Global and regional emissions estimates of 1,1-difluoroethane (HFC-152a, CH3CHF2) from in situ and air archive observations

Abstract

High frequency, in situ observations from 11 globally distributed sites for the period 1994-2014 and archived air measurements dating from 1978 onward have been used to determine the global growth rate of 1,1-difluoroethane (HFC-152a, CH₃CHF₂). These observations have been combined with a range of atmospheric transport models to derive global emission estimates in a top-down approach. HFC-152a is a greenhouse gas with a short atmospheric lifetime of about 1.5 years. Since it does not contain chlorine or bromine, HFC-152a makes no direct contribution to the destruction of stratospheric ozone and is therefore used as a substitute for the ozone depleting chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). The concentration of HFC-152a has grown substantially since the first direct measurements in 1994, reaching a maximum annual global growth rate of 0.84±0.05pptyr^'1 in 2006, implying a substantial increase in emissions up to 2006. However, since 2007, the annual rate of growth has slowed to 0.38±0.04pptyr^'1 in 2010 with a further decline to an annual average rate of growth in 2013-2014 of '0.06±0.05pptyr^'1. The annual average Northern Hemisphere (NH) mole fraction in 1994 was 1.2ppt rising to an annual average mole fraction of 10.1ppt in 2014. Average annual mole fractions in the Southern Hemisphere (SH) in 1998 and 2014 were 0.84 and 4.5ppt, respectively. We estimate global emissions of HFC-152a have risen from 7.3±5.6Ggyr^'1 in 1994 to a maximum of 54.4±17.1Ggyr^'1 in 2011, declining to 52.5±20.1Ggyr^'1 in 2014 or 7.2±2.8Tg-CO₂eqyr^'1. Analysis of mole fraction enhancements above regional background atmospheric levels suggests substantial emissions from North America, Asia, and Europe. Global HFC emissions (so called "bottom up" emissions) reported by the United Nations Framework Convention on Climate Change (UNFCCC) are based on cumulative national emission data reported to the UNFCCC, which in turn are based on national consumption data. There appears to be a significant underestimate ( > 20Gg) of "bottom-up" reported emissions of HFC-152a, possibly arising from largely underestimated USA emissions and undeclared Asian emissions.

Atmos. Chem. Phys., 16, 365382, 2016 www.atmos-chem-phys.net/16/365/2016/ doi:10.5194/acp-16-365-2016 Author(s) 2016. CC Attribution 3.0 License.

P. G. Simmonds1, M. Rigby1, A. J. Manning2, M. F. Lunt1, S. ODoherty1, A. McCulloch1, P. J. Fraser4, S. Henne5,M. K. Vollmer5, J. Mhle3, R. F. Weiss3, P. K. Salameh3, D. Young1, S. Reimann5, A. Wenger1, T. Arnold2,C. M. Harth3, P. B. Krummel4, L. P. Steele4, B. L. Dunse4, B. R. Miller14, C. R. Lunder6, O. Hermansen6,N. Schmidbauer6, T. Saito7, Y. Yokouchi7, S. Park8, S. Li9, B. Yao10, L. X. Zhou10, J. Arduini11, M. Maione11,R. H. J. Wang12, D. Ivy13, and R. G. Prinn13

1Atmospheric Chemistry Research Group, University of Bristol, Bristol, BS8 1TS, UK

2Met Ofce Hadley Centre, Exeter, EX1 3PB, UK

3Scripps Institution of Oceanography (SIO), University of California San Diego, La Jolla, CA 92093, USA

4CSIRO Oceans and Atmosphere, Aspendale, VIC 3195, Australia

5Laboratory for Air Pollution and Environmental Technology, Swiss Federal Laboratories for Materials Science and Technology (Empa), Dubendorf, 8600, Switzerland

6Norwegian Institute for Air Research (NILU), 2027 Kjeller, Norway

7Centre for Environmental Measurement and Analysis, National Institute for Environmental Studies, Onogawa, Tsukuba, 305-8506, Japan

8Department of Oceanography, College of Natural Sciences, Kyungpook National University, Daegu, 702-701, Republic of Korea

9Kyungpook Institute of Oceanography, College of Natural Sciences, Kyungpook National University, Daegu, 702-701, Republic of Korea

10Chinese Academy of Meteorological Sciences (CAMS), Beijing, 10081, China

11Department of Basic Sciences and Foundations, University of Urbino, 61029 Urbino, Italy

12School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA

13Center for Global Change Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

14Global Monitoring Division, ESRL, NOAA, Boulder, Colorado, USA

Correspondence to: P. G. Simmonds ([email protected])

Received: 12 July 2015 Published in Atmos. Chem. Phys. Discuss.: 7 August 2015 Revised: 23 November 2015 Accepted: 1 December 2015 Published: 18 January 2016

Abstract. High frequency, in situ observations from 11 globally distributed sites for the period 19942014 and archived air measurements dating from 1978 onward have been used to determine the global growth rate of 1,1-diuoroethane (HFC-152a, CH3CHF2). These observations have been combined with a range of atmospheric transport models to derive global emission estimates in a top-down approach. HFC-152a is a greenhouse gas with a short atmospheric lifetime of about 1.5 years. Since it does not contain chlorine or bromine, HFC-152a makes no direct contribution to the destruction of stratospheric ozone and is therefore used as a substitute for the ozone de-

pleting chlorouorocarbons (CFCs) and hydrochlorouorocarbons (HCFCs). The concentration of HFC-152a has grown substantially since the rst direct measurements in 1994, reaching a maximum annual global growth rate of0.84 0.05 ppt yr1 in 2006, implying a substantial increase

in emissions up to 2006. However, since 2007, the annual rate of growth has slowed to 0.38 0.04 ppt yr1 in

2010 with a further decline to an annual average rate of growth in 20132014 of 0.06 0.05 ppt yr1. The an

nual average Northern Hemisphere (NH) mole fraction in 1994 was 1.2 ppt rising to an annual average mole fraction of 10.1 ppt in 2014. Average annual mole fractions

Published by Copernicus Publications on behalf of the European Geosciences Union.

