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NIST Standards for Measurement, Instrument Calibration, and Quantification of Gaseous Atmospheric Compounds George Rhoderick, Michael Edwin Kelley, Walter R Miller, James E Norris, Jennifer Carney, Lyn Gameson, Christina Cecelski, Kimberly Harris, Cassie Goodman, Abneesh Srivastava, and Joseph T. Hodges Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05310 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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Analytical Chemistry

NIST Standards for Measurement, Instrument Calibration, and Quantification of Gaseous Atmospheric Compounds George C. Rhoderick, Michael E. Kelley, Walter R. Miller, Jr., James E. Norris, Jennifer Carney, Lyn Gameson, Christina E. (Liaskos) Cecelski, Kimberly J. Harris, Cassie A. Goodman, Abneesh Srivastava, Joseph T. Hodges Gas Sensing Metrology Group Chemical Sciences Division Materials Measurement Laboratory National Institute of Standards and Technology 100 Bureau Drive Gaithersburg, Maryland 20899-8393 USA Email: [email protected]

Abstract There are many gas phase compounds present in the atmosphere that effect and influence the earth’s climate. These compounds absorb and emit radiation, a process which is the fundamental cause of the greenhouse effect. The major greenhouse gases in the earth’s atmosphere are carbon dioxide, methane, nitrous oxide and ozone.

Some halocarbons are also strong greenhouse gases and are linked to

stratospheric ozone depletion.

Hydrocarbons and monoterpenes are precursors and contributors to

atmospheric photochemical processes which leads to the formation of particulates and secondary photo-oxidants such as ozone leading to photochemical smog. Reactive gases such as nitric oxide and sulfur dioxide are also compounds found in the atmosphere and generally lead to the formation of other oxides. These compounds can be oxidized in the air to acidic and corrosive gases, and contribute to photochemical smog. Measurements of these compounds in the atmosphere have been ongoing for decades to track growth rates and assist in curbing emissions of these compounds into the atmosphere. To accurately establish mole fraction trends and assess the role of these gas phase compounds in atmospheric chemistry it is essential to have good calibration standards. The National Institute of Standards and Technology has been developing standards of many of these compounds for over 40 years. This paper will discuss the development of these standards.

Keywords: gas standards, greenhouse gases, ozone, reactive gases, Standard Reference Materials (SRMs) 1 ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Introduction Carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are key atmospheric compounds which are known to contribute to radiative forcing of climate through their role as greenhouse gases (GHGs). These molecules are also long-lived greenhouse gases (LLGHGs) which affect stratospheric ozone (O3) mole fractions directly or indirectly, play critical roles in tropospheric chemistry, and make the largest contribution to radiative forcing. As the most important anthropogenic GHG, CO2 contributes ≈ 65 % to total radiative forcing by LLGHGs [1,2], followed by17 % for methane and 6 % for N2O. The longest record of atmospheric CO2 measurements, based on manometric determinations in pristine air samples, began in 1957 to the present by Scripps Institute of Oceanography (SIO), La Jolla, California [3]. Global mean mole fractions in September 2017 reported by the National Oceanic and Atmospheric Administration (NOAA) observations program were 402.50 µmol mol-1 CO2, 1852.7 nmol mol-1 CH4, and 329.6 nmol mol-1 N2O [4].

Halogenated trace gases that include chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), have been linked to stratospheric ozone depletion [5].

CFCs, HCFCs, as well as

hydrofluorocarbons (HFCs) are also strong greenhouse gases [6,7]. These halocarbons have lifetimes from 1 yr to 250 yrs and 100 yr global warming potentials (GWP) from 10 to over 10000 more heatabsorptive than CO2 per unit of weight.

Measurements of atmospheric abundance of halocarbons from

the National Oceanic and Atmospheric Administration (NOAA) can be found at www.esrl.noaa.gov/gmd; from the Advanced Global Atmospheric Gases Experiment (AGAGE) at http://agage.eas.gatech.edu/, and from the University of California Irvine (UCI) at http://ps.uci.edu/~rowlandblake/. These measurements assist in efforts to determine sources and sinks, global distributions, and temporal response to natural and anthropogenic processes affecting the abundance of atmospheric halocarbons.

Non-methane hydrocarbon compounds (NMHCs), particularly in urban environments, are important precursors and contributors to atmospheric photochemical processes.

