A numerical modelling study on regional mercury budget for eastern North America

International audienceIn this study, we have integrated an up-to-date physio-chemical transformation mechanism of Hg into the framework of US EPA's CMAQ model system. In addition, the model adapted detailed calculations of the air-surface exchange for Hg to properly describe Hg re-emissions and dry deposition from and to natural surfaces. The mechanism covers Hg in three categories, elemental Hg (Hg0), reactive gaseous Hg (RGM) and particulate Hg (HgP). With interfacing to MM5 (meteorology processor) and SMOKE (emission processor), we applied the model to a 4-week period in June/July 1995 on a domain covering most of eastern North America. Results indicate that the model simulates reasonably well the levels of total gaseous Hg (TGM) and the specific Hg wet deposition measurements made by the Hg deposition network (MDN). Moreover, results from various scenario runs reveal that the Hg system behaves in a closely linear way in terms of contributions from different source categories, i.e. anthropogenic emissions, natural re-emissions and background. Analyses of the scenario results suggest that 37% of anthropogenically emitted Hg was deposited back in the model domain with 5155.2 kg of anthropogenic Hg moving out of the domain during the simulation period. Overall, the domain served as a source, which supplied a net 461.2 kg of Hg to the global background pool over the period. Our model validation and a sensitivity test further rationalized the rate constant for gaseous oxidation of Hg0 by hydroxyl radical OH used in the global scale modelling study by Bergan and Rodhe (2001). A further laboratory determination of the reaction rate constant, including its temperature dependence, stands as one of the important issues critical to improving our knowledge on the budget and cycling of Hg

Large magnetothermal conductivity of HoMnO_3 single crystals and its relation to the magnetic-field induced transitions of magnetic structure

We study the low-temperature heat transport of HoMnO_3 single crystals to probe the magnetic structures and their transitions induced by magnetic field. It is found that the low-T thermal conductivity (\kappa) shows very strong magnetic-field dependence, with the strongest suppression of nearly 90% and the biggest increase of 20 times of \kappa compared to its zero-field value. In particular, some ``dip"-like features show up in \kappa(H) isotherms for field along both the ab plane and the c axis. These behaviors are found to shed new light on the complex H-T phase diagram and the field-induced re-orientations of Mn^{3+} and Ho^{3+} spin structures. The results also demonstrate a significant spin-phonon coupling in this multiferroic compound.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.

Maize-Alfalfa Intercropping Promote Ecosystem Services Than Fertilized Single Crops

Phosphorus is a non-renewable source of fertilization, which will challenge the future of food production and cropland sustainability worldwide. Crop diversity is known to promote food production, yet its capacity to alleviate the dependence of multiple ecosystem services on non-renewable fertilization remains virtually unknown. Here, we conducted a field experiment to quantify the contribution of maize-alfalfa intercropping to support multiple ecosystem services under contrasting levels of phosphorus fertilization. We showed that unfertilized intercropping systems can support larger levels of multiple ecosystem services such as soil microbial habitat, plant-soil mutualism, nutrient cycling, and soil carbon storage compared with phosphorus-fertilized single crops. Intercropping also helped to reduce important tradeoffs in productivity and soil biodiversity compared with fertilized single crops. Together, our results provide evidence that intercropping systems are efficient in maintaining multiple ecosystem services and can help alleviate our global dependence on non-renewable fertilization

Tailoring liquid crystal honeycombs by head-group choice in bird-like bent-core mesogens

We introduce a new class of mesogens that are bird-like in shape and form honeycomb-type supramolecular liquid crystals. They have a bent pi-conjugated aromatic core as wings, a linear or branched chain as the tail and a selection of functional headgroups. Honeycombs of non-centrosymmetric trigonal type (p3m1) are obtained, along with two different complex honeycomb superlattices (p31m and p2gg) and a randomized hexagonal mesophase (p6mm). The key determinant of the self-assembled structure is the nature of interaction of the headgroup with the glycerols at the ends of the wings. The structure depends on whether the sub-columns lying along the edges of the prismatic cells contain pure or mixed headgroups and wing-end hydrogen-bonding groups. Its assembly is further controlled by reducing the tail-chain volume, inducing out-of-plane buckling of the honeycomb. These two modes of symmetry breaking lead to structural polarity both in- and out-of-plane, opening the way to applications in devices relying on properties such as ferroelectricity and second harmonic generation