[1] During summer 2002, soluble trace gases, the ionic composition of size-resolved aerosols, and meteorological conditions were measured from the National Oceanic and Atmospheric Administration ship Ronald H. Brown in coastal air along the eastern U.S. seaboard. Aerosol acidities were extrapolated from direct pH measurements in minimally diluted extracts and were also inferred from the measured phase partitioning and thermodynamic properties of HCl, HNO 3 , NH 3 , HCOOH, and CH 3 COOH. Median pHs for 0.75-through 25-mm geometric mean diameter (GMD) size fractions based on direct measurements (2.6-3.9) were higher than those inferred from HCl partitioning (1.7-3.3). Phase disequilibria caused negative deviations in median pHs inferred from HNO 3 partitioning with aerosol size fractions greater than 2.8-mm GMD; pHs inferred for smaller size fractions (median values of 1.9-3.0) were similar to those based on HCl (median values of 1.5-3.0). The pHs inferred from NH 3 partitioning were significantly lower than those estimated by other approaches; causes for this apparent bias are not known. The dominance of gas-phase HCOOH and CH 3 COOH was generally consistent with predicted phase partitioning with acidic aerosols. Typically large diel excursions in most gases implied corresponding variability in aerosol acidity. The pHs inferred from maximum and minimum mixing ratios of gases over each aerosol sampling interval suggested median 12-hour variations of $0.4-0.7 pH unit. Total acidity (H t = H + + undissociated acids) was greater than H + by 1-2 orders of magnitude in all size fractions; most H t was in the form of HSO 4 À .

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 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 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.

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