The usefulness of eddy covariance for understanding terrestrial carbon exchange has been hampered by uncertainty over the magnitude and causes of a systematic underestimation of CO2 efflux on calm nights. We combined in situ measurements of the temperature, wind and CO2 profile with nocturnal Land Surface Temperature (LST) imagery from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) to infer the patterns of cold air drainage in an Amazonian tropical forest. The meteorological tower was located on a flat plateau that sloped gently to the southwest. The vertical profile from 0 to 64 m above ground level was divisible into two air parcels at night: a warm, low CO2, upper parcel and a cool, high CO2, lower parcel that was stable with respect to the upper parcel. The nocturnal wind direction of the upper parcel was from the east, a pattern that is consistent with the general circulation, whereas the wind direction of the lower parcel was from the northeast, a pattern that implies drainage down the local topographic gradient. The nocturnal patterns of LST were closely related to local topography and land use. In general, a nearby river was warm, gullies were cold, plateau centers were cold, stream drainages were cold, pastures were particularly cold, and upper slopes and plateau edges were warm. The in situ temperature and wind observations, combined with the observed relationship between elevation and nocturnal LST and the occurrence of warm thermal belts extending inward from the edges of plateaus, imply that cold air drainage occurs on clear nights. The slope of the relationship between LST and elevation varied between nights, indicating that the degree of thermal stratification, and possibly the extent of cold air drainage, also varied. The night-tonight variation in stratification across the landscape was correlated with the vertical temperature gradient at the tower but not the above-canopy friction velocity (u*). Criteria associated with vertical temperature gradients may prove better than u* for screening nocturnal eddy covariance observations to eliminate periods that underestimate CO2 flux.

There is continuing debate about the controls on wetland evapotranspiration (E-t) and whether marshes are profligate water users. We used eddy covariance to measure the CO2 exchange and E-t by a California Tule marsh in 2003. The marsh was dominated by Typha and Scirpus, and there was a large amount of standing litter that acted as a mulch. Canopy development was broadly related to air temperature, with rapid growth in May and senescence in October. E-t was a few tenths of a mm d(-1) in winter, and 3-4 mm d(-1) in summer. The midsummer Bowen ratio was similar to 1, and the annual E, was 49 cm. The peak rate of E-t was lower than has been reported for marshes based on lysimeter studies, somewhat lower than has been reported for marshes based on micrometeorological studies, and equivalent to, or somewhat lower than, has been reported for upland grassland. The midsummer water use efficiency was 0.0025 mol CO2 mol(-1) H2O, and the delta C-13 of foliage was -27.1 parts per thousand, which are both typical for productive C-3 ecosystems. Transpiration accounted for 80% of total E-t Evaporation from water standing beneath the canopy and mulch layer was only a minor component of the marsh's hydrological budget. The low rate of evaporation from standing water was a result of cool water temperatures, which remained within a few degrees of the nocturnal minimum on most days. We believe the mulch layer acted in a way analogous to an electrical diode that allowed the upward loss of heat from the water to the atmosphere at night, and shut off the flux of heat from the atmosphere to the water during daytime. resulting in cool subcanopy water and low rates of evaporation. Our observations are inconsistent with the hypothesis that Tule marshes are inefficient water users, or that their rates of transpiration and CO2 uptake are unusual compared to upland ecosystems. (c) 2006 Elsevier B.V. All rights reserved.

Dimethylsulfide (DMS) atmospheric and oceanic concentrations and eddy covariance air/sea fluxes were measured over the N. Atlantic Ocean during July 2007 from Iceland to Woods Hole, MA, USA. Seawater DMS levels north of 55°N ranged from 3 to 17 nM, with variability related to the satellite-derived distributions of coccoliths and to a lesser extent, chlorophyll. For the most intense bloom region southwest of Iceland, DMS air/sea fluxes were as high as 300 μmol m⁻² d⁻¹, larger than current model estimates. The observations imply that gas exchange coefficients in this region are significantly greater than those estimated using most gas transfer parameterizations. South of 55°N, DMS levels were lower and the gas transfer coefficients were similar to those observed in other regions of the ocean. The data suggest that DMS emissions from the bloom region may be significantly larger than current estimates. The anomalous gas exchange coefficients likely reflect strong near-surface, water column DMS gradients influenced by physical and biological processes.

Shipboard measurements of air/sea fluxes and sea surface concentrations of dimethylsulfide (DMS) were made over the tropical and midlatitude North Pacific Ocean. Atmospheric pressure chemical ionization mass spectrometry was used to measure DMS levels in ambient air and in air equilibrated with surface seawater drawn from a depth of 5 m. Air/sea fluxes were obtained using eddy covariance. Corrections and uncertainties involved in the calculation of fluxes from shipboard data are discussed. The surface ocean DMS concentrations measured during this study ranged from 1 to 10 nM, and atmospheric mixing ratios ranged from 20 to 1000 ppt. Air/sea fluxes ranged from 0.47 to 39 μmol m⁻² d⁻¹. Most of the variance in the fluxes can be accounted for by variations in sea surface concentration (37%) and wind speed (19%). Gas transfer coefficients derived from the measurements are generally consistent with those derived from deliberate inert gas tracer experiments and eddy covariance measurements of CO₂. The gas transfer coefficients exhibit wind speed dependence, but the variance in the data is sufficiently large that they do not constrain the functionality of the wind speed dependence of gas exchange.

