A mechanism for biologically-induced iodine emissions from sea-ice

International audienceOnly recently, ground- and satellite-based measurements have reported high concentrations of IO in coastal Antarctica. The sources of such a large iodine burden in the Antarctic atmosphere remain unknown. We propose a novel mechanism for iodine release from sea-ice surfaces. The release is triggered by the biological production of iodide (I-) and hypoiodous acid (HOI) from marine algae, contained within and underneath sea-ice, and their diffusion through sea-ice brine channels to accumulate in the quasi-liquid layer on the surface of sea-ice. A multiphase chemical model of polar atmospheric chemistry has been developed to investigate the biology-ice-atmosphere coupling in the polar environment. Model simulations were conducted to interpret recent observations of elevated IO in the coastal Antarctic springtime. The results show that the levels of inorganic iodine (i.e. I2, IBr, ICl) released from sea-ice through this mechanism account for the observed IO concentrations in the Antarctic springtime environment. The model results also indicate that iodine may trigger the catalytic release of bromine from sea-ice through phase equilibration of IBr. Considering the extent of sea-ice around the Antarctic continent, we suggest that the resulting high levels of iodine may have widespread impact on catalytic ozone destruction and aerosol formation in the Antarctic lower troposphere

Multiphase modeling of nitrate photochemistry in the quasi-liquid layer (QLL): implications for NOx release from the Arctic and coastal Antarctic snowpack

We utilize a multiphase model, CON-AIR (<B>Con</B>densed Phase to <B>Air</B> Transfer Model), to show that the photochemistry of nitrate (NO₃^&minus;) in and on ice and snow surfaces, specifically the quasi-liquid layer (QLL), can account for NO_x volume fluxes, concentrations, and [NO]/[NO₂] (γ=[NO]/[NO₂]) measured just above the Arctic and coastal Antarctic snowpack. Maximum gas phase NO_x volume fluxes, concentrations and γ simulated for spring and summer range from 5.0&times;10⁴ to 6.4&times;10⁵ molecules cm^&minus;3 s^&minus;1, 5.7&times;10⁸ to 4.8&times;10⁹ molecules cm^&minus;3, and ~0.8 to 2.2, respectively, which are comparable to gas phase NO_x volume fluxes, concentrations and γ measured in the field. The model incorporates the appropriate actinic solar spectrum, thereby properly weighting the different rates of photolysis of NO₃^&minus; and NO₂^&minus;. This is important since the immediate precursor for NO, for example, NO₂^&minus;, absorbs at wavelengths longer than nitrate itself. Finally, one-dimensional model simulations indicate that both gas phase boundary layer NO and NO₂ exhibit a negative concentration gradient as a function of height although [NO]/[NO₂] are approximately constant. This gradient is primarily attributed to gas phase reactions of NO_x with halogens oxides (i.e. as BrO and IO), HO_x, and hydrocarbons, such as CH₃O₂

New Insights into Martian Atmospheric Chemistry

HO_x radicals are produced in the Martian atmosphere by the photolysis of water vapor and subsequently participate in catalytic cycles that recycle carbon dioxide (CO_2) from its photolysis product carbon monoxide (CO), providing a qualitative explanation for the stability of its atmosphere. Balancing CO_2 production and loss based on our current understanding of Martian gas-phase chemistry has, however, proven to be difficult. The photolysis of O_3 produces O(^1D), while oxidation of CO produces HOCO radicals, a new member of the HO_x family. The O(^1D) quantum yield has recently been updated, which quantifies nonzero quantum yields in the Huggins bands. In Earth’s atmosphere HOCO is considered to be unimportant since it is quickly removed by abundant oxygen molecules. The smaller amount of O_2 in the Mars’ atmosphere causes HOCO’s lifetime to be longer in Mars’ atmosphere than Earth’s (3 × 10^(-5) seconds to 1.2 days from Mars’s surface to 240 km, respectively). Limited kinetic data on reactions involving HOCO prevented consideration of its reactions directly in atmospheric models. Therefore, the impact of HOCO reactions on Martian chemistry is currently unknown. Here, we incorporate new literature rate constants for HOCO chemistry and an updated representation of the O(^1D) quantum yield in the Caltech/JPL 1-D photochemical model for Mars’ atmosphere. Our simulations exemplify perturbations to NO_y, HO_x, and CO_x species, ranging from 5 to 50%. The modified O(^1D) quantum yield and new HOCO chemistry cause a 10% decrease and a 50% increase in OH and H_2O_2 total column abundances, respectively. At low altitudes, HOCO production contributes 5% towards CO_2 production. Given recent experimentally-obtained branching ratios for the oxidation of CO, HOCO may contribute up to 70% toward the production of NO_y, where HO_x and NO_y species are enhanced up to a factor 3, which has implications for rethinking the fundamental understanding of NO_y, HO_x, and CO/CO_2 cycling on Mars. Two new reaction mechanisms for converting CO to CO_2 using HOCO reactions are proposed, which reveal that H_2O_2 is more intimately coupled to CO_x chemistry. Our simulations are in good agreement with satellite/spacecraft measurements of CO and H_2O_2 on Mars

