Nitrogen trifluoride global emissions estimated from
updated atmospheric measurements
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Citation
Arnold, T., C. M. Harth, J. Muhle, A. J. Manning, P. K. Salameh,
J. Kim, D. J. Ivy, et al. “Nitrogen trifluoride global emissions
estimated from updated atmospheric measurements.”
Proceedings of the National Academy of Sciences 110, no. 6
(February 5, 2013): 2029-2034.
As Published
http://dx.doi.org/10.1073/pnas.1212346110
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National Academy of Sciences (U.S.)
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Final published version
Accessed
Thu May 26 05:04:04 EDT 2016
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http://hdl.handle.net/1721.1/79808
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Detailed Terms
Nitrogen trifluoride global emissions estimated from
updated atmospheric measurements
Tim Arnolda,1, Christina M. Hartha, Jens Mühlea, Alistair J. Manningb, Peter K. Salameha, Jooil Kima, Diane J. Ivyc,
L. Paul Steeled, Vasilii V. Petrenkoe, Jeffrey P. Severinghausa, Daniel Baggenstosa, and Ray F. Weissa
a
Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093; bMet Office, Exeter EX1 3PB, United Kingdom; cDepartment
of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139; dCentre for Australian Weather and Climate
Research, CSIRO Marine and Atmospheric Research, Aspendale VIC 3195, Australia; and eDepartment of Earth and Environmental Sciences, University of
Rochester, Rochester, NY 14627
Nitrogen trifluoride (NF3) has potential to make a growing contribution to the Earth’s radiative budget; however, our understanding of its atmospheric burden and emission rates has been limited.
Based on a revision of our previous calibration and using an expanded set of atmospheric measurements together with an atmospheric model and inverse method, we estimate that the global
emissions of NF3 in 2011 were 1.18 ± 0.21 Gg·y−1, or ∼20 Tg CO2eq·y−1 (carbon dioxide equivalent emissions based on a 100-y
global warming potential of 16,600 for NF3). The 2011 global mean
tropospheric dry air mole fraction was 0.86 ± 0.04 parts per trillion,
resulting from an average emissions growth rate of 0.09 Gg·y−2
over the prior decade. In terms of CO2 equivalents, current NF3
emissions represent between 17% and 36% of the emissions of
other long-lived fluorinated compounds from electronics manufacture. We also estimate that the emissions benefit of using NF3 over
hexafluoroethane (C2F6) in electronics manufacture is significant—
emissions of between 53 and 220 Tg CO2-eq·y−1 were avoided during
2011. Despite these savings, total NF3 emissions, currently ∼10%
of production, are still significantly larger than expected assuming
global implementation of ideal industrial practices. As such, there
is a continuing need for improvements in NF3 emissions reduction
strategies to keep pace with its increasing use and to slow its rising
contribution to anthropogenic climate forcing.
atmospheric composition
| climate change | radiative forcing
A
longside the control of carbon dioxide (CO2), there are significant climate benefits to be gained from limiting emissions
of non-CO2 long-lived greenhouse gases (GHGs) (1). Nitrogen
trifluoride (NF3) has a global warming potential on a 100-y time
scale (GWP100) of ∼16,600 (see discussion below), and its use has
grown rapidly in the manufacture of modern electronic devices
(2). The potential for significant NF3 emissions was only recently
recognized (3), and was not considered in the first commitment
period of the Kyoto Protocol (4). Despite NF3’s recent rising
importance, difficulties in making atmospheric measurements
have prevented a thorough analysis of its global abundance and
emission rates (5, 6).
Nitrogen trifluoride’s radiative efficiency of 0.211 W·m−2·ppb−1
is comparable to many anthropogenic GHGs (7); however, its
large potential impact on climate arises from its atmospheric
lifetime, which is long on societal timescales. Prather and Hsu (3)
calculated a lifetime of 550 y (resulting in a GWP100 of 16,800);
however, three separate kinetic studies have recently suggested a
larger sink for NF3 by reaction with O(1D) in the stratosphere
(8-10). Dillon et al. (9) suggested a revised lifetime of 490 y,
yielding the GWP100 value of 16,600 we use here.