Global and regional emissions estimates of 1,1-diuoroethane (HFC-152a, CH3CHF2) from in situ and air archive observations

366 P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a

in the Southern Hemisphere (SH) in 1998 and 2014 were0.84 and 4.5 ppt, respectively. We estimate global emissions of HFC-152a have risen from 7.3 5.6 Gg yr1 in 1994

to a maximum of 54.4 17.1 Gg yr1 in 2011, declining

to 52.5 20.1 Gg yr1 in 2014 or 7.2 2.8 Tg-CO2 eq yr1.

Analysis of mole fraction enhancements above regional background atmospheric levels suggests substantial emissions from North America, Asia, and Europe. Global HFC emissions (so called bottom up emissions) reported by the United Nations Framework Convention on Climate Change (UNFCCC) are based on cumulative national emission data reported to the UNFCCC, which in turn are based on national consumption data. There appears to be a signicant underestimate (> 20 Gg) of bottom-up reported emissions of HFC-152a, possibly arising from largely underestimated USA emissions and undeclared Asian emissions.

1 Introduction

HFC-152a (CH3CHF2) is primarily sold as an aerosol and foam-blowing agent (Greally et al., 2007) and as a component of some refrigerant blends (Ashford et al., 2004). Emissions to the atmosphere show both temporal and regional variability depending on the specic application in which HFC-152a is used. Incorporation of HFC-152a into aerosol propellants results in prompt release, whereas when used as a single-component non-encapsulated blowing agent, release occurs over a period of about 2 years (McCulloch et al., 2009). Refrigerant use of HFC-152a results in release over longer periods, possibly up to 20 years. Reported emissions of HFC-152a are likely to be incomplete as a consequence of a limited number of producers and condentiality considerations. Emissions of HFC-152a for some countries are aggregated with other hydrouorocarbons (HFCs) in a category reported to the UNFCCC as unspecied mix. For example, emissions reported by the USA to the UNFCCC for HFC-152a, 227ea, 245ca and 43-10mee are shown in the database as commercially condential, and they constitute the aggregated unspecied emissions. HFC-152a emissions from the USA are estimated to be the primary contributor to the total for this gas from Annex 1 countries (Lunt et al., 2015).Previous papers (Manning and Weiss, 2007; Millet et al., 2009; Stohl et al., 2009; Barletta et al., 2011; Miller et al., 2012; Simmonds et al., 2015) have reported major differences between USA HFC-152a emission estimates derived from atmospheric measurements (top down) and emissions calculated from US reports to the UNFCCC (bottom up). The apparent under-reporting of USA emissions to the UNFCCC ranges from 2060 Gg based on annual average estimates.

HFC-152a has the smallest 100-year global warming potential (GWP100, 138) of all the major HFCs (Forster et al., 2007; Myhre et al., 2013), with a short atmospheric lifetime of 1.5 years, due to efcient reaction with tropospheric hydroxyl (OH) radicals (SPARC Report No. 6, 2013). Unlike

hydrocarbons, HFC-152a does not participate in the reaction to form ozone in the troposphere. These desirable properties have made HFC-152a especially attractive as a replacement, not only for CFCs (chlorouorocarbons) and HCFCs (hydrochlorouorocarbons), but also increasingly for HFC-134a in technical aerosol applications and mobile air-conditioners (IPCC/TEAP, 2011).

Ryall et al. (2001) using observations from Mace Head, Ireland reported the distribution of European HFC-152a emissions, concentrated in Germany, and estimated an average European total emission of 0.48 Gg yr1 for 1995

1998. Reimann et al. (2004) used a 3-year data set (2000 2002) of HFC-152a observations at the Swiss Alpine station Jungfraujoch and trajectory modeling, also noting a predominantly German source for European HFC-152a emissions. This group measured an atmospheric growth rate of0.3 ppt yr1 (ppt parts per trillion, 1012, mol mol1 or pmol mol1) from 2000 to 2002 and a December 2002 mole fraction at the Jungfraujoch station of 3.2 ppt, from which they estimated a European emission strength of 0.8 Gg yr1 for 20002002.

In the Southern Hemisphere HFC-152a monthly means, annual means and trends have been reported from observations at Cape Grim, Tasmania, for 19982004 (Sturrock et al., 2001; Fraser et al., 2014a; Krummel et al., 2014).The HFC-152a annual means have grown from 0.8 ppt(0.1 ppt yr1) in 1998 to 1.8 ppt (0.4 ppt yr1) in 2004.

More recent estimates of SE Australian HFC-152a emissions (20052012) have been calculated by interspecies correlation and model inversions and by extrapolation based on population (Fraser et al., 2014a).

Here we further expand the HFC-152a record up to the end of 2014 using in situ observations from 11 globally distributed monitoring stations (9 Advanced Global Atmospheric Gases Experiment (AGAGE) stations and 2 afliated stations), together with atmospheric transport models to independently estimate HFC-152a emissions on regional and global scales. We then compare these with HFC-152a emission estimates compiled from national reports to the United Nations Framework Convention on Climate Change (UNFCCC) and Emissions Database for Global Atmospheric Research (EC-JRC/PBL EDGAR v4.2; http://edgar.jrc.ec.europa.eu/

Web End =http://edgar.jrc.ec. http://edgar.jrc.ec.europa.eu/

Web End =europa.eu/ ), using the same techniques reported for other greenhouse gases (ODoherty et al., 2009, 2014; Miller et al., 2010; Vollmer et al., 2011; Krummel et al., 2014; Rigby et al., 2014).

