.[1] Permeation of various gases through elastomeric O-ring seals can have important effects on the integrity of atmospheric air samples collected in flasks and measured some time later. Depending on the materials and geometry of flasks and valves and on partial pressure differences between sample and surrounding air, the concentrations of different components of air can be significantly altered during storage. The influence of permeation is discussed for O 2 /N 2 , Ar/N 2 , CO 2 , d 13 C in CO 2 , and water vapor. Results of sample storage tests for various flask and valve types and different storage conditions are presented and are compared with theoretical calculations. Effects of permeation can be reduced by maintaining short storage times and small partial pressure differences and by using a new valve design that buffers exchange of gases with surrounding air or by using less permeable materials (such as Kel-F) as sealing material. General awareness of possible permeation effects helps to achieve more reliable measurements of atmospheric composition with flask sampling techniques.

Key message Variations in stable carbon and oxygen isotope compositions of co-occurring plant species reflect their different water use strategies and indicate the importance of screening species' WUE i to plan climate change adaptation strategies. Abstract The different abilities of plant species to cope with drought have been associated with structural and ecophysiological constraints. In this paper, we evaluate interspecific differences in intrinsic water use efficiency (WUE i ) and the ratio of photosynthesis (A) to stomatal conductance (g s ) in three co-occurring Mediterranean shrubs: two broad-leaved evergreen (Pistacia lentiscus and Phillyrea angustifolia) and one needle-like-leaved evergreen (Juniperus phoenicea). We used d 13 C in rings to assess inter-annual changes in WUE i while the influence of the stomatal conductance was explored through d 18 O. Our results indicate consistent differences in WUE i in the three species, largely determined by leaf traits and differences in stomatal conductance control. Juniperus phoenicea could be the most threatened by the current trend of increasing temperature and summers drought. Phillyrea angustifolia and P. lentiscus seem to be less affected by drought stress because of their tighter stomatal control and high survival rate under field conditions. Our study shows that shrubs with different leaf traits employ different plant ecophysiological strategies under drought stress.

Abstract. Seeking for baseline conditions has biased the atmospheric carbon dioxide (CO 2 ) and later on also oxygen (O 2 ) monitoring networks towards remote marine stations, missing part of the variability that is due to regional anthropogenic as well as land biotic activity. We present here a five-year record of atmospheric CO 2 concentrations and oxygen/nitrogen (O 2 /N 2 ) ratio measurements from the coastal stations Lutjewad (LUT), The Netherlands and Mace Head (MHD), Ireland, derived from flask samples. O 2 /N 2 ratios, a proxy for O 2 concentrations, concurrently measured with CO 2 concentrations, help determine regional CO 2 fluxes by separating land fluxes from sea fluxes. Mace Head is the closest marine baseline station to Lutjewad, located at the same latitude, and therefore is taken as a reference. During the studied period, from 2000 until 2005, we observed an average increase of CO 2 in the atmosphere of (1.7±0.2) ppm y −1 , and a change of the O 2 /N 2 ratio of (−20±1) per meg y −1 . The difference between the CO 2 summer minimum and the winter maximum is 14.4 ppm and 16.1 ppm at Mace Head and Lutjewad, respectively, while the paraphase variation in the O 2 signal equals 113 per meg and 153 per meg, respectively. We also studied the atmospheric potential oxygen (APO) tracer at both stations. By this analysis, evidence has been found that we need to be careful when using APO close to anthropogenic CO 2 sources. It could be biased by combustion-derived CO 2 , and models need to take into account daily and seasonal variations in the anthropogenic CO 2 production in order to be able to simulate APO over the continents.Correspondence to: R. E. M. Neubert

In spite of extensive efforts, even the most experienced laboratories dealing with isotopic measurements of atmospheric CO2 still suffer from poor inter-laboratory consistency. One of the complicating factors of these isotope measurements is the presence of N2O, giving rise to mass overlap in the isotope ratio mass spectrometer (IRMS). The aim of the experiment reported here has been twofold: first, the re-establishment of the correction for 'mechanical' interference of N2O in the IRMS, along with its variability and drift, and the best way to quantitatively determine the correction factors. Second, an investigation into secondary effects, i.e. the influence of N2O admitted with the CO2 sample on the "cross contamination" between sample and (pure CO2) working gas. To make the suspected effects better detectable, isotopically enriched CO2 gas with different concentrations of N2O has been measured for the first time. No evidence of secondary effects was observed, from which we conclude that N2O is not a major player in the inter-laboratory consistency problems. Still, we also found that the determination of the 'mechanical' N2O correction needs to be very carefully determined for each individual IRMS, and should be periodically re-determined. We show that the determination of the correction should be performed using CO2/N2O mixtures with concentration ratios around that of the atmosphere, as the extrapolation from pure gas end member behaviour will give erroneous results due to non-linearities. For our IRMS, a VG SIRA series II, we find a correction of 0.23 per thousand for delta45CO2 and 0.30 per thousand for delta46CO2 of atmospheric samples, (with 0.85 per thousand mixing ratio). This implies that the relative ionisation efficiency (E) value associated with this machine is 0.75.

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