Use of NF3 began in the 1960s and 1970s in specialty applications, e.g., as a rocket fuel oxidizer and as a fluorine donor for
chemical lasers (11). More recently, beginning in the late 1990s,
NF3 has been used by the electronics industry in the manufacture
of semiconductor (SC), photovoltaic cell, and flat-panel display
devices (from here on all three are termed SC+). Nitrogen
www.pnas.org/cgi/doi/10.1073/pnas.1212346110
trifluoride can be broken down into reactive fluorine (F) radicals
and ions, which are used to remove the remaining silicon-containing contaminants in process chambers (12, 13). The physical
and chemical properties of NF3 make it a safe gas to transport; it
is stable and nonflammable, and faster cleaning rates and increased chamber life make it preferable to the perfluorocarbons
(PFCs) previously used, primarily hexafluoroethane (C2F6, PFC116). Nitrogen trifluoride was also chosen because of its promise
as an environmentally friendly alternative, with conversion efficiencies (to create reactive F) of ∼98% of total use, compared
with ∼30% for C2F6 (13, 14). Thus, its use has likely played an
important role in the SC industry’s strategy to meet its voluntary
PFC emission reduction commitments—a reduction in total
GHG emissions of 10% between 1995 and 2010 (14, 15).
Given that its production is increasing rapidly to meet demand
in end use (manufacture of SC+ devices), that it is a substitute
for PFCs for which emissions are already regulated, and that the
first atmospheric measurements showed a rising atmospheric
abundance, NF3 is now beginning to be included in global and
regional climate legislation (16, 17). It is therefore important to
study the atmospheric trend in NF3 alongside other GHGs in
order to provide information that can help evaluate the success
of efforts to quantify and regulate its emissions (18).
In this work we supplement and extend the only published
atmospheric NF3 record using a revised primary calibration and
an automated measurement technique (5, 6). By combining our
measurements with independent industrial production and emission data, we provide optimally derived global emission estimates
from 1978 to 2011 using an atmospheric model and an inverse
method. We discuss the significance of these emissions and estimate how successful the use of NF3 has been with respect to its
impact on Earth’s radiative budget compared with earlier SC+
industry technology.
Results and Discussion
Atmospheric Measurements. We have measured archived air samples from the Northern Hemisphere (NH) and Southern Hemisphere (SH) to complement and compare with the measurements
made previously in our laboratory (6). Additionally, we have extended the in situ record of NF3 in ambient air at Scripps Institution
of Oceanography (SIO), La Jolla, CA (32.87°N, 117.25°W),
from April 2011 to March 2012, and have analyzed the entire
record to estimate average monthly mole fractions representative
Author contributions: T.A. and R.F.W. designed research; T.A., C.M.H., J.M., and A.J.M.
performed research; D.J.I., L.P.S., V.V.P., J.P.S., and D.B. contributed new reagents/analytic
tools; T.A., J.M., A.J.M., P.K.S., and J.K. analyzed data; and T.A., J.M., and R.F.W. wrote
the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1
To whom correspondence should be addressed. E-mail: tarnold@ucsd.edu.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1212346110/-/DCSupplemental.
PNAS | February 5, 2013 | vol. 110 | no. 6 | 2029–2034
EARTH, ATMOSPHERIC,
AND PLANETARY SCIENCES
Edited by Susan Solomon, Massachusetts Institute of Technology, Cambridge, MA, and approved December 20, 2012 (received for review August 2, 2012)
of the well-mixed lower troposphere at that latitude (Fig. 1). La
Jolla receives polluted air masses from the surrounding region,
so to estimate these average well-mixed mole fractions (that are
needed for global emissions calculations) we identified the measurements that were not influenced by pollution by studying the
history of the arriving air masses using the NAME (Numerical
Atmospheric dispersion Modelling Environment) Lagrangian
particle model (19). The reported in situ baseline abundance
(dry air mole fraction) over the 12-mo period increased from
0.91 ± 0.05 parts per trillion (ppt) in April 2011 to 0.97 ± 0.03 ppt
in March 2012 (error range is given as 1- σ of the filtered monthly
air data). During December, none of the air masses arriving at La
Jolla were classified as baseline, consistent with the recording of
some of the highest NF3 mole fractions measured in air to date.
Fig. 2A shows the atmospheric NF3 archive measurements
made to date and the average baseline monthly mole fractions
calculated from the 2011 in situ measurement data from La Jolla.
The full set of archived air measurements is also documented in
Table S1. Measurements of archived air tanks filled before 1975
and air entrapped in ancient ice collected from Taylor Glacier,
Antarctica, found undetectable levels of NF3, and we conclude
that preindustrial NF3 levels were no greater than 0.008 ppt.