2 Experimental methods

2.1 Instrumentation and calibration

High frequency, in situ measurements of HFC-152a were made by gas chromatography-mass spectrometry (GCAgilent 6890) coupled with quadrupole mass selective detec-

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P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a 367

tion (MSD-Agilent 5973/5975). Measurements commenced at Mace Head, Ireland in 1994 and Cape Grim, Tasmania in 1998, using a custom-built automated pre-concentration system (adsorption desorption system ADS) to selectively and quantitatively retain halogenated compounds from 2 L air samples. Based on a Peltiercooled pre-concentration microtrap cooled to 50 C during the adsorption phase, the

ADS provided on-site calibrated air samples every 4 h, i.e., six per day (Simmonds et al., 1995). In 2004, the ADSGC-MS was replaced with a more advanced custom-built pre-concentration system (Medusa) with enhanced cooling to 180 C and the relatively mild adsorbent HayeSep D

(Miller et al., 2008; Arnold et al., 2012). Agilent 5973 MSDs (mass selective detector) were also upgraded to the Agilent 5975 MSDs over the course of the Medusa observations.Analysis of each 2 L sample of ambient air was alternated with analysis of a 2 L reference gas (designated as a working standard) to correct for short-term instrumental drift, resulting in 12 (Medusa) individually calibrated air measurements per day. Working standards were prepared for each station by compressing ambient air into 34 L electropolished stainless steel canisters (Essex Industries, Inc., Missouri) using modied oil-free compressors (SA-6, RIX, California). Exceptions to this were the Cape Grim and Zeppelin stations, where the working standards were lled using a cryogenic lling technique. Research-grade helium, which was used as a carrier gas in the Medusa systems, was further puried by passage through a heated getter type purier (Valco Instruments, Houston, TX). The carrier gas was analyzed for blanks on a regular basis and blank levels of HFC-152a were below the limit of detection at all eld stations.

Table 1 lists the geographical location and the time when routine ambient measurements of HFC-152a began at each monitoring station. Stations with the longest observational records that deployed both ADS and Medusa GC-MS instruments include Mace Head (MHD), Jungfraujoch (JFJ), Ny-lesund (ZEP) and Cape Grim (CGO). Medusa GCMS instruments were installed at ve other AGAGE stations Trinidad Head (THD), Gosan (GSN), Ragged Point, (RPB), Shangdianzi (SDZ), and Cape Matatula (SMO) between 2003 and 2010. In addition two AGAGE afliated stations Monte Cimone (CMN) and Hateruma (HAT), which use comparable GC-MS instruments, but a different pre-concentration design for sample enrichment, commenced HFC-152a measurements in 2001 and 2004, respectively. Importantly, all 11 stations listed in Table 1 report HFC-152a measurements relative to the Scripps Institution of Oceanography (SIO-05) calibration scale (as dry gas mole fractions in pmol mol1).

The estimated accuracy of the calibration scale for HFC-152a is 4 %: a more detailed discussion of the measurement technique and calibration procedure is reported elsewhere (Miller et al., 2008; ODoherty et al., 2009; Mhle et al., 2010). HFC-152a was determined using the MS in selected ion monitoring mode (SIM) with a target ion CH3CF+2

(m/z 65) and qualier ion CH3CF+ (m/z 46). To ensure that potential interferences from co-eluting species did not compromise the analysis, the ratio of the target to qualier ion was continuously monitored. Measurement precision was calculated as the daily standard deviation (1) of the ratios of each standard response to the average of the closest-in-time preceding and subsequent standard responses. Typical daily precisions vary from station to station with a range of 0.10.4 ppt. Individual station precisions were used to estimate the precision of each in situ measurement.

2.2 Northern and Southern Hemisphere archived air samples

In order to extend the HFC-152a data record back before the commencement of high-frequency measurements, analyses of Northern Hemisphere (NH) and Southern Hemisphere (SH) archived air samples dating back to 1978, were carried out using three similar Medusa GC-MS instruments at the Scripps Institution of Oceanography (SIO), La Jolla, California, the Commonwealth Scientic and Industrial Research Organisation (CSIRO), Aspendale, Australia and the Cape Grim Baseline Air Pollution Station, Tasmania. The SH samples are part of the Cape Grim air archive (CGAA) described in Langenfelds et al. (1996), and Krummel et al. (2007). The NH samples analyzed for this paper were lled during background conditions mostly at Trinidad Head, but also at La Jolla, California; Cape Meares, Oregon; Ny lesund, Svalbad and Point Barrow, Alaska (some samples are courtesy of the National Oceanic and Atmospheric Administration (NOAA).

In addition, eight SH samples were measured at SIO and compared with SH samples of similar age measured at CSIRO (February 1995, July 1995, November 1995, June 1998, July 2004, February 2006, August 2008, and December 2010, [Delta1]x = 0.010.07 ppt [Delta1]t = 133 days) and

three NH samples were measured at CSIRO and compared with NH samples of the same age measured at SIO (May 1989 and April 1999, [Delta1]x = 0.020.06 ppt, [Delta1]t = 1

11 days). The good agreement between SIO and CSIRO archived air stored in different types of tanks (stainless steel tanks, Essex Industries, Inc and Silcosteel treated tanks, Restek Corporation) serves both as proof of the good consistency of the individual Medusa GC-MS instruments and the integrity of the tanks used. Samples were analyzed in replicate typically 36 times each and several NH tanks were re-measured over a number of years.

2.3 Selection of baseline data

Baseline in situ monthly mean HFC-152a mole fractions were calculated by excluding values enhanced by local and regional pollution inuences, as identied by the iterative AGAGE pollution identication algorithm, (see Appendix in ODoherty et al., 2001). Briey, baseline measurements

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368 P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a

Table 1. Overview of the 11 measurement stations used in this study, their coordinates and periods for which data are available.

Station Latitude Longitude ADS data* Medusa data**

Ny-lesund, Norwaya 78.9 N 11.9 E 20012010 September 2010present Mace Head, Irelanda 53.3 N 9.9 W 19942004 June 2003present

Jungfraujoch, Switzerlanda 46.5 N 8.0 E 20002008 May 2008present Monte Cimone, Italyb 44.2 N 10.7 E June 2001presentb

Trinidad Head, Californiaa 41.0 N 124.1 W March 2005present Shangdianzi, Chinaa,c 40.4 N 117.7 E May 2010August 2012

Gosan, Jeju Island, Koreaa 33.2 N 126.2 E November 2007present

Hateruma, Japanb 21.1 N 123.8 E May 2004presentb

Ragged Point, Barbadosa 13.2 N 59.4 W May 2005present Cape Matatula, Samoaa 14.2 S 170.6 W May 2006present

Cape Grim, Tasmaniaa 40.7 S 144.7 E 19982004 Jan 2004present

a AGAGE stations; b Afliated stations use a different pre-concentration system (non-Medusa) than the AGAGE stations, but comparable GC-MS analytical instruments (see Yokouchi et al., 2006; Maione et al., 2014). c Shangdianzi was only operational for a short period and is not included in the modeling studies. * Period of HFC-152a data record using ADS-GC-MS. ** Period of HFC-152a data record using Medusa-GC-MS.

are assumed to have a Gaussian distribution around the local baseline value, and an iterative process is used to lter out the points that do not conform to this distribution. A second-order polynomial is tted to the subset of daily minima in any 121-day period to provide a rst estimate of the baseline and seasonal cycle. After subtracting this polynomial from all the observations a standard deviation and median are calculated for the residual values over the 121-day period. Values exceeding 3 standard deviations above the baseline are thus identied as non-baseline (polluted) and removed from further consideration. The process is repeated iteratively to identify and remove additional non-baseline values until the new and previous calculated median values agree within 0.1 %. For the core AGAGE stations, in situ baseline data and archive air data, extending the record to periods prior to the in situ measurement period, are then combined for each hemisphere, and outliers are rejected by an iterative lter.

