Chemistry of Halogens at the Isles of Shoals (CHAiOS)
The Chemistry of Halogens at the Isles of Shoals (CHAiOS) study, conducted at the AIRMAP site on Appledore Island, Maine, evaluated the influence of halogen radicals on the chemical evolution of pollutant outflow along the New England coast. Funded by the National Science Foundation's Atmospheric Chemistry Program, CHAiOS was a component of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) mega-campaign during which several separate field programs intensively studied the photochemical, heterogeneous chemical and radiative environment of the troposphere over North America, the North Atlantic Ocean, and western Europe during July and August 2004. The major findings of CHAiOS are:
Maps and photo showing the location of the Isles of Shoals and Appledore Island, approximately 10 kilometers off the coast of New Hampshire. Sampling was conducted from the roof a WW II era coastal surveillance tower on the grounds of the Shoals Marine Laboratory. The tower is the tallest structure on the island. (click to enlarge)
Fischer et al. [2006] investigated the behavior of nitric acid gas and aerosol nitrate in relation to air mass transport history and local meteorology. (While these species are end products of nitrogen oxide chemistry, which usually controls ozone production in polluted air, interest in these species is motivated mainly by their being "fixed" forms of nitrogen that become nutrients in the marine food web after being deposited to the ocean surface.) Peak nitric acid and super-µm nitrate concentrations were associated with westerly and southwesterly flow regimes, respectively. Although total nitrate (nitric acid plus nitrate) concentrations were higher under westerly flow, higher median dry-deposition rates of total nitrate were associated with southwesterly flow. Sea-salt concentrations were three times greater during southwesterly flow when wind speeds tended to be greater; this shifted the phase partitioning toward aerosol nitrate. Consequently, under westerly flow, nitric acid deposited more quickly than aerosol nitrate, while for southwesterly flow, the fluxes from the two phases were comparable. Sea salt therefore exerted an important influence on fixed nitrogen deposition to the coastal ocean surface.
Keene et al. [2007] found that production from sea salt was the primary source for inorganic chlorine and bromine species in the atmosphere even though sea-salt mass averaged four to eight times lower than that typically observed over the open North Atlantic Ocean.
The WW II coastal surveillance tower on Appledore Island from which air was sampled during CHAIOS. The roof elevation is approximately 40 meters above mean sea level. (click to enlarge)
Pechtl and von Glasow [2007] used a numerical one-dimensional model to show that continental pollution that is advected over the ocean leads to substantial hydrogen chloride (HCl) acid displacement from newly emitted sea salt aerosol. Therefore the recirculation of continental airmasses over the ocean and back to the coast might explain previous measurements of high nighttime levels of molecular chlorine (Cl2) in onshore winds. The model runs also predicted chlorine atom concentrations similar to those estimated by Keene et al. (see above) and Pszenny et al. (see below), supporting the case for a substantial contribution of daytime Cl-radical chemistry to the oxidation of volatile organic compounds under conditions representative of the CHAiOS field campaign.
Pikelnaya et al. [2007] identified an aerosol layer between 1 and 2 km altitude, illustrating the potential for aerosol remote sensing by Multi-axis Differential Optical Absorption Spectroscopy (MAX-DOAS). Comparison with Long-Path (LP) DOAS measurements performed simultaneously showed that MAX-DOAS accurately measures trace gases that are well mixed within the boundary layer. MAX-DOAS measurements also provided valuable information on the vertical distribution of pollutants in and above the boundary layer, as demonstrated by the identification of elevated formaldehyde levels in a layer between 1 km and 2 km altitude.
Pszenny et al. [2007] estimated chlorine atom concentrations from variability-lifetime relationships for selected non-methane hydrocarbons and inferred that chlorine atoms enhanced hydrocarbon oxidation – an important early step in ozone production in polluted air – by 15-30% over that due hydroxyl radical.
Russell et al. [2007] frequently observed nanoparticle growth events in particle size distributions measured at Appledore Island. Many of the events occurred during the morning when plentiful .- and .-pinene and ozone made production of condensable products of photochemical oxidation probable. Iodine appeared not to play a significant role in particle formation, in contrast to findings of previous studies at European coastal sites.
The UNH/MWO and UVA CHAiOS field team. From left to right: Allen Smith (UVA), Alex Pszenny (UNH & MWO), John Maben (UVA), Emily Fischer (MWO) and Bill Keene (UVA). (click to enlarge)
Smith et al. [2007] performed an analysis similar to that of Fischer et al. (see above) of another component of "fixed" nitrogen, the ammonia system. The transport of emissions from intensive agricultural activities in the eastern United States was an important source of total ammonia (ammonia gas and aerosol ammonium) over the Gulf of Maine during summer. Under cleaner northwest flow, total ammonia concentrations were relatively low and partitioned roughly equally between phases. Under the more polluted midwest flow, total ammonia concentrations were substantially higher and dominated by particulate ammonium. Aerosol ammonium was associated primarily with the highly acidic sub-µm size fractions with low deposition velocities, causing dry-deposition fluxes to be dominated by gas phase ammonia. Consequently, phase partitioning with pollutant-derived sulfur aerosol substantially increased both the atmospheric lifetime of total ammonia against dry deposition and the relative importance of removal via wet- versus dry-deposition pathways. Total ammonia accounted for a third of the dry-deposition flux of inorganic nitrogen (total ammonia plus total nitrate) to the Gulf of Maine during summer. The combined dry deposition of total ammonia and wet deposition of ammonium via precipitation contributed 40% of the corresponding total atmospheric N flux.
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