The controversial claim that attached leaves of terrestrial plants emit CH 4 aerobically remains to be corroborated. Here, we report CH 4 fluxes and CO 2 exchange rates for leaves of the C 4 species Zea mays using a high-accuracy traceable online analytical system. In contrast to earlier results for Z. mays, our measurements provide no evidence for substantial aerobic CH 4 emissions from photosynthesizing leaves illuminated with photosynthetically active radiation (k 5 400-700 nm), or from dark-respiring leaves. Preliminary measurements with the same system indicated a similar lack of aerobic CH 4 emissions in the light or dark from leaves of the C 3 species Nicotiana tabacum. These findings are supported by independent high-precision 13 C-labeling studies that also failed to confirm substantial aerobic CH 4 emissions from plants. Nevertheless, we are not able to exclude the possibility that CH 4 emissions from plants may be linked to nonenzymatic processes with an action spectrum lying outside the wavelength range for photosynthesis.

A number of manufacturing processes are adversely affected by the presence of trace amounts of water vapour in purge and process gases. It is important for the operators of such processes to monitor the levels of water vapour in these gases continuously. There are many analytical methods for determination of trace water vapour which all require regular, traceable calibration. In this paper, a new standard facility for generating an adjustable level of trace water vapour is described. It uses continuous accurate measurements of the mass loss from a permeation device coupled with a dilution system based on an array of critical flow orifices. Details of the verification of the facility are presented along with results of comparisons with the National Physical Laboratory thermally-based dew/ frost point humidity standards facility.

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Accurate measurements of sulphur dioxide at the concentrations found in ambient air have become essential to support monitoring and legislation concerned with air quality. In general, the primary element of quality assurance for field instruments is regular calibration using certified gas mixtures. The concentration range chosen for this Key Comparison (240 nmol/mol to 320 nmol/mol) is defined by appropriate European standards and is typical of similar levels used around the world.The travelling standards used for the comparison were prepared commercially by a supplier with a proven track record of preparing stable mixtures of the relevant gases. The coordinating laboratory (NPL) carried out stability checks on the mixtures and determined the amount fraction using a primary gravimetric permeation facility. These data were used to determine the drift rate (and uncertainty) of each standard.The results for the 11 participants in CCQM-K26.b and the one participant in CCQM-P50.b are presented in this report. Degrees of equivalence have been calculated based on a reference value (corresponding to the KCRV) derived from the primary gravimetric facility used by the coordinating laboratory.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

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