Tides, Winds, and Reactive Halogens A Study of Multiphase Chemistry in the Marine Boundary Layer

Abstract

Reactive halogens in the atmosphere (inorganic compounds containing chlorine, bromine, and/or iodine)have the ability to catalytically destroy ozone (O3) as well as react with NOx (= NO + NO2) , influencing concentrations and ratios of important tropospheric pollutants. They can also react with and change the ratio of OH and HO2 ( = HOx), altering the oxidative capacity of the atmosphere. Furthermore, heterogeneous reactions involving halogenated species can change the chemical composition of particulates, especially for sea-salt aerosols (SSAs). Reactive bromine (Bry) specifically is known to have a chemical mechanism unique from that of chlorine and iodine, and is more efficient in the catalytic destruction of ozone on a per-molecule basis compared to chlorine. There are generally fewer in-situ observations of Bry species because they are globally expected to be present at much lower concentrations compared to reactive chlorine. In this work we present a new transverse Ion-Molecule reaction Region, the so-called ”t-IMR”, for use with a Time-of-Flight Chemical Ionization Mass Spectrometer (ToF-CIMS). The t-IMR samples ambient air at a laminar high volume flow (at a rate of 10 L/min), and demonstrates a reduction in artificial signals/background from wall effects by multiple orders of magnitude across compound volatilities compared to previous low-pressure designs. The t-IMR utilizes an applied electric potential to accelerate ions and subsequent clusters across the IMR cavity, the strength of which is optimized to retain both high instrument sensitivity and total ion flux to the mass spectrometer. The t-IMR CIMS is calibrated directly to obtain sensitivity values for Br2 and (experimentally) for HOBr, the results of which confirm the use of theoretical ion cluster binding enthalpies to generate sensitivities for other Bry components. A dynamic water-vapor-dependent sensitivity for Br2 is also developed and applied. Five reactive bromine components are observed at the Tudor Hill Marine Atmospheric Observatory (THMAO) located in Bermuda as part of the Bermuda boundary Layer Experiment on the Atmospheric Chemistry of Halogens (BLEACH) campaign, including Br2, BrCl, BrO, HOBr, and for the first time to our knowledge, BrONO2. Local air masses originating from over the ocean, as well as those influenced by anthropogenic activity and pollution from the island of Bermuda, provide multiple environments in which to investigate Bry concentrations and partitioning. HOBr and BrONO2 are observed above detection limits almost exclusively during the day, evidencing the currently understood formation mechanisms for both which require active photochemistry and the presence of BrO. BrONO2 shows a clear dependence on NO2 concentrations, and a diurnal profile shape that qualitatively aligns with previous modeling studies. Comparison with the GEOS-Chem model output (with reactive halogen chemistry and SSA debromination mechanisms included) shows that the detailed atmospheric chemistry model over-predicts levels of the five observed Bry constituents, most of which by an order of magnitude or higher. The model is in much better agreement with observed bulk particulate sodium, bromine, and chlorine concentrations, suggesting that the SSA mass concentrations in the model are likely not the source of the discrepancy, but rather partitioning among reactive bromine. The relationship of total measured reactive bromine (Bry* = 2 Br2 + BrCl + BrO + HOBr + BrONO2) with local wind speed, tide height, and other measurements at THMAO is examined. The magnitude of the most recent low tide height and wind speed are found to be independently significant predictors of Bry* concentrations. This implies a local coastal source of reactive bromine at THMAO in addition to that expected from SSAs.