Global Emissions and Atmospheric Burden. We used an inverse
method to estimate emissions, using a 12-box atmospheric chemistry transport model to couple the sensitivity of semihemispheric
mole fractions to global emissions (20). The emissions calculated
by the inversion were then used as input into the model, and the
monthly mole fraction output and growth rate for the two semihemispheres are plotted alongside the atmospheric measurements
in Fig. 2, with annual average mole fractions and growth rates
summarized in Table 1. The inversion-derived emissions were able
to reconstruct the atmospheric history within uncertainty for the
majority of measurements (Fig. S1).
The onset of significant emissions is evident at the beginning
of the 1990s, followed by an approximately linear increase through
that decade (Fig. 3A). This finding is consistent with the beginning
of NF3 use in the SC industry in the early 1990s. A significant increase in the emissions rate begins at the start of the 2000s with
annual emissions increasing roughly linearly at a rate of ∼0.09
Gg·y−2 between 1999 and 2011, reaching 1.18 ± 0.21 Gg·y−1 in
2011. The emissions reported in our previous study (6), and
corrected for the revised primary calibration (5), were ∼0.70 and
∼0.77 Gg·y−1 in 2006 and 2008, respectively. In this work we calculate emissions of 0.83 ± 0.19 Gg·y−1 in 2006 (∼18% higher) and
1.01 ± 0.19 Gg·y−1 in 2008 (∼30% higher). The significantly larger
emissions we now calculate are due to an increased number of
Fig. 1. Time series of NF3 in situ measurements of dry air mole fraction (in
ppt) during 2011 and 2012 at Scripps Institution of Oceanography (La Jolla,
CA). Measurements representative of well-mixed midlatitudinal Northern
Hemispheric air (black circles) were separated from the full data set (gray
circles) by using the NAME particle dispersion model, and these data were
then used in the calculations of global abundances and emissions.
2030 | www.pnas.org/cgi/doi/10.1073/pnas.1212346110
Fig. 2. (A) NF3 atmospheric dry air mole fractions (in ppt) measured in
Northern Hemisphere (NH) and Southern Hemisphere (SH) archive air samples, and mean monthly abundances calculated from in situ measurements
at La Jolla, CA, 2011−2012. Data from Weiss et al. (6) are shown rescaled per
the SIO-12 scale. Error bars are calculated as the total measurement uncertainty, which includes measurement error and the ability of the measurement to reflect well-mixed air. Trends were calculated using a 12-box
model with emissions from the inversion as input. (B) Annual growth rates
were calculated for each month by using the first derivative of the cubic
spline fit of the model’s monthly mole fraction output.
measurements, a longer data record, and the improved modeling
and emissions calculations.
The growth in the average global atmospheric mole fraction
from near the detection limit (0.02 ppt) in 1980 to 0.86 ppt in
2011 is the result of a continuous increase in the growth rate to
almost 0.1 ppt·y−1 over the past two decades (Fig. 2B). The largest
increase in the growth rate occurred between 2000 and 2006,
when there was a significant relative rise in NF3 production and
use (2, 23). Using a radiative efficiency of 0.211 W·m−2·ppb−1 from
Robson et al. (7), an average instantaneous radiative forcing in
2011 is calculated at 1.8 × 10−4 W·m−2. Thus, NF3 is currently
contributing ∼0.010% of the radiative forcing due to rising
CO2 concentrations since 1750, and 0.017% of the forcing due
to non-CO2 GHGs (1, 26).
Significance of Current NF3 Emission Levels. Although the instantaneous radiative forcing due to NF3 is currently small, its
atmospheric lifetime essentially makes emissions cumulative
Arnold et al.
