Atmospheric observation-based global SF 6 emissions – comparison of top-down and bottom-up estimates

I. Levin, T. Naegler, R. Heinz, D. Osusko, E. Cuevas, A. Engel, J. Ilmberger, R. L. Langenfelds, B. Neininger, C. v. Rohden, L. P. Steele, R. Weller, D. E. Worthy, and S. A. Zimov Institut für Umweltphysik, University of Heidelberg, INF 229, 69120 Heidelberg, Germany Centro de Investigación Atmosférica de Izaña, Instituto Nacional de Meteorologı́a (INM), C/La Marina, 20, Planta 6, 38071 Santa Cruz de Tenerife, Spain Institut für Atmosphäre und Umwelt, J. W. Goethe Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt/Main, Germany Centre for Australian Weather and Climate Research/CSIRO Marine and Atmospheric Research (CMAR), Private Bag No. 1, Aspendale, Victoria 3195, Australia MetAir AG, Flugplatz, 8915 Hausen am Albis, Switzerland Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany


Introduction
SF 6 is an extremely stable man-made gas, having a very high global warming potential.The industrial production of SF 6 began in 1953 for use as an insulation gas in high voltage facilities (Ko et al., 1993;Maiss and Brenninkmeijer, 1998).Anthropogenic emissions from the electricity sector (through leaks and intentional emissions from these installations) continue to form the largest source of SF 6 to the atmosphere, with additional contributions from magnesium production, semiconductor manufacturing as well as other minor sources (Olivier et al., 2005).SF 6 is primarily destroyed in the mesosphere; its atmospheric lifetime is estimated to range from 800 to 3200 yr (Ravishankara et al., 1993;Morris et al., 1995).Therefore, practically all SF 6 emitted to the atmosphere actually accumulates there, allowing us to directly infer its global emissions from the observed atmospheric concentration increase (Maiss and Levin, 1994).Assuming that the distribution of emissions is well known (e.g. from inventories Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion such as EDGAR, 2009), SF 6 has been widely used as a tracer to compare and validate atmospheric transport models (e.g., Levin and Hesshaimer, 1996;Denning et al., 1999;Kjellstr öm et al., 2000;Waugh and Hall, 2002;Peters et al., 2004;Gloor et al., 2007;B önisch et al., 2008;Patra et al., 2009).In this study, global SF 6 emissions from 1979 to 2008 are estimated using its recent accumulation rate in the atmosphere based on new observational data.These topdown source estimates, hereafter called inferred emissions, are compared to global annual emissions published in the most recent compilation of the EDGAR data base (EDGAR, 2009) which uses a so-called bottom-up approach, based on statistical information on the sources and their global distribution.2009).The difficulties to validate, on the regional scale, reported bottom-up emissions with spatially distributed observations and atmospheric transport modelling will also be discussed.

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A description of the analysis technique and the development of the Heidelberg SF 6 calibration scale is described by Maiss et al. (1996)

