A. Saiz-Lopez

Novel iodine chemistry in the marine boundary layer

[1]xa0The atmospheric chemistry of iodine is important for several reasons, including the influence of iodine oxides on the oxidising capacity of the troposphere, the formation of new particles, and the enrichment of iodine in marine aerosols and the transport of this essential dietary element to the continents. It is shown here that a substantial iodine source is I2, most likely emitted from macro-algae at low tide. This source accounts for the daytime production of new particles in the coastal marine boundary layer, and also explains the discovery of significant night-time levels of iodine oxides.

Impact of halogen monoxide chemistry upon boundary layer OH and HO2 concentrations at a coastal site

The impact of iodine oxide chemistry upon OH and HO2 concentrations in the coastal marine boundary layer has been evaluated using data from the NAMBLEX (North Atlantic Marine Boundary Layer Experiment) campaign, conducted at Mace Head, Ireland during the summer of 2002. Observationally constrained calculations show that under low NOx conditions experienced during NAMBLEX (NO HOI + O-2 accounted for up to 40% of the total HO2 radical sink, and the subsequent photolysis of HOI to form OH + I comprised up to 15% of the total midday OH production rate. The XO + HO2 (X = Br, I) reactions may in part account for model overestimates of measured HO2 concentrations in previous studies at Mace Head, and should be considered in model studies of HOx chemistry at similar coastal locations.

The mass balance of mercury in the springtime arctic environment

[1]xa0The load of mercury in the Arctic environment is to a large extent controlled by atmospheric mercury depletion events. At Barrow, Alaska, these depletion events have been linked with the near-surface air formation of reactive gaseous mercury (Hg(II)) (RGM), to a much lesser extent in a particulate-bound form, and the accumulation of total mercury in the snow pack (>100 ng/L in late Spring). This transport of Hg from atmospheric conversion, to deposition, to bio-available forms is likely to be the predominant pathway for mercury into Arctic biota. For the first time we combine flux rate measurements, atmospheric chemistry measurements, and air mass trajectories to give a comprehensive two-week window into the springtime dynamics and mass balance of Arctic mercury. We have conducted polar-sunrise to snowmelt mercury monitoring at Barrow from 1998 to 2004, and the time period March 25th–April 7th (Julian days 84–97), 2003 appears typical for this time of year. A clear link was observed between air of marine origin, the build-up of BrO together with removal of gaseous elemental mercury (GEM)), and the formation of RGM. This provides the most direct evidence so far for Br and Hg chemistry as the direct source of RGM. The fluxes of RGM and GEM were determined and the net flux calculated.

Bromine oxide in the mid-latitude marine boundary layer

[1]xa0We report direct observations of bromine oxide (BrO) in the mid-latitude marine boundary layer (MBL), using long-path Differential Optical Absorption Spectroscopy (DOAS). The measurements were made at the Mace Head observatory on the west coast of Ireland. Over six days of observations, the BrO concentration varied from below the detection limit (≈0.8 parts per trillion (ppt)) at night, to a maximum daytime concentration of 6.5 ppt. At the average daytime concentration of 2.3 ppt, BrO causes significant O3 depletion in the MBL through catalytic cycles involving the iodine oxide and hydroperoxy radicals, and also oxidises dimethyl sulfide much more rapidly than the hydroxyl radical. A post-sunrise pulse of BrO was observed, consistent with the build up of photolabile precursors produced by heterogeneous reactions on sea-salt aerosol during the previous night. This indicates that significant bromine activation occurs over the open ocean.

Variability of the mesospheric nightglow sodium D2/D1 ratio

[1]xa0Measurements of the intensity ratio of the 589.0/589.6 nm sodium doublet in the terrestrial nightglow over an 8-year period, involving >300 separate determinations, have established that it is variable, the value RD = I(D2)/I(D1) lying between 1.2 and 1.8. Sky spectra from the Keck I telescope with the High-Resolution Echelle Spectrometer (HIRES) echelle spectrograph and the Keck II telescope with the Echellette Spectrograph and Imager (ESI) echelle spectrograph were used in this analysis. The result contrasts with the accepted view, from earlier measurements at midlatitude, that the ratio is 2.0, as expected on statistical grounds. The lack of dependence of the ratio on viewing elevation angle, and hence Na slant column, allows self-absorption to be ruled out as a cause of the variability. The data suggest a semiannual oscillation in the ratio, maximum at the equinoxes and minimum at the solstices. Airborne measurements over the North Atlantic (40°–50°N) in 2002 show an even larger range in the nightglow ratio and no correlation with the upper mesospheric temperature determined from the OH 6–2 bands. A laboratory study confirms that the ratio does not depend on temperature; however, it is shown to be sensitive to the [O]/[O2] ratio. It is therefore postulated that the variable ratio arises from a competition between O reacting with NaO(A3Σ+), produced from the reaction of Na with O3, to yield D-line emission with a D2/D1 ratio greater than about 2.0, and quenching by O2 to produce NaO(X2Π), possibly with vibrational excitation, which then reacts with O to produce emission with a ratio of less than 1.3.