Factors concerning NO2 uptake by the absorbent triethanolamine (TEA) in NO2 diffusion tubes are examined. Although the nominal freezing point of TEA is 17.9-21.2 degrees C, we show that, for a range of aqueous TEA solutions (0-20%, H2O), no freezing occurs even at -10 degrees C. Therefore NO2 collection efficiency is unlikely to be impaired by low temperature exposure. The recovery of TEA from the meshes of exposed samplers is determined as approximately 98%, even after 42 days, showing that the stability in situ of TEA is unaffected by long-term exposure. A model of a diffusion tube sampling array for simultaneous exposures, with a 0.1 m sampler spacing, shows that NO2 uptake by individual samplers is not affected by the presence of neighbouring tubes in the array. This is confirmed by sampler precision at two Cambridge sites. Four sampler preparation methods are compared for differences in NO2 uptake of exposed samplers. All methods employ TEA as absorbent, transferred by either dipping meshes in a TEA-acetone solution or pipetting aliquots of a TEA-H2O solution onto the meshes. For samplers prepared by three of the methods, no difference in NO2 uptake is found, but for samplers prepared using a 50% v/v TEA-H2O solution, a mean reduction of 18% is found. Student's t-tests show that the difference is highly significant (P < or = 0.001). Reasons for the difference are discussed.
Measurement of nitrogen dioxide using passive diffusion tube over 22 months in Cambridge, U.K. are analysed as a function of sampler exposure time, and compared with NO2 concentrations obtained from a co-located chemiluminescence analyser. The average ratios of passive sampler to analyser NO2 at a city centre site (mean NO2 concentration 22 ppb) are 1.27 (n = 22), 1.16 (n = 34) and 1.11 (n = 7) for exposures of 1, 2 and 4-weeks, respectively. Modelling the generation of extra NO2 arising from chemical reaction between co-diffusing NO and O3 in the tube gave a ratio (modelled/measured) of 1.31 for 1-week exposures. Such overestimation is greatest when NO2 constitutes, on average, about half of total NOx (= NO + NO2) at the monitoring locality. Although 4week exposures gave concentrations which were not significantly different from analyser NO2, there was no correlation between the datasets. At both the city-centre site and another semi-rural site (mean NO2 concentration 11 ppb) the average of the aggregate of four consecutive 1-week sampler exposures or of two consecutive 2-week sampler exposures was systematically greater than for a single 4-week exposure. The results indicate two independent and opposing systematic biases in measurement of NO2 by passive diffusion sampler: an exposure-time independent chemical overestimation with magnitude determined by local relative concentrations of NO and O3 to NO2, and an exposuretime dependent reduction in sampling efficiency. The impact of these and other potential sources of systematic bias on the application of passive diffusion tubes for assessing ambient concentrations of NO2 in short (1-week) or long (4-week) exposures are discussed in detail.
Summary Plasmids have been described in almost all bacterial species analysed and have proven to be essential genetic tools. In many bacteria these extrachro‐mosomal DNAs are cryptic with no known markers or function, which makes their characterization and genetic exploitation extremely difficult. Here we describe a system that will allow the rescue of any circular DNA (plasmid or phage) using an in vitro transposition system to deliver both a selectable marker (kanamycin) and an Escherichia coli plasmid origin of replication. In this study, we demonstrate the rescue of four cryptic plasmids from the opportunistic pathogen Mycobacterium avium. To evaluate the host range of the rescued plasmids, we have examined their ability to be propagated in Mycobacterium smegmatis and Mycobacterium bovis BCG, and their compatibility with other mycobacterial plasmids. In addition, we use a library of transposon insertions to sequence one plasmid, pVT2, and to begin a genetic analysis of plasmid genes. Using this approach, we identified a putative conjugative relaxase, suggesting this myco‐bacterial plasmid is transferable, and three genes required for plasmid establishment and replication.
Two studies at three sites in the UK provided confirmation that systematic positive bias in NO2 diffusion tube measurement occurred because of changes to "within-tube" chemistry, rather than eddy diffusion at the mouth of the tube. In the first study in Cambridge, UK, sampler overestimation for 1 and 2 week exposures was compared to corresponding time-averaged monitor measurements (NO-NO2-NOx, O3) and weather variables. Noninearity between sampler and monitor NO2 measurements was interpreted in terms of spatial and temporal variations in relative and absolute availability of NO, NO2 and O3 at the site. A maximum overestimation occurred for an exposure mean NO2/NOx approximately 0.5. The separate contributions of reduced NO2 photolysis and eddy diffusion were compared in Study II using samplers of two materials, acrylic and quartz, and of different lengths (40, 55, 71 and 120 mm) at three sites: Norwich background, Cambridge intermediate, London kerbside. For compared sites, NO2 measured by acrylic samplers was significantly higher than for equivalent quartz samplers. For quartz samplers [NO2]mean was only just above the monitor at Norwich and London; sampler/monitor NO2 = 1.04 (P = 0.59) and 1.01(P = 0.76), respectively. For acrylic samplers the order of [NO2]mean was 40 mm > 120 mm > 71 mm > or = 55 mm. Excepting 40 mm samplers, this accords with a chemical bias where co-diffusing NO and 03 molecules in longer tubes have more time to react to form excess NO2. Bias in 40 mm samplers is discussed. Eddy diffusion is negligible for standard samplers because [NO2]mean was equivalent for 55 mm and 71 mm acrylic samplers and close to monitor NO2 for 71 mm quartz tubes. Both studies showed that sampler accuracy was dependent on location. Significantly, overestimation was greatest (approximately 3-4 ppb) where the NO2 annual mean was approximately 20 ppb, close to the UK and EU air quality standard of 21 ppb.
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