Abstract. Mercury is a contaminant of global concern. It is transported in the atmosphere primarily as gaseous elemental mercury, but its reactivity and deposition to the surface environment, through which it enters the aquatic food chain, is greatly enhanced following oxidation. Measurements and modelling studies of oxidised mercury in the polar to sub-tropical marine boundary layer (MBL) have suggested that photolytically produced bromine atoms are the primary oxidant of mercury. We report year-round measurements of elemental and oxidised mercury, along with ozone, halogen oxides (IO and BrO) and nitrogen oxides (NO2), in the MBL over the Galápagos Islands in the equatorial Pacific. Elemental mercury concentration remained low throughout the year, while higher than expected levels of oxidised mercury occurred around midday. Our results show that the production of oxidised mercury in the tropical MBL cannot be accounted for by bromine oxidation only, or by the inclusion of ozone and hydroxyl. As a two-step oxidation mechanism, where the HgBr intermediate is further oxidised to Hg(II), depends critically on the stability of HgBr, an additional oxidant is needed to react with HgBr to explain more than 50% of the observed oxidised mercury. Based on best available thermodynamic data, we show that atomic iodine, NO2, or HO2 could all play the potential role of the missing oxidant, though their relative importance cannot be determined explicitly at this time due to the uncertainties associated with mercury oxidation kinetics. We conclude that the key pathway that significantly enhances atmospheric mercury oxidation and deposition to the tropical oceans is missing from the current understanding of atmospheric mercury oxidation.

Abstract. Emitted mainly by the oceans, iodine is a halogen compound important for atmospheric chemistry due to its high ozone depletion potential and effect on the oxidizing capacity of the atmosphere. Here we present a comprehensive data set of iodine oxide (IO) measurements in the open marine boundary layer (MBL) made during the Malaspina 2010 circumnavigation. Results show IO mixing ratios ranging from 0.4 to 1 pmol mol−1 (30% uncertainty) and, complemented with additional field campaigns, this data set confirms through observations the ubiquitous presence of reactive iodine chemistry in the global marine environment. We use a global model with organic (CH3I, CH2ICl, CH2I2 and CH2IBr) and inorganic (HOI and I2) iodine ocean emissions to investigate the contribution of the different iodine source gases to the budget of IO in the global MBL. In agreement with previous estimates, our results indicate that, globally averaged, the abiotic precursors contribute about 75 % to the IO budget. However, this work reveals a strong geographical pattern in the contribution of organic vs. inorganic precursors to reactive iodine in the global MBL.

Ship-based Multi-Axis Differential Optical Absorption Spectroscopy measurements of iodine monoxide (IO) and atmospheric and seawater Gas Chromatography-Mass Spectrometer observations of methyl iodide (CH₃I) were made in the Eastern Pacific marine boundary layer during April 2010 as a part of the HaloCarbon Air Sea Transect-Pacific (HaloCAST-P) scientific cruise. The presence of IO in the open ocean environment was confirmed, with a maximum differential slant column density of 5 × 10¹³ molecules cm⁻² along the 1° elevation angle (corresponding to approximately 1 pptv) measured in the oligotrophic region of the Southeastern Pacific. Such low IO mixing ratios and their observed geographical distribution are inconsistent with satellite estimates and with previous understanding of oceanic sources of iodine. A strong correlation was observed between reactive iodine (defined as IO + I) and CH₃I, suggesting common sources. In situ measurements of meteorological parameters and physical ocean variables, along with satellite-based observations of Chlorophyll <i>a</i> (Chl <i>a</i>) and Chromophoric Dissolved Organic Matter (CDOM) were used to gain insight into the possible sources of iodine in this remote environment. Surprisingly, reactive iodine showed a negative correlation (> 99% confidence) to Chl <i>a</i> and CDOM across the cruise transect. However, a significant positive correlation (> 99% confidence) with sea surface temperature (SST) and salinity instead suggests a widespread abiotic source related to the availability of aqueous iodine and to temperature

[1] Observations of gas-phase iodine species were made during a field campaign in the eastern Pacific marine boundary layer (MBL). The Climate and Halogen Reactivity Tropical Experiment (CHARLEX) in the Galápagos Islands, running from September 2010 to present, is the first long-term ground-based study of trace gases in this region. Observations of gas-phase iodine species were made using long-path differential optical absorption spectroscopy (LP-DOAS), multi-axis DOAS (MAX-DOAS), and resonance and off-resonance fluorescence by lamp excitation (ROFLEX). These measurements were supported by ancillary measurements of ozone, nitrogen oxides, and meteorological variables. Selective halocarbon and ultrafine aerosol concentration measurements were also made. MAX-DOAS observations of iodine monoxide (IO) display a weak seasonal variation. The maximum differential slant column density was 3.8 Â 10 13 molecule cm À2 (detection limit~7 Â 10 12 molecule cm À2 ). The seasonal variation of reactive iodine IO x (= I + IO) is stronger, peaking at 1.6 pptv during the warm season (February-April). This suggests a dependence of the iodine sources on the annual cycle in sea surface temperature, although perturbations by changes in ocean surface iodide concentration and solar radiation are also possible. An observed negative correlation of IO x with chlorophyll-a indicates a predominance of abiotic sources. The low IO mixing ratios measured (below the LP-DOAS detection limit of 0.9 pptv) are not consistent with satellite observations if IO is confined to the MBL. The IO x loading is consistent with the observed absence of strong ozone depletion and nucleation events, indicating a small impact of iodine chemistry on these climatically relevant factors in the eastern Pacific MBL.

Abstract. Iodine monoxide (IO) differential slant column densities (DSCD) have been retrieved from a new multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument deployed at the Izaña subtropical observatory as part of the Network for the Detection of Atmospheric Composition Change (NDACC) programme. The station is located at 2370 m a.s.l., well above the trade wind inversion that limits the top of the marine boundary layer, and hence is representative of the free troposphere. We report daily observations from May to August 2010 at different viewing angles. During this period, the spectral signature of IO was unequivocally detected on every day of measurement. A mean IO DSCD of 1.52×10 13 molecules cm −2 was observed at the 5 • instrument elevation angle (IEA) on clear days using a single zenith reference for the reported period, with a day-to-day variability of 33 % at one standard deviation. Based on the simulation of the DSCDs using radiative transfer calculations with five different hypothesized IO profiles, the IO mixing ratio is estimated to range between 0.2 and 0.4 pptv in the free troposphere. Episodes of Saharan dust outbreaks were also observed, with large increases in the DSCDs at higher IEA, suggesting an enhancement of IO inside the dust cloud.