide at the aqueous solution-air interface as seen from ozone uptake kinetics and X-ray photoelectron spectroscopy.
abstract: A more detailed understanding of the heterogeneous chemistry of halogenated species in the marine boundary layer is required. Here, we studied the reaction of ozone (O3) with NaBr solutions in the presence and absence of citric acid (C6H8O7) under ambient conditions. Citric acid is used as a proxy for oxidized organic material present at the ocean surface or in sea spray aerosol. On neat NaBr solutions, the observed kinetics is consistent with bulk reaction-limited uptake, and a second-order rate constant for the reaction of O3 + Br(-) is 57 ± 10 M(-1) s(-1). On mixed NaBr-citric acid aqueous solutions, the uptake kinetics was faster than that predicted by bulk reaction-limited uptake and also faster than expected based on an acid-catalyzed mechanism. X-ray photoelectron spectroscopy (XPS) on a liquid microjet of the same solutions at 1.0 × 10(-3)-1.0 × 10(-4) mbar was used to obtain quantitative insight into the interfacial composition relative to that of the bulk solutions. It revealed that the bromide anion becomes depleted by 30 ± 10% while the sodium cation gets enhanced by 40 ± 20% at the aqueous solution-air interface of a 0.12 M NaBr solution mixed with 2.5 M citric acid in the bulk, attributed to the role of citric acid as a weak surfactant. Therefore, the enhanced reactivity of bromide solutions observed in the presence of citric acid is not necessarily attributable to a surface reaction but could also result from an increased solubility of ozone at higher citric acid concentrations. Whether the acid-catalyzed chemistry may have a larger effect on the surface than in the bulk to offset the effect of bromide depletion also remains open.
authors.affiliation: †Laboratory of Radiochemistry and Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland.|‡Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland.
authors.forename: Ming-Tao
authors.initials: MT
authors.lastname: Lee
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Brown
authors.affiliation: †Laboratory of Radiochemistry and Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland.
authors.forename: Shunsuke
authors.initials: S
authors.lastname: Kato
authors.affiliation: †Laboratory of Radiochemistry and Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland.
authors.forename: Armin
authors.initials: A
authors.lastname: Kleibert
authors.affiliation: †Laboratory of Radiochemistry and Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland.|‡Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland.
authors.forename: Andreas
authors.initials: A
authors.lastname: Türler
authors.affiliation: †Laboratory of Radiochemistry and Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland.
authors.forename: Markus
authors.initials: M
authors.lastname: Ammann
mesh_terms: D000388:Air; D001965:Bromides / Q000737:chemistry*; D019343:Citric Acid / Q000737:chemistry; D007700:Kinetics; D009792:Oceans and Seas; D010126:Ozone / Q000737:chemistry*; D056951:Photoelectron Spectroscopy; D017670:Sodium Compounds / Q000737:chemistry*; D012996:Solutions; D013499:Surface Properties; D013501:Surface-Active Agents / Q000737:chemistry; D014867:Water / Q000737:chemistry*
chemical_list: D001965:Bromides; D017670:Sodium Compounds; D012996:Solutions; D013501:Surface-Active Agents; D014867:Water; D019343:Citric Acid; D010126:Ozone; C027938:sodium bromide