Table 1. Average global tropospheric NF3 dry air mole fractions
(x), mole fraction growth rates (dx/dt), and global emissions flux
(F) with associated 1- σ uncertainties
Year
1980
1985
1990
1995
1998
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
x, ppt
0.020
0.021
0.037
0.080
0.124
0.164
0.189
0.221
0.263
0.312
0.370
0.436
0.508
0.588
0.673
0.762
0.855
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.021
0.023
0.026
0.029
0.030
0.031
0.031
0.032
0.033
0.034
0.034
0.035
0.036
0.037
0.038
0.039
0.039
dx/dt, ppt·y−1
0.000
0.001
0.006
0.012
0.018
0.022
0.028
0.037
0.045
0.054
0.062
0.069
0.076
0.083
0.087
0.091
0.095
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.006
0.006
0.006
0.006
0.006
0.006
0.007
0.007
0.008
0.008
0.008
0.008
0.008
0.008
0.008
0.009
0.009
F, Gg·y−1
0.00
0.01
0.07
0.15
0.21
0.27
0.32
0.44
0.54
0.64
0.74
0.83
0.92
1.01
1.07
1.11
1.18
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.14
0.13
0.13
0.14
0.15
0.15
0.16
0.17
0.18
0.19
0.19
0.19
0.19
0.19
0.19
0.20
0.21
with an effect lasting beyond usual societal timescales. We
calculate a revised GWP100 of NF3 of 16,600 using the lifetime
calculated by Dillon et al. (9), and use this to calculate CO2equivalent (CO 2-eq) emissions (Fig. 3A). Emissions of NF3
in 2011 were 19.6 ± 3.5 Tg CO2-eq·y-1, equating to 0.06% of
the CO2 emissions due to fossil fuel combustion and cement
production (∼34,000 Tg·y-1) in terms of the contribution to
radiative forcing over the following 100 y (27).
The SC+ industry uses a suite of fluorinated gases for a range
of highly specialized tasks, although the effective mix varies
widely between countries and companies (14). Thus, accurately
quantifying emissions using “bottom-up” methods (from inventories, i.e., independent of atmospheric measurements) is
difficult, and “top-down” methods (dependent on atmospheric
measurements) presently only allow us to quantify emissions
integrated globally over all industries. Using a combination of
top-down and bottom-up data, we have attempted to compare
the recent CO2-eq emissions of NF3 with those of other more
widely studied fluorinated GHGs (Table 2).
The recent emissions of NF3 (in CO2-eq), which are almost
entirely from production and end use in SC+ manufacture, are
larger than those of the other fluorinated compounds from SC+
manufacture, except for CF4 and possibly C2F6. Estimating
emissions of CF4 and C2F6 from SC+ manufacture is very difficult
because these gases are also emitted during primary aluminum
(Al) manufacture (28). Assuming that the bottom-up estimates
for CF4 and C2F6 from SC+ and Al industries are underestimated [Mühle et al. (29) showed that the bottom-up estimates
from either SC+ or Al manufacture, or both, were too low], for
the lower SC+ emissions bound we used the emissions fraction
SC+/total from the Emissions Database for Global Atmospheric
Research (EDGAR) v4.2 database multiplied by the most recent
global estimates from atmospheric studies (25, 29). For the upper SC+ emissions bound we subtracted the bottom-up estimates
from Al manufacture from the top-down global total estimates
(28, 29). Given that global emissions for C2F6 have stabilized and
the fact that NF3 is continuing to replace C2F6 (2, 29), it is likely
that CO2-eq emissions of NF3 already, or will in the near future,
exceed those of C2F6 from SC+ manufacture.
Emissions of NF3 in 2010 accounted for between 17% and
36% of emissions (in CO2-eq) of the most widely used and
Arnold et al.
Fig. 3. (A) Top-down NF3 emissions estimated optimally using the atmospheric measurements and independent emission growth rates as constraints. The a priori emissions used to calculate the emissions growth rates
for the inversion were estimated using data from independent sources (2,
14, 23, 24). The EDGAR v4.2 emission estimates are also shown (25). The gray
shading represents the ± 1- σ uncertainty of the top-down emissions estimate. CO2-eq emissions were calculated using a 100-y global warming potential of 16,600 (right axis). (B) The ratio of (top-down) emissions to
production illustrates the improved efficiency of processes during production and/or end use over time. The gray shading represents the ±1- σ
uncertainty of this ratio considering uncertainty in the top-down emissions
estimate and global production.
emitted fluorinated compounds from the SC+ industry, up from
between 13% and 28% in 2005 (Table 2). As well as considering
the pace at which NF3 replaces other fluorinated gases, the rise
in future NF3 emissions depends on the SC+ market (likely to
grow significantly) and on progress in reducing the amount released. The global NF3 production rates are not well documented, and estimates have been revised over successive years
(3, 7, 23). The latest production estimate from industry is ∼12 Gg
in 2011 (2); however, an independent report suggests sales are
closer to ∼10.5 Gg (24). Using a 2011 production estimate of 12
Gg, and scaling emissions in previous years using the relative
year-to-year changes documented in more detailed sources (23,
24), we calculate an emissions/production ratio of over 0.20 for
the early 2000s, decreasing to 0.10 ± 0.03 in 2011 (Fig. 3B).