3 Modeling studies

We pursued several approaches to determine emissions at global, continental and regional scales. The methodologies have been published elsewhere and are summarized below. The global, continental and some regional estimates incorporate a priori estimates of emissions, which were subsequently adjusted using the observations.

There are several sources of information on production and emissions of HFC-152a; none of which, on their own, provides a complete database of global emissions. The more geographically comprehensive source of information is provided by the parties to the UNFCCC, but only includes Annex 1 countries (developed countries). The 2014 database covers years 1990 to 2012 and are reported in Table 2(II) s1 in the common reporting format (CRF) available at

http://unfccc.int/national_reports/annex ighg inventories/national inventories submissions/items/8108.php

Web End =http://unfccc.int/national_reports/annexighginventories/ http://unfccc.int/national_reports/annex ighg inventories/national inventories submissions/items/8108.php

Web End =nationalinventoriessubmissions/items/8108.php . An alternative inventory estimate was also obtained from the Emissions Database for Global Atmospheric Research (EDGAR v4.2; http://edgar.jrc.ec.europa.eu/

Web End =http://edgar.jrc.ec.europa.eu/ ), a database that estimates global emission inventories of anthropogenic greenhouse gases (GHGs) on a country, region and grid basis up to 2008.

To infer top-down emissions we select observations from the various observing sites listed in Table 1 and four chemical transport models. These 11 sites are sensitive to many areas of the world in which HFC-152a emissions are reported; however, other areas of the globe that are not well monitored by this network are also likely to have signi-cant emissions (such as South Asia, South Africa, and South America).

3.1 Global emissions estimates using the AGAGE two-dimensional 12-box model

To estimate global-average mole fractions and derive growth rates, a two-dimensional model of atmospheric chemistry and transport was employed. The AGAGE 12-box model simulates trace gas transport in four equal mass latitudinal sections (divisions at 3090 N, 030 N, 300 S and 90

30 S) and at three heights (vertical divisions at 200, 500 and 1000 hPa). The model was originally developed by Cunnold et al. (1983) (nine-box version), with subsequent improvements by Cunnold et al. (1994) and Rigby et al. (2013, 2014).Emissions were estimated between 1989 and 2014 using a Bayesian method in which an a priori constraint (EDGAR v4.2) on the emissions growth rate was adjusted using the baseline-ltered AGAGE observations (Rigby et al., 2011a, 2014). Global emissions were derived that included estimates of the uncertainties due to the observations, the prior and the lifetime of HFC-152a, as detailed in the supplementary material in Rigby et al. (2014). Note that historically and here the

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Table 2. Estimates of global emissions of HFC-152a (Gg yr1 1) based on AGAGE in situ measurements using the AGAGE 2-D 12-

box model. Emission inventories as reported in UNFCCC (United Nations Framework Convention on Climate Change) National Inventory Reports (DoE, 2014; National Inventory Report 2012, submission), EDGAR (v4.2) database and recalculated from the UNFCCC data as described in the text.

Year AGAGE UNFCCC as reported EDGAR (4.2) UNFCCC including unspecied (Gg yr1) (Gg yr1) (Gg yr1) contribution (Gg yr1)

1994 7.3 5.6

1995 7.9 7.4 1.0 7.3 8.8

1996 9.1 8.4 1.1 8.9 13.3

1997 11.3 8.6 1.3 10.3 15.4

1998 12.5 10.9 1.2 11.7 13.3

1999 14.4 11.2 1.4 13.2 14.0

2000 16.6 12.2 2.2 15.2 13.0

2001 18.4 13.4 3.5 15.9 15.4

2002 22.5 14.7 4.5 18.6 17.6

2003 26.3 15.3 4.7 20.6 17.7

2004 29.2 15.6 4.8 21.7 18.1

2005 35.8 14.7 4.3 23.0 16.5

2006 43.3 14.9 4.4 24.9 16.7

2007 48.1 17.6 4.4 26.4 16.8

2008 48.9 16.7 4.3 28.0 16.4

2009 48.0 16.4 4.6 17.6

2010 53.4 17.5 4.9 18.6

2011 54.4 17.1 5.0 19.3

2012 53.2 18.5 5.2 20.5

2013 52.5 17.8

2014 52.5 20.1

12-box model only uses observations from the core AGAGE sites, Mace Head, Trinidad Head, Ragged Point, Cape Matatula, and Cape Grim.

3.2 Global and continental emissions estimates using a combined Eulerian and Lagrangian model

We used the methodology outlined in Lunt et al. (2015) and Rigby et al. (2011b) to derive emissions of HFC-152a from continental regions. The high-resolution, regional UK Met Ofce Numerical Atmospheric-dispersion Modelling Environment (NAME), Manning et al. (2011) was used to simulate atmospheric HFC transport close to a subset of AGAGE monitoring sites, which were strongly inuenced by regional HFC sources (domains shown by red boxes in Fig. 1). Simultaneously, the inuence of changes to the global emissions eld on all measurement stations was simulated using the global Model for OZone and Related Tracers, MOZART (Emmons et al., 2010). We estimated annual emissions for the period 20072012 and aggregated the derived emissions elds into continental regions, separating countries that either do (Annex-1), or do not (non-Annex-1) report detailed, annual emissions to the UNFCCC. Emissions were estimated using a hierarchical Bayesian inverse method (Ganesan et al., 2014; Lunt et al., 2015) and all high-frequency observations from 10 of the 11 sites listed in Table 1, exclud-

ing Shangdianzi due to the short time series. The hierarchical Bayesian method includes uncertainty parameters (e.g., model mismatch errors and a priori uncertainties) in the estimation scheme, reducing the inuence of subjective choices on the outcome of the inversion.

