Concentrations and snow-atmosphere fluxes of reactive nitrogen at Summit, Greenland

Concentrations and fluxes of NOy (total reactive nitrogen), ozone concentrations and fluxes of sensible heat, water vapor, and momentum were measured from May 1 to July 20, 1995 at Summit, Greenland. Median NOy concentrations declined from 947 ppt in May to 444 ppt by July. NOy fluxes were observed into and out of the snow, but the magnitudes were usually below 1 μmol m−2 h−1 because of the low HNO3 concentration and weak turbulence over the snow surface. Some of the highest observed fluxes may be due to temporary storage by equilibrium sorption of peroxyacetylnitrate (PAN) or other organic nitrogen species on ice surfaces in the upper snowpack. Sublimation of snow at the surface or during blowing snow events is associated with efflux of NOy from the snowpack. Because the NOy fluxes during summer at Summit are bidirectional and small in magnitude, the net result of turbulent NOyexchange is insignificant compared to the 2 μmol m−2 d−1 mean input from fresh snow during the summer months. If the arctic NOy reservoir is predominantly PAN (or compounds with similar properties), thermal dissociation of this NOy is sufficient to support the observed flux of nitrate in fresh snow. Very low HNO3 concentrations in the surface layer (1% of total NOy) reflect the poor ventilation of the surface layer over the snowpack combined with the relatively rapid uptake of HNO3 by fog, falling snow, and direct deposition to the snowpack

Air-snow exchange of HNO3 and NOy at Summit, Greenland

Ice core records of NO3− deposition to polar glaciers could provide unrivaled information on past photochemical status and N cycling dynamics of the troposphere, if the ice core records could be inverted to yield concentrations of reactive N oxides in the atmosphere at past times. Limited previous investigations at Summit, Greenland, have suggested that this inversion may be difficult, since the levels of HNO3 and aerosol-associated NO3− over the snow are very low in comparison with those of NO3− in the snow. In addition, it appears that some fraction of the NO3− in snow may be reemitted to the atmosphere after deposition. Here we report on extensive measurements of HNO3, including vertical gradients between 1.5 and 7 m above the snow, made during the summers of 1994 and 1995 at Summit. These HNO3 data are compared with NO3− concentrations in surface snow and the first measurements of the concentrations and fluxes of total reactive nitrogen oxides (Ny) on a polar glacier. Our results confirm that HNO3 concentrations are quite low (mean 0.5 nmol m−3) during the summer, while NO3− is the dominant ion in snow. Daytime peaks in HNO3− appear to be due at least partly to emissions from the snow, an assertion supported by gradients indicating a surface source for HNO3− on many days. Observed short-term increases in NO3− inventory in the snow can be too large to be readily attributed to deposition of HNO3− suggesting that deposition of one or more other N oxides must be considered. We found that the apparent fluxes of HNO3 and NOy were in opposite directions during about half the intervals when both were measured, with more cases of HNO3 leaving the snow, against an NOy flux into the snow, than the reverse. The concentrations of NOy are generally about 2 orders of magnitude greater than those of HNO3; hence deposition of only a small, non-HNO3, fraction of this pool could dominate NO3− in snow, if the depositing species converted to NO3−, either in the snowpack or upon melting for analysis

Monellin (MNEI) at 1.15 Å resolution

The crystal structure of the sweet protein MNEI at 1.15 Å resolution reveals networks of alternate conformations and stably bound negative ions

What can tracer observations in the continental boundary layer tell us about surface-atmosphere fluxes?

International audienceWe analyze the potential for inferring spatially resolved surface fluxes from atmospheric tracer observations within the mixed layer, such as from monitoring towers, using a receptor oriented transport model (Stochastic Time-Inverted Lagrangian Transport model ? STILT) coupled to a simple biosphere in which CO2 fluxes are represented as functional responses to environmental drivers (radiation and temperature). Transport and biospheric fluxes are coupled on a dynamic grid using a polar projection with high horizontal resolution (~20 km) in near field, and low resolution far away (as coarse as 2000 km), reducing the number of surface pixels without significant loss of information. To test the system, and to evaluate the errors associated with the retrieval of fluxes from atmospheric observations, a pseudo data experiment was performed. A large number of realizations of measurements (pseudo data) and a priori fluxes were generated, and for each case spatially resolved fluxes were retrieved. Results indicate strong potential for high resolution retrievals based on a network of tall towers, subject to the requirement of correctly specifying the a priori uncertainty covariance, especially the off diagonal elements that control spatial correlations. False assumptions about the degree to which the uncertainties in the a priori fluxes are spatially correlated may lead to a strong underestimation of uncertainties in the retrieved fluxes, or, equivalently, to biased retrievals. The framework presented here, however, allows a conservative choice of the off diagonal elements that avoids biasing the retrievals