These emissions/production ratios (calculated from top-down
emissions estimates) can be compared with industry estimates
from NF3 production and different types of end use facilities
PNAS | February 5, 2013 | vol. 110 | no. 6 | 2031
EARTH, ATMOSPHERIC,
AND PLANETARY SCIENCES
For comparison with other relevant species, see table 2.1 in ref. 21, chap. 2
and table S1-1 in ref. 22, chap. 1.
Table 2. Global emissions of the main greenhouse gases used
in semiconductor, photovoltaic cell and flat-panel display
manufacture (together termed SC+)
From all sources,
Tg CO2-eq·y−1
Gas
NF3 (this work)
CF4 (29)
C2F6 (29)
SF6 (30–32)
CHF3 (33)
c-C4F8 (34, 35)
From leakage during
production and
SC+ manufacture,
Tg CO2-eq·y−1*
2005
2010†
2005
2010†
12.3 ± 3.1
77
30
140
180
5−10
18.4 ± 3.3
77
27
160
130
6−11
12.3 ± 3.1
6−46
16−25
6.2
0.9
3−6
18.4 ± 3.3
6−52
14−24
7.7
0.6
4−7
100-y GWPs used to calculate CO2-eq emissions: NF3 16,600, CF4 7,390,
C2F6 12,200, SF6 22,800, CHF3 14,800, and c-C4F8 10,300. Uncertainties are
given at the 1- σ level for NF3, otherwise ±20% (1- σ) should be assumed,
and very poorly constrained emissions are given with a range.
*Calculated from a combination of the top-down global emission estimates
and the ratios of emissions from SC+ to the global totals calculated from
EDGAR v4.2 (25). The upper bounds for the CF4 and C2F6 used emission
estimates from the production of primary aluminum (28) subtracted from
the top-down emissions (29).
†
From 2008 (for CF4 and C2F6) and 2009 (for CHF3) emission estimates.
(14, 23). SC+ facilities are typically characterized by the size of
the wafer they manufacture: The most modern facilities produce
300-mm-diameter wafers and use NF3 typically with conversion
efficiencies of ∼98% (with some abating the remaining NF3);
however, older facilities (producing smaller wafers) have conversion efficiencies of only ∼70% and often do not use abatement
measures (14). We therefore expect that a large contribution to
global emissions occurs from older SC+ manufacturing plants.
The reduction in the global top-down–derived emissions/production
ratio is evidence of reduced fugitive release during production
and/or significant improvements in the utilization efficiency on
a global level. However, there is indication in the emissions/
production ratios that these improvements are slowing, which, if
true, means that conversion to newer facilities is slowing or that
the new technology is not as efficient as originally proposed.
Accordingly, if NF3 production rates follow demand for production of SC+ devices, emissions are likely to rise significantly
in the near future.
Alternative Historical Emissions Scenario Analysis from Electronics
Manufacture. NF3 emissions are replacing at least some emis-
sions that would have occurred from continued use of C2F6,
which is typically converted to reactive F with efficiencies of
only ∼30% (13, 14). Understanding how the introduction of
NF3 changed total emissions of PFCs from SC+ manufacture
required a hypothetical analysis of avoided C2F6 emissions. By
using our top-down emissions and industry production estimates
to constrain the amount of NF3 used during end use, we assessed
the amount of C2F6 that would have been used, and hence the
emissions that would have occurred, had NF3 not been available
(see Fig. S2 and SI Materials and Methods for full method details).
We estimate that these avoided emissions are significant and in
2011 totaled between 3× and 11× the global CO2-eq emissions
from use of NF3 (Fig. 4).