3.3 High-resolution regional emissions estimates using InTEM

A method for estimating emissions from observations and atmospheric transport modeling with NAME referred to as In-TEM, Inversion Technique for Emission Modelling (Manning et al., 2011), uses a simulated annealing method (Press et al., 1992) to search for the emission distribution that produces a modeled times series that has the best statistical match to the observations from certain AGAGE stations (e.g., Mace Head, Cape Grim). NAME was driven with output from the operational analysis of the UK Met Ofce Numerical Weather Prediction model, the Unied Model, at global horizontal resolution of 1740 km (year dependent). InTEM estimates the spatial distribution of emissions across a dened geographical area, and can either start from a random emission distribution or be constrained by an inventory-dened distribution. Emission totals from specic geographical areas are calculated by summing the derived emissions from each grid (non-uniform) in that region.

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370 P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a

Figure 1. Location of AGAGE and afliated stations. Ny-lesund, Zeppelin, Norway (ZEP); Mace Head, Ireland (MHD); Jungfraujoch, Switzerland (JFJ); Monte Cimone, Italy (CMN); Trinidad Head, USA (THD); Shangdianzi, China (SDZ); Gosan, South Korea (GSN); Hateruma, Japan (HAT); Ragged Point, Barbados (RPB); Cape Matatula, American Samoa (SMO); and Cape Grim, Tasmania (GCO). Red boxes indicate local regions where the NAME model was used with increased resolution compared to the global MOZART model, Annex 1 countries are shaded blue and non-Annex 1 countries are shaded yellow. Note: 1 Shangdianzi (SDZ) was not used in any of the modeling studies due to the relatively short time series.

The uncertainty estimation used within InTEM is described in detail elsewhere (Manning et al., 2011). Briey, the uncertainty space was explored by (a) solving the inversion multiple times with a range of baseline mole fractions within the baseline uncertainty estimated during the baseline tting process and (b) by altering the 3-year inversion time window by 1 month throughout the data period thereby solving over a particular 1-year period many times using different observations. In total for each annual estimate, up to 111 inversions were performed; the median and 5th and 95th percentiles were used as the nal total and spread. For the Australian estimates data between 2002 and 2011 were used, for the NW European estimates data between November 1994 and December 2013 were used.

3.4 High-resolution European emission estimates using the FLEXPART model

A regional Bayesian inversion system using backward simulations of a Lagrangian particle dispersion model FLEX-PART (Stohl et al., 2005) was applied to the HFC-152a observations from Mace Head, Jungfraujoch and Mt. Cimone for the period 2006 to 2014. The inversion technique follows the description by Stohl et al. (2009) and was previously applied to regional halocarbon emissions from Europe (Keller et al., 2012; Maione et al., 2014) and China (Vollmer et al., 2009). For these emission estimates, the background was determined by applying the Robust Extraction of Baseline Signal (REBS) lter described in detail by Ruckstuhl

et al. (2012). The transport model FLEXPART was driven with output from the operational analysis of the Integrated Forecast System (IFS) of the European Centre for Medium Range Weather Forecast (ECMWF) using a spatial resolution of 0.2 0.2 for a nested domain covering the larger

area of the European Alps and a spatial resolution of 1 1

elsewhere.

The FLEXPART model was applied to the HFC-152a observations from Mace Head, Jungfraujoch and Mt. Cimone for the period 2006 to 2014. Prior to 2006, the model resolution of Integrated Forecast System (IFS) was not sufciently ne to realistically simulate the transport to the two high altitude sites Jungfraujoch and Mt. Cimone. Therefore, no attempt was made here to apply the inversion system to years before 2006. As prior information of the HFC-152a emissions we used country totals as submitted to UNFCCC. These were spatially disaggregated following the HFC-152a distribution given in EDGAR (v4.2). For countries not reporting HFC-152a emissions to UNFCCC we used the values given in EDGAR. The EDGAR inventory was only available up to the year 2008 beyond this year the EDGAR 2008 distribution was used. The uncertainty of the prior emissions was set so that the region total uncertainty equalled 20 % of the region total emissions. The regional inversion grid covered a region similar to that shown in Fig. 1.

3.5 Regional emissions estimates using the inter-species correlation (ISC) methods

We also present regional emissions estimates using inter-species correlation (ISC) methods (Yokouchi et al., 2005).Emissions of a number of trace gases from the Melbourne/Port Phillip region (CFCs, HCFCs, HFCs, carbon tetrachloride: Dunse et al., 2001, 2002, 2005; ODoherty et al., 2009; Fraser et al., 2014a, b), including HFC-152a (Gre-ally et al., 2007), have been estimated utilizing in situ high frequency measurements from Cape Grim and ISC with coincident carbon monoxide (CO) measurements.

ISC works best for co-located sources however extensive modeling has shown that by the time the Melbourne/Port Phillip plume reaches Cape Grim (300 km from the source) it is well mixed and the likely inhomogeneity of the source regions (for CO and HFC-152a in this case) does not have a signicant inuence on the derived emissions. It should be noted that in order to obtain a signicant sampling of Port Phillip pollution episodes at Cape Grim, data from 3 years (for example 20112013) are used to derive annual emissions (for 2012). (InTEM also uses data from 3 years to derive annual emissions.) The ISC uncertainties given in the paper include (1) the uncertainties in the estimates of CO emissions from Melbourne/Port Phillip (2) the uncertainties in the overall correlation between CO and HCFC-152a as seen in pollution episodes at Cape Grim (3) the uncertainties in the geographic extent of the HFC-152a and CO source regions impacting on Cape Grim and their entrained population.

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P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a 371

Using HCFC-22 as the reference tracer, Li et al. (2011) reported that China is the dominant emitter of halocarbons in East Asia. North American HFC-152a emissions have been estimated from atmospheric data using interspecies correlation based techniques with CO (Millet et al., 2009; Barletta et al., 2011) and fossil fuel CO2 (Miller et al., 2012) as the reference emissions.