A new top-down estimate of global SF 6 emissions
Observations at the four globally distributed stations and along meridional transects over the Atlantic Ocean show relatively uniform SF 6 mixing ratios north of 30 • N, and south of about 15 • S, connected by a nearly linear north-to-south decrease in the tropics (Maiss et al., 1996;Geller et al., 1997).If we assume that observations from these background stations and the Syktyvkar aircraft location represent the zonal mean tropospheric SF 6 mixing ratios in their respective latitudinal bands, it is possible to reconstruct the temporal evolution of the tropospheric SF 6 distribution from this network.Further, combination with observed stratospheric SF 6 profiles (Supplementary Fig. A2) yields an estimate of the temporal development of the global SF 6 distribution on a latitude -altitude grid, which -when integrated over the entire atmosphere -gives the temporal development of the global atmospheric SF 6 inventory (see Supplementary Material Sect.3).
With an atmospheric lifetime of 800-3200 yr (Ravishankara et al., 1993;Morris et al., 1995), the total atmospheric SF 6 sink in 2005 was 0.04-0.17Gg (1 Gg=10 9 g), i.e. approximately 1-4% of the observed annual atmospheric increase of ca.5-6 Gg.Considering that the oceanic sink is one order of magnitude smaller than the atmospheric sink (Ko et al., 1993), the total SF 6 sink can be neglected.Therefore, we claim that the first temporal derivative of the global atmospheric SF 6 inventory provides a direct observation-based estimate of global SF 6 emissions which are presented in Fig. 2a.Inferred global SF 6 emissions steadily increase from ca. 2.1±0.13Gg/a in 1978 to 6.4±0.4Gg/a in 1995 (Table 2).In 1996, just two years after the UNFCCC agreement went into force, global emissions start to drop and reach a minimum of 5.4±0.32Gg/a in 1998; but they increase again to 6.8±0.4Gg/a in 2007 and 2008.Our inferred annual emissions up to 2005 compare rather well (within a 2σ error margin of our estimates) with independent estimates from other observational studies (see Supplementary Fig. A3 and respective references in the Supplementary Material).Also the emissions estimated by EDGAR (2009) (Fig. 2a, solid black line) compare well with our data within ±20%, but it is not clear to us how independent the EDGAR (2009) emissions inventory is from observed atmospheric mixing ratio changes.Their SF 6 emissions as estimated by EDGAR are displayed in Fig. 2b (dashed-dotted black line).These data are compared here with the residual emissions as calculated from the difference between the total observation-inferred SF 6 source and Annex I reported SF 6 emissions.These so-called non-reported emissions are also plotted in Fig. 2b, either based on originally reported or Japan-corrected emissions.We call these "scenarios" of SF 6 emissions "UNFCCC-based".From 1995 to 2000 nonreported emissions in the UNFCCC-based scenario are more than three times higher than EDGAR estimates for Non-Annex I countries, and in 2000 they already account for about 2/3 of the global SF 6 source.Both, the increase of non-reported emissions and their respective share in the global emissions are surprisingly large, in particular if all these non-reported emissions would have to be assigned to emissions from Non-Annex I countries.The dominant sector for SF 6 use and emission is electricity production (Olivier and Berdowski, 2001).Interestingly, the share of Non-Annex I countries in global electricity production was only 1/3 in 2000 (BP, 2009).Assuming that the UNFCCC-based SF 6 emission scenario is correct therefore implies that the annual SF 6 emissions per GWh electricity production would be two to more than three times larger in Non-Annex I countries than in Annex I countries.The large discrepancies between the two emission scenarios, EDGAR and UNFCCC-based are further investigated in the next section.Introduction

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Full With the adjusted EDGAR distribution, the mixing ratios at Alert ( 82• N) and Neumayer (71 • S) (inlay in Fig. 3a) as well as the north-south gradient (Fig. 3b, thick red line) are also correctly reproduced.However, model simulations using the UNFCCCbased emission scenario slightly underestimate the observed north-south gradient for the period of 1995 to the present (Fig. 3b, thick blue line).This could be an indication that in the UNFCCC-based scenario the distribution of sources is not correct, i.e. that emissions are overestimated in the Southern Hemisphere or in the tropics and underestimated in mid latitudes of the Northern Hemisphere.Except for Australia and New Zealand, inhabited regions of all Annex I countries are located in northern mid-latitudes (30 • N-60 • N), while large areas of Non-Annex I countries (i.e.Southern China, India and Brazil) are located in subtropical and tropical regions of the Northern and Southern Hemispheres.Consequently, a shift of SF 6 emissions from Annex I to Non-Annex I countries (e.g. after the beginning of the 1990s) would imply a southward redistribution of the global SF 6 source.This in turn should result in a smaller SF 6 mixing ratio difference between the north and the south, as simulated with the UNFCCC-based emission scenario after about 1992 (Fig. 3b).
Besides incorrect distribution of sources, model-data mismatch can, however, also be caused by several other factors: Meridional mixing in the model may be overestimated, resulting in under-estimation of inter-hemispheric SF 6 differences and vice versa.Also the observations at Alert and Neumayer may not necessarily be representative for the large GRACE model boxes.Atmospheric transport between the different boxes in GRACE, which was kept constant from year to year, has been optimized using bomb 14 CO 2 , 10 Be/ 7 Be, but also SF 6 with estimates of the total SF 6 source taken from Levin and Hesshaimer (1996), assumed to be spatially distributed according to the electricity production (Prather et al., 1987).However, this transport optimization still allows for an uncertainty of hemispheric residence times on the order of ±25%.Indeed, increasing hemispheric residence times by 25% would bring model simulations with the UNFCCC-based scenario almost into agreement with the observations (upper Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion boundary of the blue hatched area in Fig. 3b); in this case the EDGAR distribution would over-estimate the north-south difference (upper boundary of the red hatched area in Fig. 3b).At this stage, using the observed north-south difference of SF 6 we thus cannot firmly reject one of the two SF 6 emission scenarios.This is mainly because of the lack of really independent validation of transport properties in our model, i.e. independent from any prior calibration with SF 6 or other tracers, whose emissions also have source distributions similar to that of electricity production, such as fluorocarbons (Prather et al., 1987).A possible tracer for transport validation may be 85 Krypton (Jacob et al., 1987;Levin and Hesshaimer, 1996) which is mainly emitted from nuclear fuel reprocessing plants.However, these 85 Krypton emissions are largely concentrated in a few large point sources (Winger et al., 2005), a distribution not suitable for transport validation of our coarse resolution GRACE model.