These processes lead to the

formation of particulates and secondary photo-oxidants such as ozone [8,9]. Many anthropogenic sources emit NMHCs into the atmosphere. These and other considerations result in considerable interest in measuring the levels of NMHCs at ground level and in the upper atmosphere [10,11,12,13]. Increases in the amount of global atmospheric ethane and propane caused largely by recently increased U.S. oil and natural gas production also have been reported [14,15]. These and other studies show that typical NMHCs mole fractions range from tens of nmol mol-1 (parts-per-billion; ppb) in urban atmospheres to well below 10 pmol mol-1 (parts-per-trillion; ppt) in remote environments. 2 ACS Paragon Plus Environment

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Terpenes, that include isoprene, isoprenoids and monoterpenes, have an important role in atmospheric chemistry because they lead to the formation of secondary photo-oxidants such as ozone that result in photochemical smog [16,17]. Production and loss mechanisms of tropospheric ozone are influenced by atmospheric abundances of monoterpenes. Monoterpenes may also control number of OH radicals involved in the oxidation of methane and other greenhouse gases [18], and are known to contribute to secondary organic aerosols (SOA) and therefore PM 2.5. The origins of these terpene compounds can either be biogenic (BVOC) or anthropogenic (AVOC) [19]. Numerous studies have shown variation between vegetative compounds in strength of emissions [20], indicating the unreliability of using a single biological compound for estimating the combined emissions of an ecology [21]. Biogenic emissions from various compounds of trees around the world have been reported [22,23,24,25,26,27,28,29,30].

Reactive gases such as NO and SO2 are found in the atmosphere and generally lead to the formation of other oxides (NOx and SOx) [31]. Reactions of NO in the atmosphere are very complex. Basically, NO is oxidized by ozone (O3) to NO2 on a scale of tens of minutes, while NO2 is split by UV light to yield NO and an O atom which then combines with O2 to give O3. Thus, during the day there is a quasi-equilibrium of NO, NO2 and O3 depending on the amount of sunlight. NO2 is an acidic, corrosive, toxic gas, and acts as a respiratory irritant to humans. Together these nitrogen oxides contribute to photochemical smog. Because of their potential as pollutants, NOx has been monitored in the atmosphere [32]. Atmospheric reactions of SO2 are very complex [33] resulting in sulfur in the atmosphere being oxidized usually to sulfuric acid, which also reacts with ammonia to form some bisulfates. The sulfuric acid and bisulfates then are flushed out of the atmosphere during rain events. SOx react with other compounds in the atmosphere to form small particles which contribute to pollution in the form of haze. The largest source of SO2 emissions into the atmosphere comes from the burning of fossil fuels by power plants [34,35], but are also emitted from freight transportation burning high sulfur content fuels, as well as by volcanoes. These SO2 and SOx compounds contribute to acid rain which can harm ecosystems and cause erosion to buildings.

Formaldehyde is another important reactive gas which contributes to smog formation as an intermediate compound. In addition to being naturally occurring, it is also emitted into the atmosphere during the production of building products. Furthermore, formaldehyde is formed in the atmosphere upon oxidation of methane and other hydrocarbons from automobiles, forest fires and tobacco smoke all of which contribute to smog formation. Formaldehyde is then further broken down by sunlight.

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Mercury (Hg) has the potential to be a severe health risk to humans and wildlife depending on its form [36]. While metal processing, burning of coal, and medical and other waste contribute to the amount of Hg in the environment, the largest source comes from atmospheric deposition. Volcanoes, oceans and geologic deposits are the main natural sources of atmospheric Hg. Deposition of atmospheric Hg results in fallout of inorganic forms to water and sediments. One of these forms, methylmercury, is highly toxic and can enter the food chain or release back to the atmosphere via volatilization. Because of the environmental impact of mercury and the neurotoxic risk associated with the trophic transfer of methyl mercury to humans, the reduction of mercury emissions from coal-fired electric utilities, the largest anthropogenic source, is now receiving attention worldwide.

