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, 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

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

Toward a More Open Trade Policy

Progress in achieving a healthier and more open trade system can only come from the recognition of the policy shortcomings of all sides. This study includes 8 proposals for breaking trade deadlock, with the U.S. taking the lead in a new approach to achieving freer world markets.https://openscholarship.wustl.edu/mlw_papers/1056/thumbnail.jp

Increasing background ozone in surface air over the United States

The long-term trend of background O3 in surface air over the United States from 1980 to 1998 is examined using monthly probability distributions of daily maximum 8-hour average O3 concentrations at a large ensemble of rural sites. Ozone concentrations have decreased at the high end of the probability distribution (reflecting emission controls) but have increased at the low end. The cross-over takes place between the 30th and 50th percentiles in May–August and between the 60th and 90th percentiles during the rest of the year. The increase is statistically significant at a 5% level in spring and fall, when it is 3–5 ppbv. The maximum increase is in the Northeast. A possible explanation is an increase in the O3 background transported from outside the United States. Better understanding of the causes of the increase is needed because of its implications for meeting O3 air quality standards

Self-interest And Public Interest: The Motivations Of Political Actors

Self-Interest and Public Interest in Western Politics showed that the public, politicians, and bureaucrats are often public spirited. But this does not invalidate public-choice theory. Public-choice theory is an ideal type, not a claim that self-interest explains all political behavior. Instead, public-choice theory is useful in creating rules and institutions that guard against the worst case, which would be universal self-interestedness in politics. In contrast, the public-interest hypothesis is neither a comprehensive explanation of political behavior nor a sound basis for institutional design

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

Formaldehyde, glyoxal, and methylglyoxal in air and cloudwater at a rural mountain site in central Virginia

As part of the Shenandoah Cloud and Photochemistry Experiment (SCAPE), we measured formaldehyde (HCHO), glyoxal (CHOCHO), and methylglyoxal (CH3C(O)CHO) concentrations in air and cloudwater at Pinnacles (elevation 1037 m) in Shenandoah National Park during September 1990. Mean gas‐phase concentrations of HCHO and CHOCHO were 980 and 44 pptv, respectively. The concentration of CH3C(O)CHO rarely exceeded the detection limit of 50 pptv. Mean cloudwater concentrations of HCHO and CHOCHO were 9 and 2 μM, respectively; the mean CH3C(O)CHO concentration was below its detection limit of 0.3 μM. The maximum carbonyl concentrations were observed during stagnation events with high O3, peroxides, and CO. Outside of these events the carbonyls did not correlate significantly with O3, CO, or NOy. Carbonyl concentrations and concentration ratios were consistent with a major source for the carbonyls from isoprene oxidation. Oxidation of CH4 supplies a significant background of HCHO. The carbonyl concentrations were indistinguishable in two size fractions of cloudwater having a cut at d=18 μm. Gas‐ and aqueous‐phase concentrations of HCHO from samples collected during a nighttime cloud event agree with thermodynamic equilibria within a factor of 2. Samples collected during a daytime cloud event show HCHO supersaturation by up to a factor of 4. Positive artifacts in the cloudwater samples due to hydrolysis of hydroxymethylhydroperoxide (HOCH2OOH) could perhaps account for this discrepancy

Seasonal cycles of isoprene concentrations in the Amazonian rainforest

International audienceTropical forests are an important global source of volatile organic compounds (VOCs) and other atmospheric trace gases. The high biodiversity in tropical rainforests complicates the extrapolation of biogenic volatile organic compound (BVOC) emissions from leaf-level measurements to landscape and regional or global scales. In Amazônia, a significant fraction of the carbon emitted from the biosphere to the atmosphere is emitted in the form of BVOCs, and the knowledge of these emissions is important to our understanding of tropical and global atmospheric chemistry and carbon cycling. As part of the Large scale Biosphere-atmosphere experiment in Amazônia (LBA). VOC concentrations were measured at two sites near Santarém, Para State, Brazil. The two sites are located in the National Forest of Tapajós, the first corresponding to primary forest and the second to a forest, that was selectively logged. The samples were collected simultaneously at heights of 65 and 55 m (20 and 10 m above forest canopy, respectively). The average isoprene mixing ratio was 2.2?2.5 ppb at the two sites and the standard deviations within a site ranged from 1 to 1.2 ppb. A strong seasonality of isoprene mixing ratio was observed and associated with the wet and dry seasons. The lowest mixing ratios were found during the transition between the wet to dry season, while at the start of the biomass burning season the mixing ratios increase. A qualitative analysis of a one dimensional model demonstrates that the seasonal cycle in concentrations reflects changes in isoprene production by the ecosystem, not changes in boundary layer dynamics or chemistry. The magnitude of the cycle indicates that the physiological capacity of the ecosystem to emit isoprene may depend on water availability although phenological changes could also contribute to the observed variations. A simple 1-D model that assumes mean daytime isoprene fluxes of 1.5 mg m?2h?1 and 0.9 mg m?2h?1 scaled by an algorithm depending on precipitation at the primary forest and selectively logged sites, respectively, is able to reproduce the observed vertical gradients

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