The largest contribution to that uncertainty range is from the
abatement-level estimates—the proportion that is destroyed after use but before emission to the atmosphere. Given that old
and new facilities convert NF3 to reactive fluorine with different
efficiencies, and that information is unavailable on the proportion of NF3 supplied to the different facilities, we were unable
2032 | www.pnas.org/cgi/doi/10.1073/pnas.1212346110
Fig. 4. Global emission savings by using NF3 vs. C2F6 for semiconductor,
photovoltaic cell, and flat-panel display (together termed SC+) manufacture
(where a positive emissions saving indicates reduced emissions with use of
NF3 vs. C2F6). The shading covers the range of possible values for each of the
abatement scenarios and was calculated using the estimated uncertainties in
CF4 emission factors (resulting from C2F6 use), NF3 production figures, and
our global top-down NF3 emissions. Scenarios 1 and 2 (in green and red,
respectively) are described in the text. For the purpose of comparison, scenario
3 assumes that if C2F6 had been used, its emissions would have been completely abated and that actual emissions of NF3 were as calculated in this study.
to infer the possible global levels of abatement that might have
occurred. We therefore considered two cases at the extreme ends
of possible abatement levels (Fig. 4). In scenario 1 we considered
zero abatement, i.e., that NF3 has not been abated and if C2F6
had been used it would not have been abated. Under this scenario we calculated a maximum emissions saving of 220 Tg
CO2-eq·y−1 in 2011. However, this finding is likely an overestimate because we know that abatement in some facilities does
occur for NF3 and would have occurred for C2F6, and with increasing abatement the emissions benefit from using NF3 over
C2F6 decreases. In scenario 2 we assumed a 98% abatement
efficiency for C2F6 and NF3 in new facilities, and from this we
estimated a minimum emissions saving of 53 Tg CO2-eq·y−1 in
2011. However, this finding is likely to be an underestimate
because we expect that not all modern facilities would have been
abating optimally, and with lower abatement levels the emissions
benefit from using NF3 over C2F6 increases. A third scenario is
plotted in Fig. 4 to show the negative emissions saving had C2F6
been completely destroyed during end use; this serves to illustrate how large the estimated emissions savings in scenarios 1
and 2 are relative to actual current emissions of NF3. Although it
is not possible to further constrain the avoided emissions, our
analysis shows that the shift from C2F6 to NF3 has been significantly beneficial in terms of reducing the total CO2-eq impact of
SC+ industry PFC emissions over the past decade.
Conclusions
Our expanded data set and revised primary calibration show that
the global atmospheric abundance of NF3, a potent anthropogenic GHG, continues to rise, reaching a mean concentration in
the global background troposphere of 0.86 ± 0.04 ppt during
mid-2011, and that its annual emission rate has risen throughout
the past decade to ∼1.2 Gg·y−1 in 2011. Thorough bottom-up
Arnold et al.
Materials and Methods
Instrumentation and Measurement Method. We have adapted a Medusa
preconcentration GC/MS system to make automated measurements of
NF3 at SIO (5, 36). The measurement cycle lasts 65 min, and each sample
is bracketed by analyses of a compressed reference gas, resulting in a calibrated ambient air measurement every 130 min (5). For archive samples, at
least three separate measurements were made, and the measurement error
was calculated as the 1- σ SD of these replicate analyses.
The measured archived air samples include NH samples collected at La Jolla
from 1973 to 1991 and at the Trinidad Head, CA, Advanced Global Atmospheric Gases Experiment (AGAGE) station (41.05°N, 124.15°W) from 1998 to
2010, and SH subsamples from the Cape Grim Air Archive collected at the
Baseline Air Pollution Station at Cape Grim, Tasmania (40.68°S, 144.69°E)
(37). The ice core samples were collected in the 2011–2012 field season at
Taylor Glacier, Antarctica, and the air was extracted on-site using previously
described methods (38, 39). The air samples had ages of ∼11,360 B.P (Preboreal),
∼11,500 B.P (transition from Preboreal to Younger Dryas), and ∼11,600 B.P.
(Younger Dryas; B.P. relative to 1950 A.D.). We found undetectable levels of NF3
in 2-L samples of these extractions. To further constrain the preindustrial global
abundance below the routine detection limit, we preconcentrated and measured 6 L of air from one Preboreal age sample.