It is essential to have good calibration standards to accurately establish mole fraction trends, to assess the role of these gas phase compounds in atmospheric chemistry, and to relate measurement records from many laboratories and researchers. Over the past 40+ years the Gas Sensing Metrology Group (GSMG) (previously the Gas Metrology Group) at the National Institute of Standards and Technology (NIST) has developed gravimetric primary standard mixture (PSM) suites for many compounds covering all these groups that are associated with atmospheric reactions among gas phase compounds. At the same time, improvements in the accuracy and smaller uncertainties have been achieved for these PSMs. These PSMs have in turn been used to certify secondary gas standards, either as Standard Reference Materials (SRMs) or Special Certified standards, which are used by measurement communities for calibration of their measurement systems.

Here we bring together and discuss the historical development of National

Institute of Standards and Technology (NIST) gas standards.

Discussion Major Greenhouse Gases: carbon dioxide, methane, nitrous oxide. The development of carbon dioxide (CO2) PSMs began in the 1970’s. In the early 1980’s a new suite of 18 CO2 PSMs was developed in response to a United States Department of Energy (DOE) request [37]. This suite has been maintained, refined, and added to with new PSMs with lower associated uncertainties. The original 1980’s suite of PSMs was used to certify the CO2-in-air SRMs. These SRMs were individually certified with the following typical mole fractions and expanded uncertainties (95 % confidence interval): SRMs 1670 at (336.2 ± 0.4) µmol mol-1, 1671 (342.4± 0.4) µmol mol-1 and 1672 (351.1 ± 0.4) µmol mol-1. Two of these SRMs were re-produced in the 1990s with an additional decimal place added to the mole fraction and expanded uncertainty: 1671 (345.31 ± 0.36) µmol mol-1 and 1672 (355.12 ± 0.36) µmol mol-1.


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After the introduction of the three SRMs above, four more CO2-in-air SRMs were developed and certified in 1985: SRMs 2607 at (341.4 ± 0.4) µmol mol-1 and 2608 (341.4 ± 0.4) µmol mol-1, and 2609 at (375.4 ± 0.4) µmol mol-1 and 2610 (375.2 ± 0.4) µmol mol-1. SRMs 2608 and 2610 were prepared in the standard SRM-size aluminum cylinders with nominal 5.8 L cylinders. In anticipation of larger volumes needed by the World Metrological Organization (WMO) for calibrating instruments, SRMs 2607 and 2609 were prepared in nominal 30 L aluminum cylinders. The CO2 mole fraction in each cylinder, in all seven of these SRMs, was individually certified. At the same time as the CO2 gravimetric standards development, a suite of nitrous oxide (N2O) in air was developed. The N2O suite was used to assign a “batch certified” N2O value to the previously discussed CO2-in-air SRMs: 2607 and 2608 (301 ± 3 nmol mol-1 (ppb)), and 2609 and 2610 (333 ± 3 nmol mol-1). These three CO2 and four CO2/N2O SRMs were maintained into the mid 2000’s. However, two recent SRMs that contain atmospheric levels of CO2 were developed and will be discussed later.

Since the late 1970’s, NIST has maintained a primary standard mixture (PSM) suite for methane (CH4) in air from 1 µmol mol-1 (ppm) to 10000 µmol mol-1, with a focus on atmospheric levels of (1 to 10) µmol mol-1. Beginning in the late 1970’s the original CH4 PSM suite was used to certify the CH4-in-air SRMs: SRMs 1658 (1 µmol mol-1), 1659 (10 µmol mol-1), and 1660 (4 µmol mol-1 and includes 1 µmol mol-1 propane).

These three CH4 SRMs originally had associated 95 % confidence interval expanded

uncertainties of ± 1 % relative, but those uncertainties have been steadily reduced over the years as PSMs having much lower uncertainties were added to the suite. This improvement results from significant advances in state-of-the-art mass balance systems and analytical instruments that lead to lower detectable levels of trace CH4 in the balance gas. Two other SRMs were developed, 2750 (50 µmol mol-1) and 2751 (100 µmol mol-1), which could be used for measurements of methane in environments with elevated CH4 levels, such as around natural gas wells and transfer systems. All five of these CH4-in-air SRMs are still maintained.