Conclusions
Rigorously assessing the reliability of the EDGAR or the UNFCCC-based emission distribution, and validation of reported emissions by Annex I countries, may perhaps, be possible with a high resolution atmospheric transport model; however, such a model needs to be very well validated to correctly simulate atmospheric transport and mixing.Also, this would require a denser observational network.At present we are thus left with only the evidence from total non-reported SF 6 emissions as well as from specific SF 6 emissions per electricity production, which suggests, that Annex I reported UNFCCC emissions during the 1990s (and possibly until today) may be too low.This suggestion is confirmed by the EDGAR (2009) data base which assumes similar emission factors for Annex I and Non-Annex I countries.It is likely for the last ten years that SF 6 leakage rates are larger e.g. in China and other Non-Annex I countries than in Annex I countries, but it seems implausible to assume that Non-Annex I SF 6 emissions per electricity production are 2 up to 4 times higher than for Annex I countries.Annex I reported emissions may be too low because of intrinsic uncertainties of estimated SF 6 Introduction

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Full stored in end-use applications in the US and Europe (Maiss and Brenninkmeijer, 1998), and possibly due to underestimated emissions from economies in transition.The accelerating increase of global SF 6 emissions since the end of the 1990s may be linked to rising emissions from Non-Annex I countries, which qualitatively agree with their economic growth (e.g.China) (GDP, 2008).However, as long as these countries are not obliged to report their emissions to UNFCCC, there will remain large inaccuracies in related bottom-up emission estimates.
Our study clearly shows that top-down verification of reported emissions is without alternative for greenhouse gases budgeting.On a country level, such validation can, however, only be achieved with a dense network of high-precision atmospheric observations in combination with adequately calibrated atmospheric transport models.Top-down validation of total global SF 6 emissions is accurately possible without a sophisticated transport model, even with only one or a few globally distributed background stations.This mechanism should, therefore, be included as an additional measure in the Kyoto reporting process, as it provides the only and ultimate proof of total reported emissions (changes), at least for gases with well-known sinks such as SF 6 , and other fluorinated and chlorinated compounds.
Acknowledgements.This work would not have been possible without the invaluable help from the technical personnel at the sampling sites, in particular, the late Laurie Porter at Cape Grim, Pedro Carretero at Iza ña, the changing teams at the polar stations Alert and Neumayer as well as A. Varlagin at the Russian aircraft site.We wish to thank D. Wagenbach for many helpful discussions on the manuscript.This long-term work was partly funded by a number of agencies in Germany and Europe, namely the Ministry of Education and Science, Baden-W ürttemberg, Germany; the German Science Foundation, the German Minister of Environment; the German Minister of Science and Technology; the German Umweltbundesamt, and the European Commission, Brussels, as well as national funding agencies in Canada and Spain.Sampling at the Cape Grim Baseline Air Pollution Station has been supported by funding from the Australian Bureau of Meteorology.P. Fraser and P. Krummel of CMAR are acknowledged for their longterm support of the Cape Grim Air Archive program, and gathering the SF 6 inter-comparison results at Cape Grime (P.K.).Full

