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 aScripps 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 Edited by Susan Solomon, Massachusetts Institute of Technology, Cambridge, MA, and approved December 20, 2012 (received for review August 2, 2012) Nitrogen trifluoride (NF3) has potential to make a growing contri- trifluoride can be broken down into reactive fluorine (F) radicals bution to the Earth’s radiative budget; however, our understand- and ions, which are used to remove the remaining silicon-con- ing of its atmospheric burden and emission rates has been limited. taining contaminants in process chambers (12, 13). The physical Based on a revision of our previous calibration and using an ex- and chemical properties of NF3 make it a safe gas to transport; it panded set of atmospheric measurements together with an atmo- is stable and nonflammable, and faster cleaning rates and in- spheric model and inverse method, we estimate that the global creased chamber life make it preferable to the perfluorocarbons ± · −1 ∼ emissions of NF3 in 2011 were 1.18 0.21 Gg y ,or 20 Tg CO2- (PFCs) previously used, primarily hexafluoroethane (C2F6, PFC- − eq·y 1 (carbon dioxide equivalent emissions based on a 100-y 116). Nitrogen trifluoride was also chosen because of its promise global warming potential of 16,600 for NF3). The 2011 global mean as an environmentally friendly alternative, with conversion effi- tropospheric dry air mole fraction was 0.86 ± 0.04 parts per trillion, ciencies (to create reactive F) of ∼98% of total use, compared · −2 resulting from an average emissions growth rate of 0.09 Gg y with ∼30% for C2F6 (13, 14). Thus, its use has likely played an over the prior decade. In terms of CO2 equivalents, current NF3 important role in the SC industry’s strategy to meet its voluntary emissions represent between 17% and 36% of the emissions of PFC emission reduction commitments—a reduction in total other long-lived fluorinated compounds from electronics manufac- GHG emissions of 10% between 1995 and 2010 (14, 15). fi ture. We also estimate that the emissions bene t of using NF3 over Given that its production is increasing rapidly to meet demand fl fi — hexa uoroethane (C2F6) in electronics manufacture is signi cant in end use (manufacture of SC+ devices), that it is a substitute · −1 emissions of between 53 and 220 Tg CO2-eq y were avoided during for PFCs for which emissions are already regulated, and that the ∼ 2011. Despite these savings, total NF3 emissions, currently 10% first atmospheric measurements showed a rising atmospheric of production, are still significantly larger than expected assuming abundance, NF3 is now beginning to be included in global and global implementation of ideal industrial practices. As such, there regional climate legislation (16, 17). It is therefore important to is a continuing need for improvements in NF emissions reduction 3 study the atmospheric trend in NF3 alongside other GHGs in strategies to keep pace with its increasing use and to slow its rising order to provide information that can help evaluate the success contribution to anthropogenic climate forcing. of efforts to quantify and regulate its emissions (18). In this work we supplement and extend the only published atmospheric composition | climate change | radiative forcing atmospheric NF3 record using a revised primary calibration and an automated measurement technique (5, 6). By combining our longside the control of carbon dioxide (CO2), there are sig- measurements with independent industrial production and emis- Anificant climate benefits to be gained from limiting emissions sion data, we provide optimally derived global emission estimates of non-CO2 long-lived greenhouse gases (GHGs) (1). Nitrogen from 1978 to 2011 using an atmospheric model and an inverse fi trifluoride (NF3) has a global warming potential on a 100-y time method. We discuss the signi cance of these emissions and es- scale (GWP100)of∼16,600 (see discussion below), and its use has timate how successful the use of NF3 has been with respect to its grown rapidly in the manufacture of modern electronic devices impact on Earth’s radiative budget compared with earlier SC+ (2). The potential for significant NF3 emissions was only recently industry technology. recognized (3), and was not considered in the first commitment Results and Discussion period of the Kyoto Protocol (4). Despite NF3’s recent rising importance, difficulties in making atmospheric measurements Atmospheric Measurements. We have measured archived air sam- have prevented a thorough analysis of its global abundance and ples from the Northern Hemisphere (NH) and Southern Hemi- EARTH, ATMOSPHERIC, emission rates (5, 6). sphere (SH) to complement and compare with the measurements AND PLANETARY SCIENCES − − Nitrogen trifluoride’s radiative efficiency of 0.211 W·m 2·ppb 1 made previously in our laboratory (6). Additionally, we have ex- is comparable to many anthropogenic GHGs (7); however, its tended the in situ record of NF3 in ambient air at Scripps Institution large potential impact on climate arises from its atmospheric of Oceanography (SIO), La Jolla, CA (32.87°N, 117.25°W), lifetime, which is long on societal timescales. Prather and Hsu (3) from April 2011 to March 2012, and have analyzed the entire record to estimate average monthly mole fractions representative calculated a lifetime of 550 y (resulting in a GWP100 of 16,800); however, three separate kinetic studies have recently suggested a 1 larger sink for NF3 by reaction with O( D) in the stratosphere (8-10). Dillon et al. (9) suggested a revised lifetime of 490 y, 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 yielding the GWP100 valueof16,600weusehere. 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 Use of NF3 began in the 1960s and 1970s in specialty appli- the paper. cations, e.g., as a rocket fuel oxidizer and as a fluorine donor for The authors declare no conflict of interest. chemical lasers (11). More recently, beginning in the late 1990s, This article is a PNAS Direct Submission. NF3 has been used by the electronics industry in the manufacture 1To whom correspondence should be addressed. E-mail: [email protected]. fl of semiconductor (SC), photovoltaic cell, and at-panel display This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. devices (from here on all three are termed SC+). Nitrogen 1073/pnas.1212346110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1212346110 PNAS | February 5, 2013 | vol. 110 | no. 6 | 2029–2034 Downloaded by guest on September 25, 2021 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 meas- urements 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 chem- istry 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 semi- hemispheres 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 in- crease in the emissions rate begins at the start of the 2000s with Fig.