Beginning in 2010 a major expansion in the PSM suites of CO2, CH4 and N2O was undertaken. A new suite of CO2 PSMs was developed starting from a well-characterized, pure source CO2. Studies were undertaken to quantify small (< 0.1 µmol mol-1) drift in some CO2 mole fraction standards, which was attributed to absorption/desorption issues at cylinder walls. These studies helped develop more accurate PSMs with lower associated uncertainties [38]. New CH4 PSMs were prepared starting from 99.9993 % CH4 (as determined by NIST) with a major focus on the atmospheric level of 5000 nmol mol-1 down to 300 nmol mol-1 [39]. The new PSMs had substantially lower combined uncertainties (k =1), of ± 1.6 5 ACS Paragon Plus Environment


nmol mol-1 at 5000 nmol mol-1, ± 0.75 nmol mol-1 at 1800 nmol mol-1 and ± 0.52 nmol mol-1 at 300 nmol mol-1, than the previous PSM suites (± 2 nmol mol-1) at 1800 nmol mol-1. Additionally, a few standards were developed and maintained at the 10 nmol mol-1 (ppb) level to accurately determine the trace levels of methane found in the balance air used to prepare those PSMs. A new suite of N2O PSMs was developed with major focus at the atmospheric level (300 to 400) µmol mol-1 [40]. The new CO2, CH4 and N2O PSM suites were compared to their respective historical suites to insure consistency. These new suites of GHG PSMs were then used to certify two new atmospheric SRMs: 1720 - Northern Continental Air and 1721 - Southern Oceanic Air, in 2015 [41,42]. SRM 1720 is a unique gas SRM as it involved a collaboration with the NOAA in which they filled the samples from high-altitude air at Niwot Ridge, Colorado, and value assigned the mole fractions for CO2, CH4 and N2O using their manometric and gravimetric standards. NIST-certified mole fraction values referenced to the respective PSM suites are specified on the SRM certificate with the NOAA values provided as supplemental information because NOAA is the Central Calibration Laboratory (CCL) for the World Meteorological Organization (WMO). This provides those needing to report values to the WMO to use SRM 1720 as a certified reference. Similarly, SRM 1721 was prepared at Barring Head, New Zealand, pumping air straight off the ocean winds into the cylinders. The expanded 95 % confidence interval uncertainties are the lowest ever certified (% relative to the mole fraction) for NIST SRMs containing atmospheric-level mole fraction compounds: CO2 ± 0.13 µmol mol-1 (0.045 % relative), CH4 ± 1.8 nmol mol-1 (0.14 % relative) and N2O ± 0.20 nmol mol-1 (0.062 % relative). In SRM 1721 Southern Oceanic Air the expanded uncertainty for CH4 is ± 1.0 nmol mol-1 (0.05 % relative). Typical certified mole fractions for SRM 1720 and 1721 are shown in Table 1.

Table 1. Typical certified mole fractions in SRM 1720 and 1721. Greenhouse

Certified Mole

Certified Mole

Gas

Fraction

Fraction

Component

SRM 1720

SRM 1721

Carbon

(394.32 ± 0.13)

(387.97 ± 0.13)

-1

dioxide (CO2)

µmol mol

Nitrous oxide

(322.80 ± 0.20) -1

(N2O)

nmol mol

Methane

(1878.7 ± 1.8)

(CH4)

nmol mol

-1

µmol mol-1 (323.98 ± 0.20) nmol mol-1 (1760.6 ± 1.0) nmol mol-1

NIST has collected considerable stability data on these three-gas phase compounds. The data show that CH4 and N2O are stable for at least 40 and 20 years respectively. On the other hand, CO2 is complicated 6 ACS Paragon Plus Environment

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when it comes to the level of stability one discusses. It is considered stable for at least 20 years when one has assigned an uncertainty of 0.5 % relative on the mole fraction. Where much lower uncertainties (0.1 µmol mol-1) are required, as in ambient atmospheric measurements. CO2 has been shown to increase in mole fraction, and this is dependent on the pressure and volume to surface ratio of the cylinder that contains the mixture. Studies show that CO2 in air mixtures contained in 6 L aluminum cylinders will increase in mole fraction by as much as 1 µmol mol-1 or more by the time the pressure has decreased to 3.4 KPa. However, CO2 in air mixtures contained in 30 L aluminum cylinders have shown increases of only 0.05 µmol mol-1, within the 0.1 µmol mol-1 assigned uncertainty, when the pressure decreases to the same 3.4 KPa. The SRMs discussed above are contained in 30 L aluminum cylinders and thus have a 6year expiration date with a caution to not use once the pressure falls to 3 KPa.