An Improved Multipyranometer Array for the Measurement of Direct and Diffuse Solar Radiation

This paper describes an improved multipyranometer array (MPA) for the continuous remote measurement of direct and diffuse solar radiation. The MPA described in this paper is an improvement over previously published MPA studies due to the incorporation of an artificial horizon that prevents reflected ground radiation from striking the tilted sensors. In this paper a description of the NIST-traceable calibration facility is provided and preliminary results are presented that compare the MPA predicted beam to beam measurements from a precision normal incidence pyrheliometer and diffuse measurements from a precision shadow-band pyranometer respectively

Development of Simplified Calculations for a Multipyranometer Array for the Measurement of Direct and Diffuse Solar Radiation

This paper describes the development of simplified procedures for a multipyranometer array (MPA) for the continuous measurement of direct and diffuse solar radiation. The MPA described in this paper is an improvement over previously published MPA studies due several new features, including: the incorporation of an artificial horizon that prevents reflected ground radiation from striking the tilted sensors, and a routine that corrects the spectral response of photovoltaic-type sensors used in the MPA. An optimal solution procedure has also been developed that eliminates invalid data which are inherent in the simultaneous solution of the solar equations from the four MPA sensors. In this paper a description of the NIST-traceable calibration facility is provided and results are presented that compare the improved MPA-predicted beam to side-by-side measurements from a precision Normal Incidence Pyrheliometer (NIP)

Electron electric dipole moment experiment using electric-field quantized slow cesium atoms

A proof-of-principle electron electric dipole moment (e-EDM) experiment using slow cesium atoms, nulled magnetic fields, and electric field quantization has been performed. With the ambient magnetic fields seen by the atoms reduced to less than 200 pT, an electric field of 6 MV/m lifts the degeneracy between states of unequal mF and, along with the low (approximately 3 m/s) velocity, suppresses the systematic effect from the motional magnetic field. The low velocity and small residual magnetic field have made it possible to induce transitions between states and to perform state preparation, analysis, and detection in regions free of applied static magnetic and electric fields. This experiment demonstrates techniques that may be used to improve the e-EDM limit by two orders of magnitude, but it is not in itself a sensitive e-EDM search, mostly due to limitations of the laser system.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.

Ecosystem fluxes of hydrogen: a comparison of flux-gradient methods

Our understanding of biosphere–atmosphere exchange has been considerably enhanced by eddy covariance measurements. However, there remain many trace gases, such as molecular hydrogen (H[subscript 2]), that lack suitable analytical methods to measure their fluxes by eddy covariance. In such cases, flux-gradient methods can be used to calculate ecosystem-scale fluxes from vertical concentration gradients. The budget of atmospheric H[subscript 2] is poorly constrained by the limited available observations, and thus the ability to quantify and characterize the sources and sinks of H[subscript 2] by flux-gradient methods in various ecosystems is important. We developed an approach to make nonintrusive, automated measurements of ecosystem-scale H2 fluxes both above and below the forest canopy at the Harvard Forest in Petersham, Massachusetts, for over a year. We used three flux-gradient methods to calculate the fluxes: two similarity methods that do not rely on a micrometeorological determination of the eddy diffusivity, K, based on (1) trace gases or (2) sensible heat, and one flux-gradient method that (3) parameterizes K. We quantitatively assessed the flux-gradient methods using CO[subscript 2] and H[subscript 2]O by comparison to their simultaneous independent flux measurements via eddy covariance and soil chambers. All three flux-gradient methods performed well in certain locations, seasons, and times of day, and the best methods were trace gas similarity for above the canopy and K parameterization below it. Sensible heat similarity required several independent measurements, and the results were more variable, in part because those data were only available in the winter, when heat fluxes and temperature gradients were small and difficult to measure. Biases were often observed between flux-gradient methods and the independent flux measurements, and there was at least a 26% difference in nocturnal eddy-derived net ecosystem exchange (NEE) and chamber measurements. H[subscript 2] fluxes calculated in a summer period agreed within their uncertainty and pointed to soil uptake as the main driver of H[subscript 2] exchange at Harvard Forest, with H[subscript 2] deposition velocities ranging from 0.04 to 0.10 cm s[superscript −1].National Science Foundation (U.S.) (NSF Graduate Research Fellowship)United States. National Aeronautics and Space Administration (Advanced Global Atmospheric Gases Experiment (AGAGE))Massachusetts Institute of Technology. Joint Program on the Science & Policy of Global ChangeMartin Family Society of Fellows for SustainabilityMassachusetts Institute of Technology (Ally of Nature Research Fund)Massachusetts Institute of Technology (William Otis Crosby Lectureship)Massachusetts Institute of Technology (Warren Klein Fund