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Figure 2a also shows the total share of annual SF 6 emissions which Annex I countries officially reported to UNFCCC (2009) from 1990 to 2006 (solid blue line).We applied a downward correction to the figure Japan reported to UNFCCC before 1994 (dashed blue line) because they probably overestimated pre-1994 emissions due to an inade-

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Comparison of observed mixing ratios with model simulationsA comparison of observed mixing ratios with atmospheric transport model estimates that are based on different emission scenarios may help to decide which of them is more likely correct.Here we have deployed the two emission distribution estimates, EDGAR and the corrected UNFCCC-based in a coarse-resolution two-dimensional atmospheric box model GRACE(Levin et al., 2009).Thereby, simulated tropospheric SF 6 mixing ratios were compared with observations at Alert (82• N), Cape Grim (41• S)and Neumayer (71 • S).The core of the GRACE model consists of an atmospheric module with 28 boxes, representing zonal mean tracer mixing ratios in six zonal, and either four (tropics) or five (extra-tropics) vertical subdivisions.Air mass (and tracer) exchange between the atmospheric boxes is controlled by three processes: 1) (turbulent) diffusive exchange between neighbouring boxes, 2) the Brewer-Dobson circulation, and 3) seasonal lifting and lowering of the extra-tropical tropopause.While the meridional distribution of SF 6 emissions for the EDGAR scenario can be directly taken from the data base (EDGAR, 2009), we used for the source distribution of the UNFCCC-based scenario for Annex I countries the officially reported emissions corrected for Japan (UNFCCC, 2009).For Non-Annex I countries the residual from the global inferred SF 6 source (dotted blue line in Fig.2b) was distributed according to the electrical power production of these countries (see Supplementary Material Sect.4.3 and Fig.A4).Simulated SF 6 mixing ratios from 1976 to 2009 for the mid latitude box of GRACE in the Southern Hemisphere are displayed in Fig.3a, together with the observations at Cape Grim (41 • S) starting in 1978.There is generally good agreement of the long-term trends between EDGAR-based simulations and the observations, with model results being slightly lower than observations up to 1992 and again from about 2003 onwards.When using the relative latitudinal distribution of SF 6 emissions from EDGAR adjusted to our observation-inferred annual totals, we obtain almost perfect agreement with observations for the whole period from 1978 until present.This confirms our top-down method, but also shows how sensitive global tropospheric SF 6 mixing ratio trends can

Fig. 1 .
Fig. 1.Global observations of tropospheric SF 6 .Symbols and left axis: atmospheric SF 6 mixing ratios (given in ppt=parts per trillion, i.e. picomoles of SF 6 per mole of dry air) observed in the Northern and Southern Hemispheres.The overlapping flask and cylinder data from Neumayer are virtually indistinguishable (note that 12 data points in total have been rejected as outliers from the Figure).Lines and right axis: SF 6 growth rates calculated for individual stations from de-seasonalized measurements using a fit routine from Nakazawa et al. (1997) (colour codes: same as original data).
Jigme, UNFCCC, pers, comm., see Supplementary Material  Sect.4.2).While in 1990 total reported emissions by Annex I countries still correspond to 80% of the global inferred emissions, they decrease in 1995 to 43%, and to less than 24% in 2005.The EDGAR (2009) data base not only provides global emissions but also emissions per country.The annual sum of all Annex I emissions as estimated by EDGAR for 1976 to 2005 is also plotted in Fig.2a(dashed-dotted black line).These emissions are higher by more than 90% compared to what was officially reported to UNFCCC for 1995, and by more than a factor of two from 1997 until 2005.Newly industrialized countries not included in Annex I of the UNFCCC, such as China, India, Brazil, and others, are not required to report SF 6 emissions to UNFCCC.

Table 2 .
Observed global atmospheric SF 6 inventory and inferred annual SF 6 source.The 1σ uncertainties of the inventory are ±3-4 Gg, while the uncertainties of the annual emissions are ±6%, neglecting oceanic and atmospheric sinks.Note that the inventory refers to the middle of each year, whereas the source refers to the period 1 January-31 December.