Validation of northern latitude Tropospheric Emission Spectrometer stare ozone profiles with ARC-IONS sondes during ARCTAS: sensitivity, bias and error analysis

We compare Tropospheric Emission Spectrometer (TES) versions 3 and 4, V003 and V004, respectively, nadir-stare ozone profiles with ozonesonde profiles from the Arctic Intensive Ozonesonde Network Study (ARCIONS, http://croc.gsfc.nasa.gov/arcions/ during the Arctic Research on the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field mission. The ozonesonde data are from launches timed to match Aura's overpass, where 11 coincidences spanned 44° N to 71° N from April to July 2008. Using the TES "stare" observation mode, 32 observations are taken over each coincidental ozonesonde launch. By effectively sampling the same air mass 32 times, comparisons are made between the empirically-calculated random errors to the expected random errors from measurement noise, temperature and interfering species, such as water. This study represents the first validation of high latitude (>70°) TES ozone. We find that the calculated errors are consistent with the actual errors with a similar vertical distribution that varies between 5% and 20% for V003 and V004 TES data. In general, TES ozone profiles are positively biased (by less than 15%) from the surface to the upper-troposphere (~1000 to 100 hPa) and negatively biased (by less than 20%) from the upper-troposphere to the lower-stratosphere (100 to 30 hPa) when compared to the ozonesonde data. Lastly, for V003 and V004 TES data between 44° N and 71° N there is variability in the mean biases (from −14 to +15%), mean theoretical errors (from 6 to 13%), and mean random errors (from 9 to 19%)

Potential significance of photoexcited NO2 on global air quality with the NMMB/BSC chemical transport model

Atmospheric chemists have recently focused on the relevance of the NO2* + H2O → OH + HONO reaction to local air quality. This chemistry has been considered not relevant for the troposphere from known reaction rates until nowadays. New experiments suggested a rate constant of 1.7 × 10−13 cm3 molecule−1 s−1, which is an order of magnitude faster than the previously estimated upper limit of 1.2 × 10−14 cm3 molecule−1 s−1, determined by Crowley and Carl (1997). Using the new global model, NMMB/BSC Chemical Transport Model (NMMB/BSC-CTM), simulations are presented that assess the potential significance of this chemistry on global air quality. Results show that if the NO2* chemistry is considered following the upper limit kinetics recommended by Crowley and Carl (1997), it produces an enhancement of ozone surface concentrations of 4–6 ppbv in rural areas and 6–15 ppbv in urban locations, reaching a maximum enhancement of 30 ppbv in eastern Asia. Moreover, NO2 enhancements are minor (xemissions are present; however, differences are small in most parts of the globe

The effect of the novel HO_2 + NO → HNO_3 reaction channel at South Pole, Antarctica

It is well established that the reaction of HO_2 with NO plays a central role in atmospheric chemistry, by way of OH/HO_2 recycling and reduction of ozone depletion by HO_x cycles in the stratosphere and through ozone production in the troposphere. Utilizing a photochemical box model, we investigate the impact of the recently observed HNO_3 production channel (HO_2+NO → HNO_3) on NO_x (NO + NO_2), HO_x (OH + HO_2), HNO_3, and O_3 concentrations in the boundary layer at the South Pole, Antarctica. Our simulations exemplify decreases in peak O_3, NO, NO_2, and OH and an increase in HNO_3. Also, mean OH is in better agreement with observations, while worsening the agreement with O_3, HO_2, and HNO_3 concentrations observed at the South Pole. The reduced concentrations of NO_x are consistent with expected decreases in atmospheric NO_x lifetime as a result of increased sequestration of NO_x into HNO_3. Although we show that the inclusion of the novel HNO_3 production channel brings better agreement of OH with field measurements, the modelled ozone and HNO_3 are worsened, and the changes in NO_x lifetime imply that snowpack NO_x emissions and snowpack nitrate recycling must be re-evaluated

