Seasonality of temperate forest photosynthesis and daytime respiration.

Terrestrial ecosystems currently offset one-quarter of anthropogenic carbon dioxide (CO2) emissions because of a slight imbalance between global terrestrial photosynthesis and respiration. Understanding what controls these two biological fluxes is therefore crucial to predicting climate change. Yet there is no way of directly measuring the photosynthesis or daytime respiration of a whole ecosystem of interacting organisms; instead, these fluxes are generally inferred from measurements of net ecosystem-atmosphere CO2 exchange (NEE), in a way that is based on assumed ecosystem-scale responses to the environment. The consequent view of temperate deciduous forests (an important CO2 sink) is that, first, ecosystem respiration is greater during the day than at night; and second, ecosystem photosynthetic light-use efficiency peaks after leaf expansion in spring and then declines, presumably because of leaf ageing or water stress. This view has underlain the development of terrestrial biosphere models used in climate prediction and of remote sensing indices of global biosphere productivity. Here, we use new isotopic instrumentation to determine ecosystem photosynthesis and daytime respiration in a temperate deciduous forest over a three-year period. We find that ecosystem respiration is lower during the day than at night-the first robust evidence of the inhibition of leaf respiration by light at the ecosystem scale. Because they do not capture this effect, standard approaches overestimate ecosystem photosynthesis and daytime respiration in the first half of the growing season at our site, and inaccurately portray ecosystem photosynthetic light-use efficiency. These findings revise our understanding of forest-atmosphere carbon exchange, and provide a basis for investigating how leaf-level physiological dynamics manifest at the canopy scale in other ecosystems.

The influence of atmospheric pressure on landfill methane emissions.

Landfills are the largest source of anthropogenic methane (CH4) emissions to the atmosphere in the United States. However, few measurements of whole landfill CH4 emissions have been reported. Here, we present the results of a multi-season study of whole landfill CH4 emissions using atmospheric tracer methods at the Nashua, New Hampshire Municipal landfill in the northeastern United States. The measurement data include 12 individual emission tests, each test consisting of 5-8 plume measurements. Measured emissions were negatively correlated with surface atmospheric pressure and ranged from 7.3 to 26.5 m3 CH4 min(-1). A simple regression model of our results was used to calculate an annual emission rate of 8.4 x 10(6) m3 CH4 year(-1). These data, along with CH4 oxidation estimates based on emitted landfill gas isotopic characteristics and gas collection data, were used to estimate annual CH4 generation at this landfill. A reported gas collection rate of 7.1 x 10(6) m3 CH4 year(-1) and an estimated annual rate of CH4 oxidation by cover soils of 1.2 x 10(6) m3 CH4 year(-1) resulted in a calculated annual CH4 generation rate of 16.7 x 10(6) m3 CH4 year(-1). These results underscore the necessity of understanding a landfill's dynamic environment before assessing long-term emissions potential.

Tunable infrared laser detection of pyrolysis products of explosives in soils.

A research program involving two applications of tunable infrared laser differential absorption spectroscopy (TILDAS) with multipass, long-path absorption cells to the detection of explosives contamination in soils is reported. In the first application, sensitive, specific real-time species concentration measurements by TILDAS have led to new understanding of the processes involved in explosives detection by the heating of contaminated soils and the quantification of the resulting pyrolysis gases. In the second, we present results of our calculations of the properties of astigmatic off-axis resonator absorption cells, which show that useful TILDAS path lengths can be achieved inside a cone penetrometer probe.

Astigmatic mirror multipass absorption cells for long-path-length spectroscopy.

A multipass absorption cell, based on an astigmatic variant of the off-axis resonator (Herriott) configuration, has been designed to obtain long path lengths in small volumes. Rotation of the mirror axes is used to obtain an effective adjustability in the two mirror radii. This allows one to compensate for errors in mirror radii that are encountered in manufacture, thereby generating the desired reentrant patterns with less-precise mirrors. Acombination of mirror rotation and separation changes can be used to reach a variety of reentrant patterns and path lengths with a fixed set of astigmatic mirrors. The accessible patterns can be determined from trajectories, as a function of rotation and separation, through a general map of reentrant solutions. Desirable patterns for long-path spectroscopy can be chosen on the basis of path length, distance of the closest beam spot from the coupling hole, and tilt insensitivity. We describe the mathematics and analysis methods for the astigmatic cell with mirror rotation and then describe the design and test of prototype cells with this concept. Two cell designs are presented, a cell with 100-m path length in a volume of 3 L and a cell with 36-m path length in a volume of 0.3 L. Tests of low-volume absorption cells that use mirror rotation, designed for fast-flow atmospheric sampling, show the validity and the usefulness of the techniques that we have developed.

Narrow optical interference fringes for certain setup conditions in multipass absorption cells of the Herriott type.

We can select narrow FSR fringes in a multipass cell of the type described by Herriott et al. by setting the mirror spacing to give a particular type of beam path: M = odd integer, N = 4M +/- 4.

Atmospheric methane measurement instrument using a Zeeman-split He-Ne laser.

We report the construction of an atmospheric methane measurement instrument based on a Zeeman-split IR He-Ne laser. The laser has a transverse magnetic field over ~2/3 of its gain length and can oscillate at an (unsplit) frequency (2947.91 cm(-1)) centered on a methane absorption line, or on either of two frequencies split by +/-0.055 cm(-1)) from the center, with low CH(4)) absorption. The laser is tuned to dwell sequentially at each frequency, giving two differential absorption measurements in each 46-ms tuning cycle. Atmospheric measurements are made using two multiple pass absorption cells, one with fast (0.75-s) and one with slow (5-s) flow response times. Fluctuations in ambient CH(4)) of ~20-ppb (rms, 1-s averaging) are detected, with interference fringe effects the dominant noise source. The instrument has operated in a field experiment (NASA GTE/ABLE-3A) in Alaska.

Switched holograms for reconfigurable optical interconnection: demonstration of a prototype device.

We report the construction of a device intended for programmable optical interconnection of multiple processors in a highly parallel computer. This device, the Holoswitch, is based on an array of optical switches (liquid crystal polarization switches and polarizing beam splitters), which direct a set of optical beams toward any of a selection of holograms. Each hologram, when selected, deflects the input beams toward an output array with any desired prerecorded permutation. The prototype device has nine different interconnection patterns recorded on a single 10-x12.7-cm DCG hologram. We have recorded permutation patterns with up to 64 optical channels in an area of 4 cm(2).

Construction of an optical-carry adder.

We have constructed a hybrid electronic/optical digital adder, based on a system suggested by B. Arazi [Proc. IEEE 73,162 (1985)], which uses an optical system for the carry-bits, giving fully parallel operation. Our bulk-optic breadboard system has six acoustooptic modulators as optical switches, with a single He-Ne laser source. It adds two 4-bit words at 10 MHz, with a 500-ns pipeline delay. The optical carry is based on a multistage optical path, with light added or allowed to pass at each stage, according to whether carries are generated or propagated at the corresponding column of the addition. For long addends, the system speed is limited by light loss through the carry path and by size effects.