4 Results and discussion

4.1 In situ observations

The time series of HFC-152a in situ observations recorded at selected AGAGE and afliated monitoring stations are shown in Fig. 2ac. Data have been ltered into baseline (black) and above baseline (red) using the AGAGE pollution algorithm, as discussed in Sect. 2.3. Figure 2a shows the mole fractions in ppt for the four stations that deployed both ADS and Medusa GC-MS instruments (Mace Head, Zeppelin, Jungfraujoch, and Cape Grim). Most notable are the substantial above baseline events at Mace Head and Jungfraujoch that are inuenced primarily by emissions from European sources. Conversely, the Zeppelin Arctic station and the SH station at Cape Grim have relatively small above baseline events implying smaller emissions from local or regional sources.

Figure 2b shows measurements at the ve other AGAGE stations (Trinidad Head, Gosan, Ragged Point, Shangdianzi, and Cape Matatula), which used only Medusa GC-MS instruments. The North American site at Trinidad Head and the Asian sites at Shangdianzi and Gosan are the most strongly inuenced by regional emissions. The tropical sites at Ragged Point, Barbados, and Cape Matatula, American Samoa show very few enhancements above the baseline and these are due mostly to local emissions occurring under nighttime inversion conditions and occasional inuences from regional emission sources (note the different y axis scales). Although the Shangdianzi station was operational for only a short period, the enhancements above baseline are signicant due to the sensitivity of this site to Chinese emissions, and comparable in magnitude to those at Gosan.

Figure 2c illustrates the time series from the two afliated AGAGE stations (Monte Cimone and Haturuma) that used comparable GC-MS instruments but with different methods of pre-concentration. Monte Cimone, like the Jungfraujoch, is also inuenced by substantial emissions from sources in continental Europe. Hateruma is inuenced by sources in China, Korea, Taiwan, and Japan (Yokouchi et al., 2006).

4.2 Atmospheric trends and seasonal cycles

Figure 3 shows the in situ measurements of HFC-152a, as baseline monthly means (excluding pollution events), obtained from the two AGAGE stations Mace Head and Cape Grim with the longest time series that deployed both ADS

and Medusa GC-MS instruments. Superimposed in Fig. 3 are the NH and SH archived ask data extending back to 1978.Annual average mole fractions at Mace Head increased from1.2 ppt in 1994 to 10.2 ppt by 2014, Cape Grim annual average mole fractions increased from 0.84 ppt in 1998 when in situ measurements rst began to 4.5 ppt in 2014. However, in the last few years the rates of growth at both sites have slowed to almost zero.

The NH archived samples are more variable than the SH archived samples. The SH archive is collected only under strict baseline conditions (Southern Ocean air) and is far removed from the major sources of HFC-152a. Conversely in the NH, where most major sources of emissions are located, sampling under strict baseline conditions is more difcult to achieve.

Figure 4a illustrates HFC-152a baseline monthly means obtained from the ve other AGAGE observing sites (Ragged Point, Gosan, Cape Matatula, Trinidad Head, and Shangdianzi using only the more advanced Medusa GC-MS. There is a large seasonal cycle at Gosan with a very deep minimum due to summertime transport from the Southern Hemisphere (Li et al., 2011). Barbados can also be inuenced by Southern Hemispheric air during the hurricane season (Archibald et al., 2015).

Figure 4b shows the baseline monthly mean mole fractions for the three mountain stations. Ny-lesund and Jungfraujoch, using combined ADS and Medusa GC-MS measurements and Monte Cimone, which used a commercial pre-concentrator GC-MS. In most years Monte Cimone exhibits enhanced mole fractions during the NH spring months (MarchMay).

The HFC-152a seasonal cycles at Mace Head and Cape Grim shown in Fig. 5a and b, are broadly representative of the Northern Hemisphere and Southern Hemisphere, respectively. The seasonal cycle at Mace Head shows a NH spring maximum (AprilMay) and late summer minimum (AugustOctober), while the SH seasonal cycle at Cape Grim exhibits a broad austral spring maximum (JulyNovember) and a late summer minimum (JanuaryApril). The summer minimum at both locations is attributed to enhanced summertime loss (OH) with possibly a contribution from seasonally varying emissions in the NH that may be out-of-phase with the NH sink. At Cape Grim an additional source of seasonality is due to seasonally varying transport between the NH and SH, which is generally in phase with the sink induced seasonal cycle. This competition between OH summertime loss and seasonally varying transport has been observed at many other AGAGE locations (Prinn et al., 1992; Greally et al., 2007;ODoherty et al., 2009, 2014; Li et al., 2011).

Figure 6 shows the mole fractions output from the AGAGE global 12-box model, along with the monthly mean semi-hemispheric average observations used in the inversion. The gure also shows the running mean growth rate, smoothed using a KolmogorovZurbenko lter with a window of approximately 12 months (Rigby et al., 2014). Most notable

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372 P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a

Figure 2. (a) Time series of HFC-152a mole fractions (ppt) recorded at the four monitoring stations with combined ADS and Medusa data. (MHD, JFJ, ZEP and CGO), (note the different y axis scales). Data have been assigned as baseline (black) and non-baseline (red) using the AGAGE pollution identication algorithm. (b) Time series of HFC-152a mole fractions (ppt), recorded with the Medusa GC-MS instruments at the ve AGAGE monitoring stations (THD, GSN, SDZ, RPB, and SMO). Data have been assigned as baseline (black) and non-baseline (red) using the AGAGE pollution identication algorithm. (c) Time series of HFC-152a mole fractions (ppt) recorded with the GC-MS instruments at the two afliated AGAGE stations CMN and HAT. Data have been assigned as baseline (black) and non-baseline (red) using the AGAGE pollution identication algorithm.

is the positive growth rate from 1995 reaching a maximum of 0.84 ppt yr1 in 2006, followed by a steady decline in

the growth rate with a minimum during the economic recession in 20082009. The positive growth rate then resumes increasing to 0.4 ppt yr1 in 2010 followed by a subsequent

decrease with an annual average growth rate in 20132014 of minus 0.06 ppt yr1.