Oxygen isotopic fractionation in the photochemistry of nitrate in water and ice

We recently reported the first multiple oxygen isotope composition of nitrate (NO_3^−) in ice cores (Alexander et al., 2004). Postdepositional photolysis and volatilization may alter the isotopic signatures of snowpack nitrate. Therefore the precise assessment of the geochemical/atmospheric significance of O-isotopic signatures requires information on the relative rates of photolysis (λ > 300 nm) of N^(16)O_3^−, N^(16)O_2^(17)O^−, and N^(16)O_2^(18)O^− in ice. Here we report on ^(17)O^- and ^(18)O^-fractionation in the 313-nm photolysis of 10-mM aqueous solutions of normal Fisher KNO3 (i.e., Δ17O = −0.2 ± 0.2‰) and 17O-enriched USGS-35 NaNO_3 (Δ^(17)O = 21.0 ± 0.4‰) between −30° and 25°C. We found that Fisher KNO_3 undergoes mass-dependent O-fractionation, i.e., a process that preserves Δ^(17)O = 0. In contrast, Δ^(17)O in USGS-35 NaNO_3 decreased by 1.6 ± 0.4‰ and 2.0 ± 0.4‰ at 25°C, 1.2 ± 0.4‰ and 1.3 ± 0.4‰ at −5°C, and 0.2 ± 0.4‰ and 1.1 ± 0.4‰ at −30°C, after 12 and 24 hours, respectively. Since the small quantum yield (∼0.2%) of NO_3^− photodecomposition into (NO_2 + OH) is due to extensive cage recombination of the primary photofragments rather than to intramolecular processes, the observed Δ^(17)O decreases likely reflect competitive O-isotope exchange of geminate OH-radicals with H_2O (Δ^(17)O = 0) and escape from the solvent cage, in addition to residual O-isotope mixing of the final photoproducts NO, NO_2, NO_2^−, with H_2O. At the prevailing low temperatures, photochemical processing will not impair the diagnostic value of O-isotopic signatures in tracing the chemical ancestry of nitrate in polar ice

An active nitrogen cycle on Mars sufficient to support a subsurface biosphere

Mars' total atmospheric nitrogen content is 0.2 mbar. One-dimensional (1D) photochemical simulations of Mars' atmosphere show that nitric acid (HNO_3(g)), the most soluble nitrogen oxide, is the principal reservoir species for nitrogen in its lower atmosphere, which amounts to a steady-state value of 6×10^(−2) kg or 4 moles, conditions of severe nitrogen deficiency. Mars could, however, support ∼10^(15) kg of biomass (∼1 kg N m^(−2)) from its current atmospheric nitrogen inventory. The terrestrial mass ratio of nitrogen in biomass to that in the atmosphere is ∼10^(−5); applying this ratio to Mars yields ∼10^(10) kg of total biomass – also, conditions of severe nitrogen deficiency. These amounts, however, are lower limits as the maximum surface-sink of atmospheric nitrogen is 2.8 mbar (9×10^(15) kg of N), which indicates, in contradistinction to the Klingler et al. (1989), that biological metabolism would not be inhibited in the subsurface of Mars. Within this context, we explore HNO_3 deposition on Mars' surface (i.e. soil and ice-covered regions) on pure water metastable thin liquid films. We show for the first time that the negative change in Gibbs free energy increases with decreasing HNO_3(g) (NO_3^−(aq)) in metastable thin liquid films that may exist on Mars' surface. We also show that additional reaction pathways are exergonic and may proceed spontaneously, thus providing an ample source of energy for nitrogen fixation on Mars. Lastly, we explore the dissociation of HNO_3(g) to form NO_3^−(aq) in metastable thin liquid films on the Martian surface via condensed phase simulations. These simulations show that photochemically produced fixed nitrogen species are not only released from the Martian surface to the gas-phase, but more importantly, transported to lower depths from the Martian surface in transient thin liquid films. A putative biotic layer at 10 m depth would produce HNO_3 and N_2 sinks of −54 and −5×10^(12) molecules cm^(−2) s^(−1), respectively, which is an ample supply of available nitrogen that can be efficiently transported to the subsurface. The downward transport as well as the release to the atmosphere of photochemically produced fixed nitrogen species (e.g. NO_2^−, NO and NO_2) suggests the existence of a transient but active nitrogen cycle on Mars

