20 research outputs found
Standards development of global warming gas species : Methane, nitrous oxide, trichlorofluoromethane, dichlorodifluoromethane
Standards Development of Global Warming Gas Species: Methane, Nitrous Oxide, Trichlorofluoromethane, and Dichlorodifluoromethane
Stability Assessment of Gas Mixtures Containing Monoterpenes in Varying Cylinder Materials and Treatments
Studies of climate change increasingly
recognize the diverse influences
exerted by monoterpenes in the atmosphere, including roles in particulates,
ozone formation, and oxidizing potential. Measurements of key monoterpenes
suggest atmospheric mole fractions ranging from low pmol/mol (parts-per-trillion;
ppt) to nmol/mol (parts-per-billion; ppb), depending on location and
compound. To accurately establish the mole fraction trends, assess
the role of monoterpenes in atmospheric chemistry, and relate measurement
records from many laboratories and researchers, it is essential to
have good calibration standards. The feasibility of preparing well-characterized,
stable gas cylinder standards for monoterpenes at the nmol/mol level
was previously tested using treated (Aculife IV) aluminum gas cylinders
at NIST. Results for 4 of the 11 monoterpenes, monitored versus an
internal standard of benzene, indicated stability in these treated
aluminum gas cylinders for over 6 months and projected long-term (years)
stability. However, the mole fraction of the key monoterpene β-pinene
decreased, while the mole fractions of α-pinene, d-limonene
(<i>R</i>-(<i>+</i>)-limonene), <i>p</i>-cymene, and camphene (a terpene not present in the initial gas mixture)
increased, indicating a chemical transformation of β-pinene
to these species. A similar pattern of decreasing mole fraction was
observed in α-pinene where growth of d-limonene, <i>p</i>-cymene, and camphene has been observed in treated gas
cylinders prepared with a mixture of just α-pinene and benzene
as the internal standard. The current research discusses the testing
of other cylinders and treatments for the potential of long-term stability
of monoterpenes in a gas mixture. In this current study, a similar
pattern of decreasing mole fraction, although somewhat improved short-term
stability, was observed for β-pinene and α-pinene, with
growth of d-limonene, <i>p</i>-cymene, and camphene,
in nickel-plated carbon steel cylinders. β-Pinene and α-pinene
showed excellent stability at over 6 months in aluminum cylinders
treated with a different process (Experis) than used in the original
study
NIST Gravimetrically Prepared Atmospheric Level Methane in Dry Air Standards Suite
The Gas Metrology Group at the National Institute of
Standards
and Technology was tasked, by a congressional climate change act,
to support the atmospheric measurement community through standards
development of key greenhouse gases. This paper discusses the development
of a methane (CH<sub>4</sub>) primary standard gas mixture (PSM) suite
to support CH<sub>4</sub> measurement needs over a large amount-of-substance
fraction range 0.3–20 000 μmol mol<sup>–1</sup>, but with emphasis at the atmospheric level 300–4000 nmol
mol<sup>–1</sup>. Thirty-six CH<sub>4</sub> in dry air PSMs
were prepared in 5.9 L high-pressure aluminum cylinders with use of
a time-tested gravimetric technique. Ultimately 14 of these 36 PSMs
define a CH<sub>4</sub> standard suite covering the nominal ambient
atmospheric range of 300–4000 nmol mol<sup>–1</sup>.
Starting materials of pure CH<sub>4</sub> and cylinders of dry air
were exhaustively analyzed to determine the purity and air composition.