The NIST PSM suites of CO2, CH4 and N2O were compared with other National Metrology Institute (NMI) standards through Consultative Committee for the Quantity of Matter in Chemistry and Biology (CCQM) key comparisons (K): CCQM-K52 for CO2 [43], CCQM-K68 for N2O [44] and CCQM-P41 for CH4. Those results show that the NIST PSMs are comparable to those from other NMIs around the globe. The 2012 CCQM-K82 comparison for CH4 showed excellent results among NMIs and significant reduction in uncertainty from the 2002 CCQM-P41 comparison [45]. A new comparison for atmospheric level CO2, CCQM-K120, is currently underway and an ambient level N2O study is planned for 2019.

Halocarbons. The GSMG has prepared and maintained PSM suites for some key halocarbons since the late 1980’s.

In 2012 those suites were augmented substantially with new PSMs.

In 2012 NIST

coordinated CCQM-K83 that showed comparable results between the participants [46]. An additional comparison was run in parallel with a limited number of laboratories, including NOAA and SIO, that had major atmospheric monitoring programs for halocarbons. Those results also showed good comparability between all participants [47].

In 2012-2014, samples of continental air were collected in cylinders at Niwot Ridge, CO, by NOAA. Those mixtures were sent to NIST and measured by the GSMG. The mole fractions of six halocarbons were determined by comparison to the suites of halocarbons PSMs. These samples constitute new SRM 1722 Halocarbons-in-Air which have been certified and were finalized in early 2017. Each halocarbon, in each cylinder in the lot, are individually certified. Table 2 lists typical mole fractions and expanded 95 % confidence interval uncertainties.

The halocarbons discussed above have all demonstrated 20-year

stability in aluminum gas cylinders within the stated uncertainties given in Table 2.



Table 2. Halocarbon mole fractions and uncertainties for typical sample in SRM 1722. Typical Certified Value pmol mol-1 524.9 ± 4.9

Halocarbon Dichlorodifluoromethane (CFC-12) Trichlorofluoromethane (CFC-11) 1,1,2-Trichlorotrifluoroethane (CFC-113) 1-Chloro-1,1-difluoroethane (HCFC-142b) Chlorodifluoromethane (HCFC-22) 1,1,1,2-Tetrafluoroethane (HFC-134a)

236.4 ± 4.5 74.7 ± 2.1 22.9 ± 1.0 236.3 ± 2.4 71.8 ± 4.6

Hydrocarbons. In the late 1980’s, the GSMG began preparing gravimetric PSMs for hydrocarbons in nitrogen covering a mole fraction range of (1 to 15) nmol mol-1. These original PSMs were in support of monitoring programs for the U.S. Environmental Protection Agency (EPA) and the California Air Resources Board. Development of these hydrocarbon PSMs was followed in 1993 by the certification of the fifteen-component hydrocarbon-in-nitrogen SRM 1800 with each component at a nominal mole fraction of 5 nmol mol-1 [48]. SRM 1800 was closely monitored over a period of 10 years and exhibited excellent stability [49]. Measurement values determined over time for several of those SRM samples were within < 1.5 % of the original certified values for all fifteen hydrocarbons. The original expanded uncertainties were ± 4 % (95 % confidence interval), hence the hydrocarbons have remained stable within those original uncertainties.

This SRM was reissued in 2004 with the addition of three more

hydrocarbons making a total of eighteen compounds.

In 2007, the GMG extended its hydrocarbon standards mole fraction range downward focusing on 50 pmol mol-1 to 250 pmol mol-1 for the same components represented in SRM 1800. This development resulted in a consistent and stable suite of hydrocarbon standards [50]. Supporting Figure S1 shows the agreement of the standards for six of the hydrocarbons and is representative of the consistency of all eighteen hydrocarbons in the PSM suite. One of these standards at nominal mole fraction of 200 pmol mol-1 was used as a sample for an international comparison between several NMIs, government agencies and academia which resulted in excellent agreement between laboratories [51]. Supporting Figure S2 illustrates the good agreement between laboratories. While a hydrocarbon SRM is not planned in the pmol mol-1 region, the primary calibration scale is in place to certify reference gas mixtures as needed to support the measurement of these hydrocarbons for many atmospheric environments. This 200 pmol mol-1 standard has also shown 8 ACS Paragon Plus Environment

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excellent stability for fifteen of the eighteen compounds over seven-years as shown in Figure 1. The three exceptions are the double-bonded hydrocarbons: propene, i-butene, and 1-pentene.

