Modelling chemistry in the nocturnal boundary layer above tropical rainforest and a generalised effective nocturnal ozone deposition velocity for sub-ppbv NOx conditions     

Thomas A. M. Pugh  James Ryder  A. Robert MacKenzie  Sarah J. Moller  James D. Lee  Carole Helfter  Eiko Nemitz  Douglas Lowe  C. Nicholas Hewitt 《Journal of Atmospheric Chemistry》2010,65(2-3):89-110

Measurements of atmospheric composition have been made over a remote rainforest landscape. A box model has previously been demonstrated to model the observed daytime chemistry well. However the box model is unable to explain the nocturnal measurements of relatively high [NO] and [O₃], but relatively low observed [NO₂]. It is shown that a one-dimensional (1-D) column model with simple O₃-NO_x chemistry and a simple representation of vertical transport is able to explain the observed nocturnal concentrations and predict the likely vertical profiles of these species in the nocturnal boundary layer (NBL). Concentrations of tracers carried over from the end of the night can affect the atmospheric chemistry of the following day. To ascertain the anomaly introduced by using the box model to represent the NBL, vertically-averaged NBL concentrations at the end of the night are compared between the 1-D model and the box model. It is found that, under low to medium [NO_x] conditions (NO_x?<?1 ppbv), a simple parametrisation can be used to modify the box model deposition velocity of ozone, in order to achieve good agreement between the box and 1-D models for these end-of-night concentrations of NO_x and O₃. This parametrisation would could also be used in global climate-chemistry models with limited vertical resolution near the surface. Box-model results for the following day differ substantially if this effective nocturnal deposition velocity for ozone is implemented; for instance, there is a 9% increase in the following days peak ozone concentration. However under medium to high [NO_x] conditions (NO_x > 1 ppbv), the effect on the chemistry due to the vertical distribution of the species means no box model can adequately represent chemistry in the NBL without modifying reaction rate coefficients.  相似文献   

Processing of atmospheric organic matter by California radiation fogs   总被引：2，自引：0，他引：2  

Jeffrey L. Collett Jr.  Pierre Herckes  Sarah Youngster  Taehyoung Lee 《Atmospheric Research》2008,87(3-4):232

Considerable effort has been put into characterizing the ionic composition of fogs and clouds over the past twenty-five years. Recently it has become evident that clouds and fogs often contain large concentrations of organic material as well. Here we report findings from a series of studies examining the organic composition of radiation fogs in central California. Organic compounds in these fogs comprise a major fraction of total solute mass, with total organic carbon sometimes reaching levels of several tens of mg/L. This organic matter is comprised of a wide variety of compounds, ranging from low molecular weight organic acids to high molecular weight compounds with molecular masses approaching several hundred to a thousand g/mole. The most abundant individual compounds are typically formic acid, acetic acid, and formaldehyde. High concentrations are also observed of some dicarboxylic acids (e.g., oxalate) and dicarbonyls (e.g., glyoxal and methylglyoxal) and of levoglucosan, an anhydrosugar characteristically emitted by biomass combustion. Many other compounds have been identified in fog water by GC/MS, including long chain n-alkanoic acids, n-alkanes, PAH, and others, although these compounds typically comprise a total of only a few percent of fog TOC. Measurements of fog scavenging of organic and elemental carbon reveal preferential scavenging of organic carbon. Tracking of individual organic compounds utilized as source type markers suggests the fogs differentially scavenge carbonaceous particles from different source types, with more active processing of wood smoke than vehicle exhaust. Observations of high deposition velocities of fog-borne organic carbon, in excess of 1 cm/s, indicate that fogs in the region represent an important mechanism for cleansing the atmosphere of pollution.  相似文献   

Weathering rates from top to bottom in a carbonate environment     

Bernard Bourdon  Sarah Bureau  Morten B. Andersen  Eric Pili  Amélie Hubert 《Chemical Geology》2009,258(3-4):275-287

This study investigates U-series, Sr isotopes, major and trace elements in a chalk aquifer system located in Eastern France. Soil and rock samples were collected along depth profiles down to 45 m in four localities as an attempt to investigate the weathering processes in the soil, the unsaturated zone and the saturated zone of the aquifer. Interstitial water was extracted from soils and rocks by a centrifugation technique. U-series offer a powerful tool to calculate weathering rates because the relative mobility of the U- and Th-isotopes can be precisely measured and it does not require the determination of a reference state as in other approaches. As expected, the data show very large mobile element depletion in the soil with large ²³⁰Th excess relative to ²³⁸U, while the rocks show more limited but not insignificant mobile element depletion. The U-series data have been used to constrain weathering rates based on a 1-D reactive transport model. Weathering rates in the near surface are about 10–100 times faster than at depth. However, when integrated over the depth of the cores, including the unsaturated and the saturated zones, this underground weathering represents more than 30% of the total weathering flux, assuming congruent dissolution of carbonates. The (²³⁴U/²³⁸U) ratios in interstitial water are consistent with solid samples showing ²³⁴U depletion near the surface and an excess ²³⁴U at depth. A leaching experiment performed on chalk shows that the excess ²³⁴U in natural waters percolating through carbonate rocks results both from preferential ²³⁴U leaching and direct recoil in the interstitial water. A new approach was used to derive the recoil ejection factor based on BET measurements and the fractal dimension of chalk surface. Consideration of preferential leaching and recoil allows a more accurate modeling of weathering rates.  相似文献   

Fe(II) adsorption on hematite (0 0 0 1)     

Kunaljeet S. Tanwar  Sarah C. Petitto  Peter J. Eng 《Geochimica et cosmochimica acta》2009,73(15):4346-2587