Gas chromatography with flame-ionization detection (GC-FID) was used
to determine a CH<sub>4</sub> response for each of the 14 PSMs where
the reproducibility of average measurement ratios as a standard error
was typically (0.04–0.26) %. An ISO 6134-compliant generalized
least-squares regression (GenLine) program was used to analyze the
consistency of the CH<sub>4</sub> suite. All 14 PSMs passed the <i>u</i>-test with residuals between the gravimetric and the GenLine
solution values being between −0.74 and 1.31 nmol mol<sup>–1</sup>; (0.00–0.16)% relative absolute. One of the 14 PSMs, FF4288
at 1836.16 ± 0.75 nmol mol<sup>–1</sup> (<i>k</i> = 1) amount-of-substance fraction, was sent to the Korea Research
Institute of Standards and Science (KRISS), the Republic of Korea’s
National Metrology Institute, for comparison. The same PSM was subsequently
sent to the National Oceanic and Atmospheric Administration (NOAA)
for analysis to their standards. Results show agreement between KRISS-NIST
of +0.13% relative (+2.3 nmol mol<sup>–1</sup>) and NOAA-NIST
of −0.14% relative (−2.54 nmol mol<sup>–1</sup>)
Development and Verification of Air Balance Gas Primary Standards for the Measurement of Nitrous Oxide at Atmospheric Levels
The
Gas Metrology Group at the National Institute of Standards
and Technology (NIST) became active in developing primary standards
at ambient levels of N<sub>2</sub>O in the 1980s, and this has continued
through to the present. In recent years, interest in NIST-traceable
standards has increasednot only at the ambient level of approximately
325 nmol mol<sup>–1</sup> (ppb) but at micromole per mole (ppm)
levels as well. In order to support two in-process dry whole air standard
reference materials (SRMs 1720 and 1721) and the NIST Traceable Reference
Materials (NTRM) program, a project was implemented in the Gas Metrology
Group to produce a complete suite of new primary standard materials
(PSMs) of N<sub>2</sub>O with synthetic air (O<sub>2</sub>/N<sub>2</sub>) as the balance gas. Six levels of dilution, approximately 1 order
of magnitude apart, were gravimetrically prepared and verified. Each
level serves as the “parent mix” for the next level.
This discussion describes the process of producing each level and
then verifying its amount-of-substance fraction. Expanded uncertainties, <i>k</i> = 2, of 0.025% relative to the gravimetric amount-of-substance
fraction were obtained at the ambient level. One standard from the
final group of standards at the ambient amount-of-substance fraction
level was compared with standards from the National Oceanographic
and Atmospheric Administration and the Scripps Institution of Oceanography,
two organizations experienced in gas standards preparation and ambient
whole air measurements, and shows agreement to 0.07 nmol mol<sup>–1</sup> (0.02% relative) and 0.20 nmol mol<sup>–1</sup> (0.06% relative),
respectively
Preparation of Accurate, Low-Concentration Gas Cylinder Standards by Cryogenic Trapping of a Permeation Tube Gas Stream
Comparison of halocarbon measurements in an atmospheric dry whole air sample
Abstract The growing awareness of climate change/global warming, and continuing concerns regarding stratospheric ozone depletion, will require continued measurements and standards for many compounds, in particular halocarbons that are linked to these issues. In order to track atmospheric mole fractions and assess the impact of policy on emission rates, it is necessary to demonstrate measurement equivalence at the highest levels of accuracy for assigned values of standards. Precise measurements of these species aid in determining small changes in their atmospheric abundance. A common source of standards/scales and/or well-documented agreement of different scales used to calibrate the measurement instrumentation are key to understanding many sets of data reported by researchers. This report describes the results of a comparison study among National Metrology Institutes and atmospheric research laboratories for the chlorofluorocarbons (CFCs) dichlorodifluoromethane (CFC-12), trichlorofluoromethane (CFC-11), and 1,1,2-trichlorotrifluoroethane (CFC-113); the hydrochlorofluorocarbons (HCFCs) chlorodifluoromethane (HCFC-22) and 1-chloro-1,1-difluoroethane (HCFC-142b); and the hydrofluorocarbon (HFC) 1,1,1,2-tetrafluoroethane (HFC-134a), all in a dried whole air sample. The objective of this study is to compare calibration standards/scales and the measurement capabilities of the participants for these halocarbons at trace atmospheric levels. The results of this study show agreement among four independent calibration scales to better than 2.5% in almost all cases, with many of the reported agreements being better than 1.0%