220 Amount-of-Substance Franction, pmol mol-1 (ppt)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


210 200

propane i-butane n-pentane n-octane nonane decane benzene n-hexane i-pentane n-butane o-xylene i-octane toluene n-heptane

ethane

190 180

propene Jan-08

170

1-pentene i-butene

Aug-11 Jan-13

160

Aug-15

150 140

Figure 1. Stability of hydrocarbons at 200 pmol mol-1 contained in an Aculife 4 treated aluminum cylinder. Uncertainty bars represent expanded uncertainty at 95 % confidence interval. Monoterpenes. In 2006 the WMO approached a group of NMIs to develop certified gas mixtures containing VOCs in support of their Global Atmosphere Watch (GAW) program. NIST was tasked with researching the feasibility of developing monoterpene in nitrogen gas standards. Monoterpenes are some of the most difficult compounds to contain in gas cylinders with good stability (i.e., longer than 2 years). It took until 2014 to find a combination of aluminum cylinder type and an internal wall treatment process to determine that a suitable container had been obtained [52]. After successfully demonstrating a stable monoterpene gas mixture had been achieved, the development of suites of standards at nominal mole fractions of 2 nmol mol-1 was initiated. Twelve key monoterpenes of interest to the WMO were assigned into three groups as shown in Supporting Table S1.

Development of PSMs started with Group 1 which contained four of the high-priority monoterpenes at 250 nmol mol-1 in nitrogen; α-pinene, 3-carene, R-limonene and 1,8-cineole. A simple hydrocarbon, such as n-hexane or n-octane as an internal standard, either of which is known to be stable in gas mixtures, was included to track the ratio of the monoterpene response to that of the hydrocarbon. Stability of the four 9 ACS Paragon Plus Environment


monoterpenes was monitored over time by gas chromatography/flame-ionization detection analysis tracking the ratio to the internal hydrocarbon standard. Measurements of this standard over time has revealed good stability for over 1300+ days (> 3.5 yrs) to date as illustrated in Supporting Figure S3. After successfully demonstrating stability for Group 1 monoterpenes at 250 nmol mol-1, a suite of 5 PSMs ranging from 1 nmol mol-1 to 3 nmol mol-1 was developed. The typical preparation uncertainties on these PSMs is ± 1.2 % relative. In 2014 the WMO accepted NIST as the Central Calibration Laboratory (CCL), and NIST began supporting laboratories with the Group 1 monoterpene standards. Development of the Group 2 PSMs has advanced to the stage where NIST can now support the need of calibration gases for 8 of the 12 monoterpenes. No SRM has been planned, but the current suite of PSMs allows NIST to certify gas mixtures of these 8 monoterpenes in a balance gas of nitrogen or air as calibration mixtures. Typical expanded uncertainties (95 % confidence interval) on these calibration mixtures are ± 2 % relative. Stability data for an eight-component mixture in air are shown in Supporting Figure S4. Six of the eight monoterpenes are exhibiting stable characteristics while one, α-terpinene is showing a trend of degradation and p-cymene may be in a growth mode. This suggests that the α-terpinene is possible chemically transforming into p-cymene.

Ozone. Ambient ozone measurements in the United States and many other countries are traceable to a NIST Standard Reference Photometer (SRP) [53]. The NIST SRP serves as the highest-level ozone reference standard in the United States, with NIST SRPs located at many U.S. Environmental Protection Agency (EPA) laboratories. The expanded uncertainty on the NIST SRPs is ± 1 nmol mol-1 from (1 to 100) nmol mol-1 and ± 1 % relative from (100 to 1000) nmol mol-1. The International Bureau of Weights and Measures (BIPM) maintains a NIST SRP as the reference standard for international measurement comparability through the International Committee of Weights and Measures (CIPM). The uncertainty of the NMIs maintaining an SRP is ± 0.3 % as determined from CCQM.QM-K1 key comparisons. Upgrades of the NIST SRP to reduce the uncertainty in ozone measurements have been completed [54,55]. Currently there are 61 NIST SRPs maintained in 26 countries around the world, shown in Figure 2, underpinning ozone measurement calibration and traceability within and between countries.