Calibration. We report NF3 concentrations on an updated calibration scale,
SIO-12, the preparation of which is described by Arnold et al. (5). This scale is
based on four primary gravimetric standards prepared in a whole-air matrix
to minimize the risk of bias due to interference with other atmospheric
constituents; its estimated overall uncertainty is 2%. For our 2008 measurements we used a preliminary volumetric calibration scale that was prepared using a digital quartz pressure gauge and a fixed volume to add ∼3
ppt of NF3 from a volumetric NF3/N2O mixture to a large aliquot of whole air
(5). At the time we recorded a small −0.597 torr zero offset in the Paroscientific 2300 torr full-scale pressure gauge but failed to apply that offset to
the unusually small 2.183 torr pressure measurement associated with the
required very small aliquot of NF3. As a result, we underreported the amount
of NF3 added to the 2008 standard by 21.5%, and thus underestimated the
amount of NF3 in the atmosphere by this same percentage. The 2008 preliminary standard has now been measured directly against the SIO-12 gravimetric scale (4), the results of which show the 2008 numbers to be 19.9% too
low. The revised gravimetric standard thus agrees with the zero-offset corrected 2008 volumetric standard within the expected ∼3% combined uncertainty of the two methods (5, 6). Accordingly, the NF3 measurements
reported by Weiss et al. (6) that are also used in the present work have been
corrected to the SIO-12 gravimetric scale. It must also be stressed that, except
for the initial SIO-2008 NF3 scale, all SIO calibration scales used for atmospheric
trace gas measurements in the AGAGE program are entirely gravimetric and
are not vulnerable to such pressure measurement uncertainties (40).
Arnold et al.
Pollution Analysis of in Situ Measurements. The NAME model was run in
backward mode using UK Met Office global resolution meteorology (∼25
km). Tracer particles were released at the sampling site over every 3-h period, and the 19-d back trajectory histories of these particles were estimated.
In a similar method described by Manning et al. (19) for calculating methane
and nitrous oxide baseline values at Mace Head, Ireland, threshold criteria
were used to classify the air masses arriving at La Jolla for each 3-h period:
air arriving via the continent, stagnant local air, or air from equatorial latitudes were classified as nonbaseline.
Global Emissions Calculation: Inverse Method and Atmospheric Chemistry
Transport Model. Given that prior emission rate estimates of NF 3 are likely
inaccurate, and because our atmospheric measurements are relatively sparse,
we used a growth-based Bayesian inverse method to optimally derive global
emissions (20). Instead of using absolute emission rates, this method uses the
more reliably understood year-to-year growth in emissions. From the total NF3
bottom-up emissions, discussed above, we calculated the annual emissions
rate growth and included this as a constraint in the inversion.
We calculated bottom-up emissions estimates from data that are publicly
documented on NF3 production levels (2, 23, 24), and emission factors that
have been estimated or directly measured (14, 23, 41). Fthenakis et al. (23)
discuss the reduction in emission factors (emitted/produced) from two of the
largest manufacturers of NF3, Air Products and Kanto Denka. Using this information we applied emission factors of 10% from 1967 to 1990, dropping
linearly with respect to time to 7% in 1997, to 5% in 2004, 2% in 2009, and
1% in 2012. Calculating a global bottom-up emissions estimate for end use is
very difficult given the different types of technology and different levels of
abatement in use (14, 41). We calculated emissions from end use with an
emission factor (emitted/supplied) of 0.02 for all years, which is in reasonable
agreement with the available details in the present literature (14, 23). The
full time-series is shown in Table S2. The EDGAR v4.2 database published 0.1°
longitude × 0.1° latitude gridded annual emissions from 1978 to 2008 (for
which the origin of information is not documented) (25); however, it
appears that those estimates are unrealistically low relative to our bottomup estimates (Fig. 3A; Table S2).
For the inversion we computed the sensitivity of atmospheric mole fractions to global emissions using the AGAGE 12-box model (42–44). Further
details on the model structure and inversion method are described in SI
Materials and Methods. We used a NF3 lifetime estimated by Dillon et al.
(9) of 490 y, which considers reaction with O(1D) and photolysis by UV in
the stratosphere, and a negligible reaction with OH radical (3, 8–10, 45,
46). The model was run multiple times with global emissions perturbed
individually by 1 Gg in each year. The sensitivity matrix for the inversion
was then constructed from these outputs so that each element contained
the mole fraction perturbation in a particular model box on a particular
month (where data exist) due to emissions in a particular year. Although
our box model could allow us to solve for semihemispheric emissions, the
low frequency in time and space of atmospheric measurements only
allowed for global emissions to be accurately resolved. The spatial distribution of emissions was taken from the annual EDGAR v4.2 gridded
estimates. These estimates begin in 1978 with 94% of emissions originating
from 30°−90° latitudes in the NH, and end in 2008 with these latitudes
contributing 47% of emissions. Remaining emissions originate from the
lower NH latitudes (0°−30°).
