Latitudinal variation in the multiphase chemical processing of inorganic halogens and related species over the eastern North and South Atlantic Oceans

Abstract. Volatile inorganic and size-resolved particulate Cl- and Br-species were measured in near-surface air over a broad range of conditions within four distinct regimes (European – EURO, North African – N-AFR, the Intertropical Convergence Zone – ITCZ, and South Atlantic – S-ATL) along a latitudinal gradient from 51° N to 18° S through the eastern Atlantic Ocean. Median dry-deposition fluxes of sea salt, oxidized N, and oxidized non-sea-salt S varied by factors of 25, 17, and 9, respectively, among the regimes. Sea-salt production was the primary source for inorganic Cl and Br. Acidification and dechlorination of sea salt primarily by HNO₃ sustained HCl mixing ratios ranging from medians of 82 (ITCZ) to 682 (EURO) pmol mol⁻¹. Median aerosol pHs inferred from HCl phase partitioning with super-μm size fractions ranged from ~3.0 for EURO to ~4.5 for ITCZ. Because SO₂ solubility over this pH range was low, S(IV) oxidation by hypohalous acids was unimportant under most conditions. Simulations with a detailed multiphase box model indicated that BrCl photolysis and ClO + NO were the major sources for atomic Cl in all regimes. Simulated midday concentrations of Cl atoms ranged from 2.1×10⁴ to 7.8×10⁴ cm⁻³ in the ITCZ and N-AFR regimes, respectively. Measured particulate Br⁻ (median enrichment factor = 0.25) was greater and volatile inorganic Br less than simulated values, suggesting that the halogen activation mechanism in the model overestimated Br-radical production and processing. Reaction with atomic Br was an important sink for modeled O₃ (5% in EURO to 46% in N-AFR). Formation of halogen nitrates accelerated the oxidation of NO_x (NO + NO₂) primarily via hydrolysis reactions involving S aerosol. Relative to simulations with no halogens, lower NO_x coupled with direct reactions involving halogens yielded lower steady state mixing ratios of OH (20% to 54%) and O₃ (22% to 62%) and lower midday ratios of OH:HO₂ (3% to 32%) in all regimes.