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Figure 2. Map displaying countries utilizing the NIST SRP for ground level ozone measurements.

Reactive Gases. For measurements of ambient air, the analytical instrument (chemiluminescence) is typically calibrated with NO in nitrogen standards to measure the total nitrogen oxides (NOx), as well as the amount of NO emitted from human processes such as road transport and electricity generation. NIST has developed numerous SRMs of NO-in-N2 covering a mole fraction range of 3000 µmol mol-1 down to 0.5 µmol mol-1, with expanded uncertainties of ± 0.2 % relative at the high end down to ± 2.0 %. The NO PSM suite extends down to 0.075 µmol mol-1 and calibrations of instruments can be extended down to 0.001 µmol mol-1 using dilution systems, provided the dilution gas is analyzed for any trace NO. While the stability of the higher mole fraction NO SRMs has been very good long term (> 8 years). Those < 50 µmol mol-1 almost always exhibit a gradual decrease in concertation and must be recertified every two years.

Numerous SRMs of SO2-in-N2 have been developed to monitor man-made emissions of SO2 at the source. The lower mole fraction SRMs can also be used to measure atmospheric level SO2. These SO2 SRMs cover a mole fraction range from 3500 µmol mol-1 down to 5 µmol mol-1, with typical expanded uncertainties across that range varying from ± 0.4 % to 1 % relative. In addition, the SO2 PSM suite can effectively extend the SO2 calibration down to 0.001 µmol mol-1 using dilution systems. The stability of SO2 in gas mixtures, as with NO, is very dependent on the moisture present. If the cylinders and the matrix gas used to prepare the NO and SO2 are very dry to begin with, then these components should be stable at least for 10 years at mole fractions above 100 µmol mol-1. Lesser shelf-lives of 4 years can be expected below 100 µmol mol-1.



Formaldehyde. Stable gas mixtures of formaldehyde (CH2O) are difficult to develop. Therefore, NIST provides traceability of formaldehyde to the International System of Units (SI) using a permeation device system (PDS). To produce a calibrant, or standard, 1,3,5-trioxane, the cyclic trimer of formaldehyde and available at a purity of 99+ %, is used instead of paraformaldehyde, the polymer of formaldehyde with a typical purity of only 95 %. By varying the dilution gas flow over the diffusion tube, a suite of CH2O standards can be produced on demand. In this manner, a sample can be determined from the calibration curve and certified traceable to the SI through NIST. NIST is effectively certifying samples in the 1µmol mol-1 to 10 µmol mol-1 range with expanded uncertainties of ± 1 % relative.

Mercury. NIST has been working jointly with the EPA to develop a traceability system that links the output of elemental mercury calibration instrumentation sited at the power utilities to the International System of Units (SI). To accomplish this, NIST provides traceability of mercury continuous emission monitors (CEMs) to the SI through the output of a mercury calibration gas generator maintained as the NIST Prime generator. The output of the NIST Prime generator is established using the primary isotope dilution using cold-vapor generation inductively coupled plasma mass spectrometry (ID-CV-PCP-MS).

A mercury detector and the NIST Prime generator are used to quantify mercury calibration gas generators submitted to NIST by vendors for certification, establishing “Vendor Prime” generators. Certification is assigned at specific settings over a range of (0.2 to 39) µg/m3 for the NIST Prime generator at low levels and from (40 to 300) µg/m3 for the high level NIST Prime generator. Expanded uncertainties are ± 3.5 % and ± 2.0 % relative at the low and high levels respectively. Vendor Primes are returned with their gas delivery certified at specific mole fraction points chosen by each vendor. Theses certified generators are used to establish traceability for the manufactured mercury CEMs.

Summary.

Many SRMs have been discussed here.

Information on SRMs available including the

compounds, the mole fractions available and their uncertainties, can be found at: https://www-s.nist.gov/srmors/BrowseMaterials.cfm?subkey=7

Future Research and Development The current PSM suites for the GHG compounds and others that contribute to atmospheric chemistry that have been discussed here will be maintained. New PSMs will be added in a timely manner to check the consistency and stability of the scales. SRMs will continue to be developed as needs arise. We will 12 ACS Paragon Plus Environment

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continue to consider the need for reference standards for other compounds that are of significant interest and importance in the chemistry of the atmosphere.

