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

Abstract

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. Processes within each regime were interpreted in conjunction with box-model calculations. 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. Aerosol pHs inferred from HCl phase partitioning with super-μm size fractions (~3 for EURO to the low 4 s for ITCZ) were similar to modeled values. Within all regimes, the dominant sources for atomic Cl were BrCl photolysis and ClO+NO. Maxima in atomic Cl ranged from 2.1 to 7.8×104 cm−3 in the ITCZ and N-AFR regimes, respectively. Because SO2 solubility over the aerosol pH range was low, S(IV) oxidation by hypohalous acids was unimportant under most conditions. Measured particulate Br− (median enrichment factor=0.25) was greater and volatile inorganic Br less than simulated values. Reaction with atomic Br was an important sink for O3 (5% in EURO to 46% in N-AFR). Formation of halogen nitrates accelerated the oxidation of NOx (NO+NO2) primarily via hydrolysis reactions involving S aerosol. Relative to simulations with no halogens, lower NOx coupled with direct reactions involving halogens yielded lower OH (by 20% to 54%) in all regimes. Halogen chemistry resulted in net O3 destruction and steady-state mixing ratios that were lower by 22% (EURO) to 62% (N-AFR) relative to "no-halogen" runs.