682 research outputs found

    Interannual, seasonal, and diel variation in soil respiration relative to ecosystem respiration at a wetland to upland slope at Harvard Forest

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    Soil carbon dioxide efflux (soil respiration, SR) was measured with eight autochambers at two locations along a wetland to upland slope at Harvard Forest over a 4 year period, 2003–2007. SR was consistently higher in the upland plots than at the wetland margin during the late summer/early fall. Seasonal and diel hystereses with respect to soil temperatures were of sufficient magnitude to prevent quantification of the influence of soil moisture, although apparent short‐term responses of SR to precipitation occurred. Calculations of annual cumulative SR illustrated a decreasing trend in SR over the 5 year period, which were correlated with decreasing springtime mean soil temperatures. Spring soil temperatures decreased despite rising air temperatures over the same period, possibly as an effect of earlier leaf expansion and shading. The synchronous decrease in spring soil temperatures and SR during regional warming of air temperatures may represent a negative feedback on a warming climate by reducing CO2 production from soils. SR reached a maximum later in the year than total ecosystem respiration (ER) measured at a nearby eddy covariance flux tower, and the seasonality of their temperature response patterns were roughly opposite. SR, particularly in the upland, exceeded ER in the late summer/early fall in each year, suggesting that areas of lower efflux such as the wetland may be significant in the flux tower footprint or that long‐term bias in either estimate may create a mismatch. Annual estimates of ER decreased over the same period and were highly correlated with SR

    Agricultural Perturbations of the Nitrogen Cycle and Related Impact on Atmospheric N_2O and Ozone

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    The available data are employed to identify the fate of agricultural nitrogen in the environment. Best estimates predict denitrification of nearly 50% of fertilizer nitrogen in less than 10 years after application. We also discuss in detail the expected demand curve for agricultural N. If population growth continues at projected levels, between 100 and 200 M tons/yr of agricultural N will be needed by the year 2000. We estimate that as a result, atmospheric N_2O could be more than doubled by 2050, and that perturbations of O_3 at that time could range from 10 to more than 20%. Major uncertainties remain however, and we emphasize the importance of further experimental research into the nitrogen cycle

    HAPTEN-SANDWICH LABELING : I. A GENERAL PROCEDURE FOR SIMULTANEOUS LABELING OF MULTIPLE CELL SURFACE ANTIGENS FOR FLUORESCENCE AND ELECTRON MICROSCOPY

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    A hapten-sandwich procedure has been developed for specific labeling of cell surface antigens for fluorescence or electron microscopy. Haptens are azo-coupled to immunoglobulins specific for a cell surface antigen; the hapten-modified cell-bound antibodies can then be visualized by adding fluorescent antihapten antibody, or by adding antihapten antibody followed by hapten-modified markers for electron microscopy. Virus or high molecular weight protein markers are lightly cross-linked before conjugation with hapten to prevent their disruption. Such stable hapten-modified markers, and the accessibility of many different purified anti-azophenyl-hapten antibodies, make it feasible to distinguish more than one membrane antigen in a given labeling experiment. When mouse lymphoid cell populations are labeled with separate markers for Ig and for thymus-associated antigens, many cells exhibit the Ig marker exclusively or the thymic marker predominantly, and some cells are completely free of label

    The atmospheric effects of stratospheric aircraft: A fourth program report

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    This document presents the fourth report from the Atmospheric Effects of Stratospheric Aircraft (AESA) component of NASA's High-Speed Research Program (HSRP). Market and technology considerations continue to provide an impetus for high-speed civil transport research. A recent AESA interim assessment report and a review of that report have shown that considerable uncertainty still exists about the possible impact of aircraft on the atmosphere. The AESA has been designed to develop the body of scientific knowledge necessary for the evaluation of the impact of stratospheric aircraft on the atmosphere. The first Program report presented the basic objectives and plans for AESA. This fourth report comes after the interim assessment and sets forth directions for the 1995 assessment at the end of AESA Phase 1. It also sets forth the goals and directions for AESA Phase 2, as reported at the 1994 Atmospheric Effects of Aviation Project (AEAP) annual meeting held in June. The focus of the Phase 2 effort is to obtain the best possible closure on the outstanding problems identified in the interim assessment and NASA/NRC review. Topics discussed in this report include how high-speed civil transports (HSCT) might affect stratospheric ozone, emissions scenarios and databases to assess potential atmospheric effects from HSCT's, calculated results from 2-D zonal mean models using emissions data, engine trace constituent measurements

    Vertical profiles of CO\u3csub\u3e2\u3c/sub\u3e above eastern Amazonia suggest a net carbon flux to the atmosphere and balanced biosphere between 2000 and 2009

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    From 2000 until January 2010 vertical profiles were collected above eastern Amazonia to help determine regional-scale (∌105–106 km2) fluxes of carbon cycle-related greenhouse gases. Samples were collected aboard light aircraft between the surface and 4.3 km and a column integration technique was used to determine the CO2 flux. Measured CO2 profiles were differenced from the CO2 background determined from measurements in the tropical Atlantic. The observed annual flux between the coast and measurement sites was 0.40 ± 0.27 gC m−2 d−1 (90% confidence interval using a bootstrap analysis). The wet season (January–June) mean flux was 0.44 ± 0.38 gC m−2 d−1 (positive fluxes defined as a source to the atmosphere) and the dry season mean flux was 0.35 ± 0.17 gC m−2 d−1 (July–December). The observed flux variability is high, principally in the wet season. The influence of biomass burning has been removed using co-measured CO, and revealed the presence of a significant dry season sink. The annual mean vegetation flux, after the biomass burning correction, was 0.02 ± 0.27 gC m−2 d−1, and a clear sink was observed between August and November of −0.70 ± 0.21 gC m−2 d−1 where for all of the dry season it was −0.24 ± 0.17 gC m−2 d−1

    On spin-rotation contribution to nuclear spin conversion in C_{3v}-symmetry molecules. Application to CH_3F

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    The symmetrized contribution of E-type spin-rotation interaction to conversion between spin modifications of E- and A_1-types in molecules with C_{3v}-symmetry is considered. Using the high-J descending of collisional broadening for accidental rotational resonances between these spin modifications, it was possible to co-ordinate the theoretical description of the conversion with (updated) experimental data for two carbon-substituted isotopes of fluoromethane. As a result, both E-type spin-rotation constants are obtained. They are roughly one and a half times more than the corresponding constants for (deutero)methane.Comment: 13 pages with single-spacing, REVTeX, no figures, accepted for publication in <J. Phys. B
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