A41G-01 INVITED
Halogen Chemistry at North American Coastal Sites
In recent years observational evidence has emerged that reactive halogen species (RHS), such as chlorine atoms, and bromine and iodine oxides, are present in coastal areas. Their chemistry can be significant as they catalytically destroy O3; oxidize hydrocarbons, dimethylsulfide, and S(IV); and modify NOx and HOx cycling. Despite their potential importance our observational database on RHS is still very limited. Most observations of RHS thus far have been made in clean areas and very few observations along the North American coast have been made. Here we will review our current understanding of RHS chemistry in both clean and polluted environments. Recent observations at coastal areas around the world will be discussed. We will also give an overview of an experiment performed by our group in Malibu, CA in October 2006 and present initial results. A suite of trace gases and environmental parameters, including halogen molecules, halogen oxides, Cl + VOC reaction products, aerosol composition, O3, NOx, CO, VOCs, meteorology, and radiation, were measured during a three week period. In addition, Cl + VOC reaction products were measured at two locations in urban Los Angeles. Clear evidence for the presence of various halogen species on the California coast was found. Observations during periods with relatively clean marine air and during times where our site was in the outflow of Los Angeles show the impact of pollution on coastal atmospheric chemistry. Our observations will be compared to earlier studies of halogen chemistry at coastal areas to further advance our understanding of halogen chemistry.
A41G-02 INVITED
Iodine: The missing halogen of polar tropospheric chemistry
Iodine is important in the Earth's troposphere for several reasons, including the catalytic destruction of ozone, altering the oxidizing capacity, and the formation of ultra-fine aerosol, all with potential impacts on climate. Nevertheless, the role of iodine chemistry has so far remained virtually unconsidered in studies of the polar troposphere. Only recently, the occurrence of high concentrations of IO over vast sea-ice covered areas of the Southern Ocean and around coastal Antarctica has been reported. The sources of such a large iodine burden in the Antarctic atmosphere remain unknown. This talk will address ground- and satellite-based observations of the IO radical over coastal Antarctica. Also, it presents a novel mechanism for biologically-induced iodine release from sea-ice surfaces that initiates an autocatalytic cycle, termed iodine explosion. Numerical modeling studies on the impact of iodine upon the chemistry of the Antarctic troposphere will also be presented.
A41G-03 INVITED
Sea-air Fluxes of Organic Iodine Compounds and Evidence for Their Large-Scale Effects in the Tropical Marine Boundary Layer
Whilst it is now clear that iodine has an important atmospheric impact in coastal regions, particularly as a precursor to new aerosol particles, the global impact is as yet unknown. This is mainly a result of the paucity of observations over the open ocean. Here we present sea-air fluxes of the organic iodine compounds CH3I, C2H5I, 1-C3H7I, CH2ICl, CH2IBr and CH2I2 calculated from simultaneous surface seawater and air measurements in/over the mid-latitude and tropical eastern North Atlantic Ocean during summer 2006 and 2007. Average combined dihalomethane (CH2IX where X is I, Br, Cl) fluxes were around 40 nmol m-2d-1 in the mid-latitude and tropical eastern North Atlantic and reached up to 120 nmol m-2d-1 in upwelling regions. Globally, the dihalomethane fluxes are extrapolated to be a factor of about 2 higher than those of CH3I alone. A box model incorporating gas phase reactions and reactions in and on sea salt aerosol, driven by the measured open ocean organic iodine fluxes, predicts a daytime maximum of around 1 pmol/mol of the iodine oxide radical (IO) in the tropical MBL. Evidence for the large-scale effects of such halogen chemistry is apparent in the trace gas measurements at the Cape Verde Atmospheric Observatory in the tropical eastern North Atlantic Ocean. Measurements of ozone from October 2006 to October 2007 reveal daily net photochemical destruction averaging 3.2 nmol/mol between 9am and 5pm; aircraft measurements in summer show that these destruction rates are sustained throughout the boundary layer. Photochemical losses of ozone caused by photolysis in the presence of water vapour combined with reactions of ozone with OH and HO2 (together, "odd-hydrogen" losses) are sufficient to explain only 55% of the photochemical ozone destruction. However, model calculations show that the observed photochemical losses can be simulated by including halogen-catalysed ozone destruction reactions, assuming 1 pmol/mol IO combined with BrO concentrations of a few pmol/mol (midday maxima).
A41G-04
Atmospheric Oxidation of Iodinated Hydrocarbons
Iodinated hydrocarbons are emitted from natural, mostly oceanic, sources. The high fluxes of these species (dominated by methyl iodide, but including larger species as well) and their short photochemical lifetime imply potentially significant impacts on the chemistry of the marine boundary layer. Recently, we have employed a variety of experimental and theoretical methods to study the oxidation of organic iodides under atmospheric conditions. Compounds studied include a model compound, CF3CH2I, as well as ethyl and propyl iodides. While our studies have generally involved Cl-atom initiated processes, many of the results can be generalized to OH-initiated attack. General concepts to be discussed, in the context of the atmospheric behavior of these species, include: 1) The rates of destruction of iodinated organics via reaction with Cl-atom, and a comparison with other loss processes; 2) Oxidation pathways and end-product distributions, with a focus on the formation of alkenes from decomposition of beta-iodoalkyl radicals; and the mechanism of the reaction of alpha-iodoalkyl radicals with molecular oxygen; and 3) The reversible formation of Cl / iodoalkane adducts, and the subsequent chemistry of these species.
