672 research outputs found
Interannual, seasonal, and diel variation in soil respiration relative to ecosystem respiration at a wetland to upland slope at Harvard Forest
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, 2003â2007. 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 shortâterm 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 longâterm 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
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
Reply to âComment by A. Appleby, D. Lilian and H. B. Singh on âAtmospheric halocarbons: A discussion with emphasis on chloroformââ
No abstract
HAPTEN-SANDWICH LABELING : I. A GENERAL PROCEDURE FOR SIMULTANEOUS LABELING OF MULTIPLE CELL SURFACE ANTIGENS FOR FLUORESCENCE AND ELECTRON MICROSCOPY
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
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Boreal forest CO2 exchange and evapotranspiration predicted by nine ecosystem process models: Intermodel comparisons and relationships to field measurements
Nine ecosystem process models were used to predict CO2 and water vapor exchanges by a 150-year-old black spruce forest in central Canada during 1994â1996 to evaluate and improve the models. Three models had hourly time steps, five had daily time steps, and one had monthly time steps. Model input included site ecosystem characteristics and meteorology. Model predictions were compared to eddy covariance (EC) measurements of whole-ecosystem CO2exchange and evapotranspiration, to chamber measurements of nighttime moss-surface CO2release, and to ground-based estimates of annual gross primary production, net primary production, net ecosystem production (NEP), plant respiration, and decomposition. Model-model differences were apparent for all variables. Model-measurement agreement was good in some cases but poor in others. Modeled annual NEP ranged from â11 g C mâ2 (weak CO2source) to 85 g C mâ2 (moderate CO2 sink). The models generally predicted greater annual CO2sink activity than measured by EC, a discrepancy consistent with the fact that model parameterizations represented the more productive fraction of the EC tower âfootprint.â At hourly to monthly timescales, predictions bracketed EC measurements so median predictions were similar to measurements, but there were quantitatively important model-measurement discrepancies found for all models at subannual timescales. For these models and input data, hourly time steps (and greater complexity) compared to daily time steps tended to improve model-measurement agreement for daily scale CO2 exchange and evapotranspiration (as judged by root-mean-squared error). Model time step and complexity played only small roles in monthly to annual predictions
The atmospheric effects of stratospheric aircraft: A fourth program report
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
From 2000 until January 2010 vertical profiles were collected above eastern Amazonia to help determine regional-scale (âŒ105â106 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 mâ2 dâ1 (90% confidence interval using a bootstrap analysis). The wet season (JanuaryâJune) mean flux was 0.44 ± 0.38 gC mâ2 dâ1 (positive fluxes defined as a source to the atmosphere) and the dry season mean flux was 0.35 ± 0.17 gC mâ2 dâ1 (JulyâDecember). 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 mâ2 dâ1, and a clear sink was observed between August and November of â0.70 ± 0.21 gC mâ2 dâ1 where for all of the dry season it was â0.24 ± 0.17 gC mâ2 dâ1
On spin-rotation contribution to nuclear spin conversion in C_{3v}-symmetry molecules. Application to CH_3F
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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