Adsorbed water and thin liquid films on Mars

At present, bulk liquid water on the surface and near-subsurface of Mars does not exist due to the scarcity of condensed- and gas-phase water, pressure and temperature constraints. Given that the nuclei of soil and ice, that is, the soil solid and ice lattice, respectively, are coated with adsorbed and/or thin liquid films of water well below 273 K and the availability of water limits biological activity, we quantify lower and upper limits for the thickness of such adsorbed/water films on the surface of the Martian regolith and for subsurface ice. These limits were calculated based on experimental and theoretical data for pure water ice and water ice containing impurities, where water ice containing impurities exhibit thin liquid film enhancements, ranging from 3 to 90. Close to the cold limit of water stability (i.e. 273 K), thin liquid film thicknesses at the surface of the Martian regolith is 0.06 nm (pure water ice) and ranges from 0.2 to 5 nm (water ice with impurities). An adsorbed water layer of 0.06 nm implies a dessicated surface as the thickness of one monolayer of water is 0.3 nm but represents 0.001–0.02% of the Martian atmospheric water vapour inventory. Taking into account the specific surface area (SSA) of surface-soil (i.e. top 1 mm of regolith and 0.06 nm adsorbed water layer), shows Martian surface-soil may contain interfacial water that represents 6–66% of the upper- and lower-limit atmospheric water vapour inventory and almost four times and 33%, the lower- and upper-limit Martian atmospheric water vapour inventory. Similarly, taking the SSA of Martian soil, the top 1 mm or regolith at 5 nm thin liquid water thickness, yields 1.10×10^(13) and 6.50×10^(13) litres of waters, respectively, 55–325 times larger than Mars’ atmospheric water vapour inventory. Film thicknesses of 0.2 and 5 nm represent 2.3×10^4 – 1.5×10^6 litres of water, which is 6.0×10^(−7) – 4.0×10^(−4)%, respectively, of a 10 pr μm water vapour column, and 3.0×10^(−6) – 4.0×10^(−4)% and 6.0×10^(−6)–8.0×10^(−4)%, respectively, of the Martian atmospheric water vapour inventory. Thin liquid film thicknesses on/in subsurface ice were investigated via two scenarios: (i) under the idealistic case where it is assumed that the diurnal thermal wave is equal to the temperature of ice tens of centimetres below the surface, allowing for such ice to experience temperatures close to 273 K and (ii) under the, likely, realistic scenario where the diurnal thermal wave allows for the maximum subsurface ice temperature of 235 K at 1 m depth between 30°N and 30°S. Scenario 1 yields thin liquid film thicknesses ranging from 11 to 90 nm; these amounts represent 4×10^(6) – 3.0×10^7 litres of water. For pure water ice, Scenario 2 reveals that the thickness of thin liquid films contained on/within Martian subsurface is less than 1.2 nm, several molecular layers thick. Conversely, via the effect of impurities at 235 K allows for a thin liquid film thickness on/within subsurface ice of 0.5 nm, corresponding to 6.0×10^4 litres of water. The existence of thin films on Mars is supported by data from the Mars Exploration Rovers (MERs) Spirit and Opportunity's Alpha Proton X-ray Spectrometer instrumentation, which have detected increased levels of bromine beneath the immediate surface, suggestive of the mobilization of soluble salts by thin films of liquid water towards local cold traps. These findings show that biological activity on the Martian surface and subsurface is not limited by nanometre dimensions of available wate

Diurnal Variation in Mars Equatorial Odd Oxygen Species: Chemical Production and Loss Mechanisms

Odd oxygen (O, O(¹D), O₃) abundance and its variability in the Martian atmosphere results from complex physical and chemical interactions among atmospheric species, which are driven mainly by solar radiation and atmospheric conditions. Although our knowledge of Mars’ ozone distribution and variability has been significantly improved with the arrival of several recent orbiters, the data acquired by such missions is not enough to properly characterize its diurnal variation. Thus, photochemical models are useful tools to assist in such a characterization. Here, both the Martian ozone vertical distribution and its diurnal variation for equatorial latitudes are studied, using the JPL/Caltech one-dimensional photochemical model and diurnally-variable atmospheric profiles. The chosen equatorial latitude-region is based on the recent and future plans of NASA and other agencies to study this region by different surface missions. A production and loss analysis is performed in order to characterize the chemical mechanisms that drive odd oxygen's diurnal budget and variability on Mars making use of the comprehensive chemistry implemented in the model. The diurnal variation shows large differences in the abundance between daytime and nighttime; and variable behavior depending on the atmospheric layer. The photolysis-driven ozone diurnal profile is obtained at the surface, whilst a sharp decrease is obtained in the upper troposphere at daytime, which originates from the large differences in atomic oxygen abundances between atmospheric layers. Finally, no clear anticorrelation between ozone and water vapor is found in the diurnal cycle, contrary to the strong correlation observed by orbiters on a seasonal timescale