One area of significance and importance are the isotopes of CO2 and CH4. Previously, the National Bureau of Standards (NBS; now the NIST), NBS19 calcium carbonate (TS-Limestone) [56] was adopted for anchoring high δ13C in CO2 amount and was assigned the value +1.95‰ relative to the Vienna Pee Dee Belemnite (VPDB) following recommendations of the IAEA and the International Union of Pure and Applied Chemistry (IUPAC) [57,58]. NIST has developed other reference materials (RM) of 13C and 18O in CO2 [59]. Future standards research and development at NIST will focus on the isotopes of CO2 and CH4, followed by CO and possibly N2O. This will involve developing gravimetric gas cylinder PSMs for the 12C and 13C isotopes of CO2 and CH4. We will start by producing a gravimetric suite of PSMs at 400 µmol mol-1 CH4 in air (N2, O2, Ar only) with varying δ13C values (+20 ‰ to -90‰ in increments of 10 ‰). The starting materials will be highly enriched 99.9%), and scrubbed real air.

13

C depleted CH4 (12C ≥ 99.99%),

13

CH4 (13C ≥

Cavity ring-down spectroscopy (CRDS), fourier transfer infrared

spectroscopy (FTIR), and isotope ratio mass spectrometry (IRMS) will be used to analytically verify the 12

CO2 and 13CO2 content in relationship to the current globally excepted primary isotopic VPDB scale.

After analytical verification of

12

CH4 and

13

CH4 by CRDS and FTIR, half of each standard will be

-1

converted to 400 µmol mol CO2 in real air by use of a platinum catalyst, thereby producing a CO2 suite of gravimetric standards with varying δ13C values (+20 ‰ to -90‰ in increments of 10 ‰), and a natural abundance of 16O, 17O, and 18O.

References 1

Hall, B.D. Bull. Amer. Meteor. Soc. 2014, 95 (7), SS33-S36.

2

NOAA/ESRL Global Monitoring Division website: www.esrl.noaa.gov/gmd/

3

Keeling, C.D.; Piper, S.C.; Whorf, T.P.; and Keeling, R.F. Tellus Series B-Chem. and Phys. Metrol. 2011, 63(1).

4

NOAA/ESRL/GMD website: http://www.esrl.noaa.gov/gmd/ccgg/trends/global.html

5

Montzka, S.A.; Reimann, S. (Corresponding Lead Authors);,Engel, A.; Krüer, K.; O’Doherty, S.J.; Sturges, W.T.; Blake, D.; Dorf, M.; Fraser, P.; Froidevaux, L.; Jucks, K.; Kreher, K.; Kurylo, M.J.; Melloouki, A.; Miller, J.; Nielsen, O.-J.; Orkin, V.L.; Prinn, R.G.; Rhew, R.; Santee, M.L.; Stohl, A.; and Verdonik, D. Ozone Depletion Substances (ODSs) and Related Chemicals, Chapter 1, In:



Scientific Assessment of Ozone Depletion: 2010 Global Ozone Research and Monitoring ProjectReport No. 52, World Metrological Organization, Geneva, Switzerland, 2011. 6

Forster, P.; Ramaswamy, V. (coordinating authors); Artaxo, P.; Berntsen, T.; Betts, R.; Fahey, D.W.; Haywood, J.; Lean, J.; Lowe, D.C.; Myhre, G.; Nganga, J.; Prinn, R.; Raga, G.; Schulz M.; Van Dorland, R. Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S.D.; Qin, M.; Manning, Z.; Chen, M.; Marquis, K.B.; Averyt, M.; Tignor; Miller, H.L. (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2007.

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Prinn, R.G.; Zander, R. (Coordinating Lead Authors); Cunnold, D.M.; Elkins, J.W.; Engel, A.; Fraser, P.J.; Gunson, M.R.; Ko, M.K.W.; Mahieu, E.; Midgley, P.M.; Russel, J.M.; Volk, C.M.; Weiss, R.F. Long-Lived Ozone-Related Comounds, Chapter 1, in: Scientific Assessment of Ozone Depletion: 1998, Global Ozone Research and Monitoring Project- Report No. 44, World Metrological Organization, Geneva, Switzerland, 1998.

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Plass-Dülmer, C.; Michl, K.; Ruf, R.; Berresheim, H. J. Chromatogr. 2002, 953:175-197.

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