The strong inter-hemispheric gradient demonstrates that emissions are predominantly in the NH, as has been illustrated for many other purely anthropogenic trace gases (Prinn et al., 2000). The globally averaged mole fraction in the lower troposphere in 2014 is estimated to be 6.84 0.23 ppt

and the annual rate of increase is 0.06 0.05 ppt yr1. As

reported by Rigby et al. (2014) the major long lived synthetic greenhouse gases (SGHGs) which include CFCs, HCFCs, HFCs, and peruorocarbons (SF6 and NF3), as well as

CH3CCl3 and CCl4 were responsible for 350 10 mW m2

of direct radiative forcing in 2012. The radiative forcing of

HFC-152a, determined from the AGAGE 12-box model in this study, was 0.61 0.02 mW m2 in 2014, which repre

sents only a tiny fraction ( 0.2 %) of the global radiative

forcing of the SGHG.

5 Top-down emission estimates

5.1 Global estimates

Estimated global emissions of HFC-152a using the 12-box model and the reported UNFCCC and EDGAR emission inventories are shown in Fig. 7 and Table 2. The blue solid line represents our model-derived emissions, with the 1 error band shown by the shaded areas. Model derived emissions

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P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a 373

emissions, all expressed as CO2 equivalents. The values shown in column 4 of Table 2 are the global totals of HFC-152a after adjusting in these ways for the quantities included in unspecied emissions.

The additional component of US emissions makes a substantial contribution to the very large difference between the UNFCCC data as reported and the adjusted values. This is partly due to the low global warming potential of HFC-152a (a factor of 10 lower than other HFCs) which magnies its mass component in the 8200 Gg CO2 equivalent of US unspecied emissions.

The AGAGE observation based global emissions are substantially higher than the emissions calculated from the UNFCCC GHG reports (2014 submission). It is not unreasonable that UNFCCC-reported emissions are lower than the AGAGE global emission estimates, since countries and regions in Asia (e.g., China, Indonesia, Korea, Malaysia, the Philippines, Taiwan, Vietnam), the Indian sub-continent (e.g., India, Pakistan), the Middle East, South Africa, and Latin America do not report to the UNFCCC. Where we include the HFC-152a component of unspecied emissions (green line in Fig. 7) results are consistent within the error bars until approximately 2003 to 2005 when they start to diverge (UNFCCC + unspecied lower). From 1996 to

2002, estimated emissions from EDGAR (v4.2) are generally consistent with AGAGE emissions, but then begin to diverge with EDGAR emissions 22 Gg below 2008 AGAGE emissions, the last year for which EDGAR reports emissions.

5.2 Regional emissions of HFC-152a inferred for Europe, United States, Asia, and Australia

Lunt et al. (2015) have reported global and regional emissions estimates for the most abundant HFCs, based on inversions of atmospheric mole fraction data, aggregated into two categories; those from Annex 1 countries and those from non-Annex 1 countries. The inversion methodology used the NAME model to simulate atmospheric transport close to the monitoring sites, and the Model for Ozone and Related chemical Tracers (MOZART, Emmons et al., 2010) to simultaneously calculate the effect of changes to the global emissions eld on each measurement site. The model sensitivities were combined with a prior estimate of emissions (based on EDGAR) and the atmospheric measurements, in a hierarchical Bayesian inversion (Ganesan et al., 2014), to infer emissions.

Using this method we infer emissions estimates for the entire world, Europe, North America, and East Asia. Table 3 lists our estimated regional emissions in Gg yr1 averaged across two time periods: 20072009 and 20102012, together with our global emission estimates averaged over the same time periods from the 12-box model. It is apparent that North American average annual emissions ( 30 Gg) are the

major contributor to the global total with Europe contributing annual average emissions from about 56 Gg yr1. East Asia

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Figure 3. HFC-152a baseline monthly mean mole fraction (ppt) recorded at Mace Head-MHD (ADS GC-MS, 19942003; Medusa GC-MS, 20042014) and at Cape Grim-CGO (ADS GC-MS, 1998 2003; Medusa GC-MS, 20042014) and from analysis of archived NH and SH air samples extending back to 1975: in situ (black), air archive NH (red) and SH (blue).

grew steadily from 1995 to 2007 with a non-statistically signicant decrease in emissions in 2009 to 48 16.4 Gg yr1,

during the economic downturn in 20082009. The mean

emission reached a maximum of 54.4 17.1 Gg yr1 in

2011, followed by a period of relatively stable emissions, the mean showing a slight decline to 52.5 20.1 Gg yr1

(7.2 2.8 Tg-CO2 eq yr1) in 2014.

The data shown in column 3 of Table 2 are the totals of submissions by the national governments to the UNFCCC (Rio Convention) as reported in Table 2(II) s1 in the Common Reporting Format (CRF), available on the UNFCCC website (http://unfccc.int/national reports/annex ighg inventories/national inventories submissions/items/8108.php

Web End =http://unfccc.int/nationalreports/annexighginventories/ http://unfccc.int/national reports/annex ighg inventories/national inventories submissions/items/8108.php

Web End =nationalinventoriessubmissions/items/8108.php ). The values were taken from the 2014 database and cover years 1995 (the baseline year for submissions) to 2012. In addition to reporting calculated emissions of HFCs 23, 32, 125, 134a, 143a, 152a, 227ea, 236fa, 245ca, and 43-10mee individually, many countries also included unspecied emissions in this database (as the sum of their CO2 equivalents). Where the unspecied component was small in relation to the national specied emissions, it was disaggregated by assuming that it had the same fractional contribution of each HFC as reported in the specied components (adjusted for their CO2 equivalence). However, in the US, although values of emissions of several HFCs are calculated specically for the individual substances, HFCs 152a, 227ea, 245ca and 43-10mee are shown in the database as commercially condential and their emissions apparently constitute the substantial aggregated unspecied emissions reported. Hence, for the US, these unspecied annual emissions were divided only between HFCs 152a, 227ea, 245ca and 43-10mee, assuming the same ratio as their reported global

374 P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a

Figure 4. (a) Medusa GC-MS baseline monthly mean mole fractions (ppt) recorded at Ragged Point, Gosan, Cape Matatula, Trinidad Head, and Shangdianzi. Observations at Shangdianzi were discontinued in August 2012. (b) Combined ADS and Medusa GC-MS baseline monthly mean mole fraction recorded at Ny-lesund, Jungfraujoch, and Monte Cimone.

