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Saiz-Lopez A.,Laboratory for Atmospheric and Climate science | Lamarque J.-F.,U.S. National Center for Atmospheric Research | Kinnison D.E.,U.S. National Center for Atmospheric Research | Tilmes S.,U.S. National Center for Atmospheric Research | And 12 more authors.
Atmospheric Chemistry and Physics

We have integrated observations of tropospheric ozone, very short-lived (VSL) halocarbons and reactive iodine and bromine species from a wide variety of tropical data sources with the global CAM-Chem chemistry-climate model and offline radiative transfer calculations to compute the contribution of halogen chemistry to ozone loss and associated radiative impact in the tropical marine troposphere. The inclusion of tropospheric halogen chemistry in CAM-Chem leads to an annually averaged depletion of around 10% (∼2.5 Dobson units) of the tropical tropospheric ozone column, with largest effects in the middle to upper troposphere. This depletion contributes approximately-0.10 W m -2 to the radiative flux at the tropical tropopause. This negative flux is of similar magnitude to the ∼0.33 W m∼2 contribution of tropospheric ozone to present-day radiative balance as recently estimated from satellite observations. We find that the implementation of oceanic halogen sources and chemistry in climate models is an important component of the natural background ozone budget and we suggest that it needs to be considered when estimating both preindustrial ozone baseline levels and long term changes in tropospheric ozone. © 2012 Author(s). Source

Boxe C.S.,Jet Propulsion Laboratory | Hand K.P.,Jet Propulsion Laboratory | Nealson K.H.,University of Southern California | Yung Y.L.,California Institute of Technology | And 2 more authors.
International Journal of Astrobiology

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. © 2012 Cambridge University Press. Source

Abbatt J.P.D.,University of Toronto | Thomas J.L.,University of Versailles | Thomas J.L.,University of California at Los Angeles | Abrahamsson K.,Gothenburg University | And 13 more authors.
Atmospheric Chemistry and Physics

The role of ice in the formation of chemically active halogens in the environment requires a full understanding because of its role in atmospheric chemistry, including controlling the regional atmospheric oxidizing capacity in specific situations. In particular, ice and snow are important for facilitating multiphase oxidative chemistry and as media upon which marine algae live. This paper reviews the nature of environmental ice substrates that participate in halogen chemistry, describes the reactions that occur on such substrates, presents the field evidence for ice-mediated halogen activation, summarizes our best understanding of ice-halogen activation mechanisms, and describes the current state of modeling these processes at different scales. Given the rapid pace of developments in the field, this paper largely addresses advances made in the past five years, with emphasis given to the polar boundary layer. The integrative nature of this field is highlighted in the presentation of work from the molecular to the regional scale, with a focus on understanding fundamental processes. This is essential for developing realistic parameterizations and descriptions of these processes for inclusion in larger scale models that are used to determine their regional and global impacts. © 2012.Author(s). Source

MacDonald S.M.,University of Leeds | Oetjen H.,University of Leeds | Oetjen H.,University of Colorado at Boulder | Mahajan A.S.,University of Leeds | And 8 more authors.
Atmospheric Chemistry and Physics

Tropical rainforests act as a huge contributor to the global emissions of biogenic volatile organic compounds (BVOCs). Measurements of their oxidation products, such as formaldehyde (HCHO) and glyoxal (CHOCHO), provide useful indicators of fast photochemistry occurring in the lower troposphere. However, measurements of these species in tropical forest locations are extremely limited. To redress this, HCHO and CHOCHO were measured using the long-path (LP) and multi-axis (MAX) differential optical absorption spectroscopy (DOAS) techniques above the rainforest canopy in Borneo during two campaigns in spring and summer 2008, as part of the Oxidant and Particle Photochemical Processes above a south-east Asian tropical rainforest (OP3) project. The results were compared with concurrent measurements of hydroxyl radical (OH), isoprene (C5H8) (which was the dominant organic species emitted in this forest environment), and various meteorological parameters. Formaldehyde was observed at a maximum concentration of 4.5 ppb and glyoxal at a maximum of 1.6 ppb, significantly higher than previous measurements in rural locations. A 1-D chemistry model was then used to assess the diurnal evolution of formaldehyde and glyoxal throughout the boundary layer. The results, which compare well with the LP-DOAS and MAX-DOAS observations, suggest that the majority of the glyoxal and formaldehyde is confined to the first 500 m of the boundary layer, and that the measured ratio of these species is reproduced using currently accepted product yields for the oxidation of isoprene by OH. An important conclusion is that the measured levels of glyoxal are consistent with the surprisingly high concentrations of OH measured in this environment. © 2012 Author(s). Source

Schonhardt A.,University of Bremen | Begoin M.,University of Bremen | Richter A.,University of Bremen | Wittrock F.,University of Bremen | And 3 more authors.
Atmospheric Chemistry and Physics

This article reports on satellite observations of iodine monoxide (IO) and bromine monoxide (BrO). The region of interest is Antarctica in the time between spring and autumn. Both molecules, IO and BrO, are reactive halogen species and strongly influence tropospheric composition. As a result, a better understanding of their spatial distribution and temporal evolution is necessary to assess accurately their role in tropospheric chemistry. Especially in the case of IO, information on its present magnitude, spatial distribution patterns and source regions is still sparse.

The present study is based on six years of SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY) data recorded in nadir viewing geometry. Multi-year averages of monthly mean IO columns are presented and compared to the distributions of BrO. Influences of the IO air mass factor and the IO absorption cross section temperature dependence on the absolute vertical columns are discussed. The long-term observations of IO and BrO columns yield new insight into the temporal and spatial variation of IO above the Antarctic region. The occurrence of IO on Antarctic sea ice in late spring (November) is discovered and presented. In addition, the comparison between IO and BrO distributions show many differences, which argues for different mechanisms and individual nature of the release of the two halogen oxide precursors. The state of the ecosystem, in particular the changing condition of the sea ice in late spring, is used to explain the observations of the IO behaviour over Antarctica and the differences between IO and BrO distributions. © 2012 Author(s). Source

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