[1] In this study, direct measurements of the air/sea flux of acetone were made over the North Pacific Ocean. The results demonstrate that the net flux of acetone is into, rather than out of the oceans. The extrapolated global ocean uptake of 48 Tg yr⁻¹ requires a major revision of the atmospheric acetone budget. This result is consistent with a recent reevaluation of acetone photodissociation quantum yields.

Shipboard measurements of eddy covariance dimethylsulfide (DMS) air–sea fluxes and seawater concentration were carried out in the North Atlantic bloom region in June/July 2011. Gas transfer coefficients (k₆₆₀) show a linear dependence on mean horizontal wind speed at wind speeds up to 11 m s⁻¹. At higher wind speeds the relationship between k₆₆₀ and wind speed weakens. At high winds, measured DMS fluxes were lower than predicted based on the linear relationship between wind speed and interfacial stress extrapolated from low to intermediate wind speeds. In contrast, the transfer coefficient for sensible heat did not exhibit this effect. The apparent suppression of air–sea gas flux at higher wind speeds appears to be related to sea state, as determined from shipboard wave measurements. These observations are consistent with the idea that long waves suppress near-surface water-side turbulence, and decrease interfacial gas transfer. This effect may be more easily observed for DMS than for less soluble gases, such as CO₂, because the air–sea exchange of DMS is controlled by interfacial rather than bubble-mediated gas transfer under high wind speed conditions.

Air/sea fluxes of dimethylsulfide (DMS) were measured by eddy correlation over the Eastern South Pacific Ocean during January 2006. The cruise track extended from Manzanillo, Mexico, along 110° W, to Punta Arenas, Chile. Bulk air and surface ocean DMS levels were also measured and gas transfer coefficients (k_DMS) were computed. Air and seawater DMS measurements were made using chemical ionization mass spectrometry (API-CIMS) and a gas/liquid membrane equilibrator. Mean surface seawater DMS concentrations were 3.8±2.2 nM and atmospheric mixing ratios were 340±370 ppt. The air/sea flux of DMS was uniformly out of the ocean, with an average value of 12±15 μmol m⁻² d⁻¹. Sea surface concentration and flux were highest around 15° S, in a region influenced by shelf waters and lowest around 25° S, in low chlorophyll gyre waters. The DMS gas transfer coefficient exhibited a linear wind speed-dependence over the wind speed range of 1 to 9 m s⁻¹. This relationship is compared with previously measured estimates of k from DMS, CO₂, and dual tracer data from the Atlantic and Pacific Ocean, and with the NOAA/COARE gas transfer model. The model generated slope of k vs. wind speed is at the low end of those observed in previous DMS field studies.

We used eddy covariance and ecological measurements to investigate the effects of reduced impact logging (RIL) on an old-growth Amazonian forest. Logging caused small decreases in gross primary production, leaf production, and latent heat flux, which were roughly proportional to canopy loss, and increases in heterotrophic respiration, tree mortality, and wood production. The net effect of RIL was transient, and treatment effects were barely discernable after only 1 y. RIL appears to provide a strategy for managing tropical forest that minimizes the potential risks to climate associated with large changes in carbon and water exchange.                            

We used eddy covariance to measure the net exchange of CO2between theatmosphere and an old-growth tropical forest in Para ́, Brazil from 1 July 2000 to 1 July2001. The mean air temperature and daily temperature range varied little year-round; therainy season lasted from late December to around July. Daytime CO2uptake under highirradiance averaged 16–19mmol·m22·s21. Light was the main controller of CO2exchange,accounting for 48% of the half-hour-to-half-hour variance. The rate of canopy photosyn-thesis at a given irradiance was lower in the afternoon than the morning. This photosyntheticinhibition was probably caused by high evaporative demand, high temperature, an intrinsiccircadian rhythm, or a combination of the three. Wood increment increased from Januaryto May, suggesting greater rates of carbon sequestration during the wet season. However,the daily net CO2exchange measured by eddy covariance revealed the opposite trend, withgreater carbon accumulation during the dry season. A reduction in respiration during thedry season was an important cause of this seasonal pattern. The surface litter was desiccatedin the dry season, and the seasonal pattern of respiration appears to be a direct result ofreduced forest floor decomposition during drought. In contrast, canopy photosynthesis wasnot directly reduced by the dry season, probably because deep rooting allows the forest toavoid drought stress