Post‐discharge tobacco cessation rates among hospitalized US veterans with and without diabetes

Aims  Smoking is a major risk factor for cardiovascular complications among patients with diabetes. Hospitalization has been shown to enhance cessation rates. The purpose of this study was to compare 6‐month post‐hospitalization tobacco cessation rates among US veterans with and without diabetes. Methods  This was a longitudinal study among inpatient veterans who used tobacco in the past month ( n  = 496). Patients were recruited and surveyed from three Midwestern Department of Veterans Affairs hospitals during an acute‐care hospitalization. They were also asked to complete a follow‐up survey 6 months post‐discharge. Bivariate‐ and multivariable‐adjusted analyses were conducted to determine differences in tobacco cessation rates between patients with and without a diagnosis of diabetes. Results  The mean age of patients was 55.2 years and 62% were white. Twenty‐nine per cent had co‐morbid diabetes. A total of 18.8% of patients with diabetes reported tobacco cessation at 6 months compared with 10.9% of those without diabetes ( P  = 0.02). Cotinine‐verified cessation rates were 12.5 vs. 7.4% in the groups with and without diabetes, respectively ( P  = 0.07). Controlling for psychiatric co‐morbidities, depressive symptoms, age, self‐rated health and nicotine dependence, the multivariable‐adjusted logistic regression showed that patients with diabetes had three times higher odds of 6‐month cotinine‐verified tobacco cessation as compared with those without diabetes (odds ratio 3.17, P  = 0.005). Conclusions  Post‐hospitalization rates of smoking cessation are high among those with diabetes. Intensive tobacco cessation programmes may increase these cessation rates further.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/92145/1/j.1464-5491.2012.03635.x.pd

Intercomparison of field measurements of nitrous acid (HONO) during the SHARP campaign

Because of the importance of HONO as a radical reservoir, consistent and accurate measurements of its concentration are needed. As part of SHARP (Study of Houston Atmospheric Radical Precursors), time series of HONO were obtained by six different measurement techniques on the roof of the Moody Tower at the University of Houston. Techniques used were long path differential optical absorption spectroscopy (DOAS), stripping coil-visible absorption photometry (SC-AP), long path absorption photometry (LOPAP® ), mist chamber/ion chromatography (MC-IC), quantum cascade-tunable infrared laser differential absorption spectroscopy (QC-TILDAS), and ion drift-chemical ionization mass spectrometry (ID-CIMS). Various combinations of techniques were in operation from 15 April through 31 May 2009. All instruments recorded a similar diurnal pattern of HONO concentrations with higher median and mean values during the night than during the day. Highest values were observed in the final 2 weeks of the campaign. Inlets for the MC-IC, SC-AP, and QC-TILDAS were collocated and agreed most closely with each other based on several measures. Largest differences between pairs of measurements were evident during the day for concentrations ~100 parts per trillion (ppt). Above ~ 200 ppt, concentrations from the SC-AP, MC-IC, and QC-TILDAS converged to within about 20%, with slightly larger discrepancies when DOAS was considered. During the first 2 weeks, HONO measured by ID-CIMS agreed with these techniques, but ID-CIMS reported higher values during the afternoon and evening of the final 4 weeks, possibly from interference from unknown sources. A number of factors, including building related sources, likely affected measured concentrations