The total measurement uncertainty used in the inversion was calculated as
the square root of the sum of the following squared errors: errors resulting
from sampling frequency, measurement-model mismatch, measurement
precision, and scale propagation (see SI Materials and Methods for further
details). The inversion was carried out 1,000 times using 1,000 alternative
sensitivity matrices and measurement vectors constructed from the randomly perturbed parameters in the model and calibration scale, respectively.
The uncertainty associated with the average emissions from the 1,000 inversions was combined with the inherent uncertainty from the inversion (taken
from the error covariance matrix) to produce the final 1- σ uncertainties.
To estimate the annual average global tropospheric mixing ratios and
related uncertainties (Table 1), the model was run 1,000 times using the
annual emissions with normal distributions based on the associated uncertainties derived from the inversions. Each run produced mole fraction outputs for the lower troposphere in each of the semihemispheres. The
growth rates and associated uncertainties were calculated similarly using
the first derivative of each of the 1,000 model outputs fitted with a cubic
spline curve.
Emissions-Saving Analysis. The following assumptions were used in this
emissions saving analysis. First, emissions of NF3 during end use only were
PNAS | February 5, 2013 | vol. 110 | no. 6 | 2033
EARTH, ATMOSPHERIC,
AND PLANETARY SCIENCES
estimates of emission rates are lacking because requirements for
reporting are only now being initiated in legislation. Nonetheless,
from our analysis of the available independent information
sources, it is clear that bottom-up emissions are currently being
significantly underestimated on a global scale.
Given the available information on production rates, we calculated that the ratio of emissions to production has decreased significantly since the introduction of NF3 in SC+ manufacture,
probably because of efficiency gains in manufacturing and end use,
and a shift toward semiconductor production in more modern facilities that use NF3 with very high efficiency (∼98%). Our integrated
global analysis therefore probably masks significant variability in
emissions performance across the globe and among industrial sectors, which makes estimating the emissions saving over use of C2F6
challenging and uncertain. Nonetheless, our analysis suggests that
the savings in recent years from the shift to NF3 are substantial.
Continued efforts are needed to increase the utilization efficiency during application and to minimize emissions in production
so that the full climate benefit of NF3 use can be realized. This shift
will become progressively more important as the demand for
NF3 rises significantly over the coming years. Together with voluntary and legislative action to curb emissions of NF3, bottom-up
inventories will need improving, alongside atmospheric monitoring
and modeling work to provide independent emissions verification,
especially on regional and national scales.
calculated by accounting for emissions during production: The upper limit
for emissions during production was taken as half the total top-down estimate,
and the lower limit was taken from bottom-up estimates of emission factors
and production (2, 23, 24). Second, utilization efficiencies were 98% and 70%
for NF3 in modern and older end-use facilities, respectively, corresponding to
utilization efficiencies of 44% and 30% for C2F6, respectively (14). Third, when
abatement was considered it was only in modern facilities and assumed
98% for both NF3 and C2F6. Fourth, 10% by mass of the C2F6 supplied was
assumed to be converted into CF4 (14), and an abatement efficiency of
40–90% of this CF4 was assumed in modern facilities (14, 47). And fifth,
the production of CF4 from silicon carbide cleaning was small and consistent between methods (14), and was therefore not considered. Further
details of the method used to calculate the emissions saving are given in
Fig. S1 and SI Materials and Methods.
ACKNOWLEDGMENTS. We thank Matt Rigby (University of Bristol) and Anita
Ganesan (Massachusetts Institute of Technology) for their discussions of
modeling and inverse methods; Stephanie Mumma for filling standard
tanks; our AGAGE colleagues at the University of Bristol, Empa, Massachusetts Institute of Technology, CSIRO, and Seoul National University for their
general enthusiasm and support; and Peter Maroulis and Robert Ridgeway
of Air Products Corporation for many useful discussions related to this work.
We are also very grateful for the time and effort spent on our submission by
the three anonymous reviewers. This research was carried out as part of the
international AGAGE research program, supported in the United States by
the Upper Atmosphere Research Program of the National Aeronautics and
Space Administration. Support was also provided by UK Department of Energy
and Climate Change Contract GA0201, CSIRO, and the Australian Government
Bureau of Meteorology. Ancient air collection from Taylor Glacier, Antarctica,
was supported by National Science Foundation Award 0839031 (to J.P.S.).
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