The surface structure of α-Fe₂O₃(0 0 0 1) was studied using crystal truncation rod (CTR) X-ray diffraction before and after reaction with aqueous Fe(II) at pH 5. The CTR results show the unreacted α-Fe₂O₃(0 0 0 1) surface consists of two chemically distinct structural domains: an O-layer terminated domain and a hydroxylated Fe-layer terminated domain. After exposing the α-Fe₂O₃(0 0 0 1) surface to aqueous Fe(II), the surface structure of both co-existing structural domains was modified due to adsorption of Fe at crystallographic lattice sites of the substrate, resulting in six-coordinated adsorbed Fe at the surface. The average Fe-O bond lengths of the adsorbed Fe are consistent with typical Fe(III)-O bond lengths (in octahedral coordination), providing evidence for the oxidation of Fe(II) to Fe(III) upon adsorption. These results highlight the important role of substrate surface structure in controlling Fe(II) adsorption. Furthermore, the molecular scale structural characterization of adsorbed Fe provides insight into the process of Fe(II) induced structural modification of hematite surfaces, which in turn aids in assessing the effective reactivity of hematite surfaces in Fe(II) rich environments.  相似文献   

Organisational geographies of power: Introduction to special issue     

James R. Faulconbridge  Sarah Hall 《Geoforum》2009,40(5):785-789

This paper introduces the special issue ‘organisational geographies of power’ by examining the theoretical and empirical context for debates. It does this, firstly, with reference to existing theoretical work on what power is and does as well as work on the construction of power relations. Building on this, the paper then, secondly, examines the way relational and production network approaches have incorporated power into work on transnational corporations before, thirdly, considering how work on neoliberalism has studied the construction of spatial power relations. In doing this the paper highlights four important questions for future research relating to the practices and spatiality of power, the exercising of power and the situated effects of power relations.  相似文献   

Dialogue, metaphors of dialogue and understandings of geography   总被引：2，自引：0，他引：2  

David Demeritt  & Sarah Dyer 《Area》2002,34(3):229-241

Increasingly, human and physical geographers alike describe their research practices as involving dialogue of some kind. However, the widespread popularity of the term belies some very different understandings of its meaning and methodological implications. In this paper we reflect on these different conceptions of 'dialogical' geography as a way, first, of illustrating the broad range of understandings of research methodology now current in the discipline and, second, of identifying their implications for three long–standing controversies in the discipline: relativism and the truth of geographical knowledge, the associated dualisms between subject/object, nature/society, and realism/constructionism, and the potential for unity between human and physical geography. We argue that, while dialogue is a potentially fruitful way of understanding and practising geography, that the defence of it, like the attacks on it, is often misconceived.  相似文献   

Applied and theoretical considerations for constructing spatially explicit individual-based models of marine larval fish that include multiple trophic levels   总被引：3，自引：2，他引：3  

Hermann  Albert J.; Hinckley  Sarah; Megrey  Bernard A.; Napp  Jeffrey M. 《ICES Journal of Marine Science》2001,58(5):1030-1041

  相似文献   

Subsistence in coastal fisheries policy: What's in a word?   总被引：1，自引：2，他引：1  

Sarah Schumann  Seth Macinko   《Marine Policy》2007,31(6):706-718

Consideration of subsistence fishing activities seems particularly relevant to coastal fisheries policy, yet formal recognition of subsistence fishing is often absent from associated policy frameworks. A critical problem is the very meaning of the term “subsistence.” A review of the literature on subsistence, dominated until recently by North American research, reveals a schism between interpretations emphasizing material aspects of subsistence and interpretations highlighting cultural aspects. The North American literature on the subject is heavily influenced by a focus on Arctic indigenous populations emphasizing cultural survival. Ultimately, subsistence can be a matter of survival in the belly, the soul, or both. International case studies suggest that different interpretations of subsistence are appropriate in different circumstances, and that appropriate policy can be fashioned only after the local context of subsistence is understood.  相似文献   

Plio-Pleistocene microtephra in DSDP site 231, Gulf of Aden   总被引：1，自引：0，他引：1  

Sarah J. Feakins  Francis H. Brown  Peter B. deMenocal 《Journal of African Earth Sciences》2007,48(5):341-352

We reconstruct a Plio-Pleistocene microscopic tephrostratigraphy for DSDP Site 231 in the Gulf of Aden. Systematic microtephrostratigraphy increases the potential for identifying tephra horizons for regional stratigraphic correlation and age control, as well as providing information about eruptive histories. Microtephra reveal three main pulses of volcanism c. 4.0–3.2 Ma, 2.4 Ma and 1.7–1.3 Ma, corresponding to peaks in volcanic activity recorded in the East African Rift System. Previous studies of DSDP Site 231 have reported six visible tephra horizons (up to 25 cm thick) with geochemical compositions matching East African tuffs. We find 68 additional microtephra horizons through microscopic examination of 1050 samples (each integrating c. 3 ka) in over 200 m of marine sediments. We report the major and minor element geochemical compositions of individual glass shards in six of these microtephra horizons and establish a robust correlation at 168.73 m to the Lokochot Tuff (3.58 Ma), which together with previously identified tephra, provides a tightly constrained chronostratigraphy for the mid Pliocene.  相似文献   

Remote sensing for groundwater modelling in large semiarid areas: Lake Chad Basin,Africa     

Marc Leblanc  Guillaume Favreau  Sarah Tweed  Christian Leduc  Moumtaz Razack  Linus Mofor 《Hydrogeology Journal》2007,15(1):97-100

  相似文献