A41G-05
Enhanced Surface Photochemistry in Chloride-Nitrate Ion Aerosol Mixtures
Heterogeneous reactions of sea salt aerosol with various oxides of nitrogen often lead to replacement of chloride ion by nitrate ion. Photolysis of aqueous nitrate at wavelengths > 290 nm leads to the production of nitrogen dioxide, hydroxyl radicals and other oxidants. The photochemistry of aerosols containing NO3- and Cl-, however, has not been investigated. Aerosols containing mixtures of NaCl and NaNO3 were used as a model system for processed sea salt aerosol. Photolysis experiments (λmax = 306 nm) were performed in 55 L Teflon chambers that contained deliquesced aerosols and production of gas phase NO2 was measured as a function of time using chemiluminescence detection. Simple alkanes were added in some experiments to probe OH radical and Cl atom production. Molecular dynamics simulations were also carried out on mixed aqueous NaCl and NaNO3 slabs to help understand ion solvation in these mixed salt systems. The simulations showed that as the Cl- to NO3- ratio increases, nitrate ions are drawn toward the interface due to the large double layer of interfacial Cl- and subsurface Na+. The systems explored both experimentally and computationally included pure NaNO3 and mixtures of Cl- and NO3- in molar ratios of 1:9, 1:1, and 9:1. The photolysis experiments showed an enhanced yield of gas phase NO2 as the chloride to nitrate ratio increased. We attribute the observed enhanced NO2 yields to an increased interfacial concentration of under-coordinated nitrate ion as the ratio of Cl- to NO3- increases. The implications of these enhanced NO2 yields as sea salt aerosols become processed in the atmosphere will also be discussed.
A41G-06
Direct Experimental Evidence for a Heterogeneous Reaction of Ozone With Bromide at the Air- Aqueous Interface
An order of magnitude discrepancy between measured and predicted Br2 production in the reaction of ozone with deliquesced NaBr aerosol, combined with recent experimental and theoretical work indicating an enhancement of bromide anions at the air-aqueous interface have led to the suggestion that an interface reaction occurs between ozone and bromide. We have used an interface-sensitive fluorescent probe to measure pH changes associated with the interfacial reaction of ozone and bromide. The rate of pH change is well described by a Langmuir-Hinshelwood surface-mediated kinetic model. When octanol is present at the interface, the rate of pH change tracks the octanol adsorption isotherm, as expected if octanol enhances the concentration of ozone at the surface, as is known from previous work. These observations constitute direct evidence for a surface reaction between ozone and bromide anions.
A41G-07
Chlorocarbon Fluxes in Coastal and Upland Ecosystems of California
Anthropogenic chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) account for ~ 70% of the organic chlorine in the global atmosphere, with CH3Cl (methyl chloride), CHCl3 (chloroform), CCl4 (carbon tetrachloride) and CH3CCl3 (methyl chloroform) supplying most of the balance. Since 1994, the total atmospheric chlorine burden has been decreasing, owing in large part to the declining concentrations of CH3CCl3 and CCl4, two chlorocarbons regulated by the Montreal Protocol. The primary sink of all four compounds is attributed to destruction processes in the atmosphere (oxidation or photodissociation). However, several recent studies have reported a potentially significant terrestrial sink for CH3Cl, CCl4 and CH3CCl3. Particularly surprising is the report that coastal salt marshes in China appear to be net sinks for all four of these chlorocarbons. If these sinks are indeed significant relative to atmospheric destruction processes, then their estimated lifetimes would need to be reduced and their source and sink budgets reassessed. In this study, we report net fluxes of CHCl3, CCl4, and CH3CCl3 from a variety of southern California ecosystems, including coast sagebrush, chamise chaparral, creosote bush scrub, shoreline, and coastal salt marsh. 75 flux chamber measurements were conducted between 1997 and 2000. We find no evidence of a significant soil sink in these ecosystems but rather a small net source of CHCl3 and CCl4. Meanwhile, previously reported CH3Cl fluxes from these ecosystems show that coastal salt marshes are large sources of this compound while shrublands act as either a net source or sink, depending on predominant vegetation, soil conditions and season. To address the possibility of simultaneous production and consumption of CH3Cl in salt marshes, we employed a stable isotope tracer technique at a northern California salt marsh during the spring of 2007. We measured gross consumption rates of CH3Cl, but gross production rates were much greater at all sites, resulting in large net emissions overall. We suggest that salt marshes are typically net sources of CH3Cl and CHCl3 but may become net sinks if ambient concentrations of these compounds are unusually high, as reported from the salt marshes in China.