Figure 5. (a) Average seasonal cycle at Mace Head, Ireland (20042014). Black line represents the average for each month of these individual years and the error bars represent the min to max range. (b) Average seasonal cycle at Cape Grim, Tasmania (20042014). Black line represents the average for each month of these individual years and the error bars represent the min to max range.

and Europe contribute emissions of 7 and 6 Gg yr1, re

spectively to the global total. The 20072009 North American emission estimate of 28 Gg yr1 agrees within the uncertainties of HFC-152a emission estimates reported in Barletta et al. (2011) and Simmonds et al. (2015). The North American estimate indicates one reason why the UNFCCC reported amount appears to be so low; more than half the global emissions appear to come from this continental region, yet the UNFCCC reports do not include specic HFC-152a emissions from the US.

5.2.1 InTEM northwestern Europe (NWEU) estimated emissions from Mace Head observations

The HFC-152a perturbations above baseline, observed at

Mace Head, are driven by emissions on regional scales that have yet to be fully mixed on the hemisphere scale. The Mace Head observations are coupled with NAME model air history maps using the inversion system InTEM to estimate surface emissions across NWEU (Manning et al., 2011). NWEU is dened as United Kingdom, Ireland, Germany, France, Benelux, and Denmark.

As shown in Fig. 8, the NWEU emission estimates for

HFC-152a from InTEM (rolling 3-yr averages) agree to within inversion uncertainties with the UNFCCC data (2013 submission) for most years. The estimates of NWEU emissions grew steadily from 1995 reaching a maximum emission of 1.6 0.21 Gg yr1 in 2003 with a subsequent decline to

0.98 0.34 Gg yr1 in 2013.

5.2.2 European estimated emissions from European observations at Mace Head Jungfraujoch and Mt.Cimone

The temporal evolution of emission estimates for different

European regions are given in Fig. 9. In contrast to the In-TEM estimates the Bayesian inversion derived emissions in NWEU were slightly smaller than the UNFCCC estimate and showed a continued decrease until 2014. Total emissions in the inversion domain ranged from 4 0.5 Gg yr1 (2 con

dence range) for 2006 to only 2.5 0.2 Gg yr1 in 2014.

This is considerably smaller than the European Annex I estimate given in Sect. 5.2, but covers a signicantly smaller geographical region. The estimate given in Sect. 5.2 encompassed all countries in Europe extending beyond the bounds

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P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a 375

Table 3. Annex 1 and non-Annex 1 global and regional emissions in Gg yr1 averaged over two 3-year periods. Values in the nal column are from the 12-box model, all other values are from the combined Eulerian and Lagrangian model of Lunt et al. (2015). The global estimates from the 12-box model are not in exact agreement with the combined Annex I and non-Annex I emissions reported in Lunt et al. 2015. However, this is not unexpected, given the vastly different transport and inversion models used to estimate these terms. We note that the uncertainty range of the combined Annex I and non-Annex I estimates does overlap with the uncertainty range from the 12-box model, and a similar growth in emissions is seen across the two averaging periods.

3-year Averages Europe North America East Asia East Asia GLOBAL GLOBAL GLOBAL Annex 1 Annex 1 Annex 1 Non-Annex 1 Annex 1 Non-Annex 1 12-box model

20072009 6.4(5.27.5)

28.0

(22.533.4)

0.4

(0.21.2)

5.8

(4.57.5)

35.2

(27.742.6)

6.6

(4.39.2)

48.5

(37.060.6)

20102012 5.2

(4.16.4)

31.6

(24.538.6)

1.0

(0.51.6)

6.0

(4.38.2)

40.2

(31.349.3)

6.6

(3.99.8)

53.9

(43.067.3)

30 N - 90 N 00 N - 30 N30 S - 00 S90 S - 30 S

10

HFC-152a (ppt)

8

6

4

2

0

1.2

Growth rate (ppt/yr)

0.8

0.4

0.0

1995 2000 2005 2010 20150.4

Figure 7. HFC-152a emissions estimates derived from observations (blue line and shading, 1 uncertainty) and inventories. The purple line shows the global emissions estimates from EDGAR (v4.2), the red line shows the emissions reported to the UNFCCC and the green line shows emissions calculated from all data reported to UNFCCC, including allowance for the HFC-152a component of unspecied emissions.

152a emissions were reported by Brunner et al. (2012) using observations from Jungfraujoch and Mace Head (but not Mt.Cimone) in an extended Kalman Filter inversion.

5.2.3 US estimated emissions

Estimates of North American emissions have been reported by several groups (see also estimates from this study in Table 3). Millet et al. (2009) report average US emissions for 20042006 of 7.6 Gg (4.810 Gg) compared with the UNFCCC average 20052006 estimate of 12.3 Gg calculated from UNFCCC data. Miller et al. (2012) provided HFC-152a emissions estimates averaged from 20042009 of 25 Gg (1150 Gg). Barletta et al. (2011) reported a 2008 HFC-152a emission estimate of 32 4 Gg. In a recent investigation of

the surface-to-surface transport of HFC-152a from North America to Mace Head, Ireland, an interspecies correlation method with HFC-125 as the reference gas was also used to

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Figure 6. Top panel: AGAGE 12-box model mole fractions (solid line) for the two NH (3090 N, MHD, and THD and 0030 N,

RGP) and the two SH (3000 S, SMO and 9030 S, CGO) latitudinal bands The points show the semi-hemispheric monthly mean observations from the ve AGAGE stations used in the inversion (MHD, THD, RPB, SMO, CGO). Lower panel: HFC-152a semi-hemisphere annualized growth rates are shown as dashed lines (see Rigby et al., 2014 for smoothing method), with the solid blue line and shading showing the global mean and its uncertainty.

of the area indicated in Fig. 1 (red box). The steady decline in emissions was interrupted by a local maximum in the years 20102012, when emissions reached 3.6 0.5 (Gg yr1). A

minimum in the posterior emissions can be seen in 2009 and was most pronounced for the Iberian Peninsula, Italy, France and Germany, which might indicate the inuence of the European recession in 20082009. For NWEU the emission estimate remains slightly below the UNFCCC estimates and those estimated by InTEM, but support the declining trend in European emissions. Despite the fact that Italy does not report HFC-152a emissions to the UNFCCC, the largest by country emissions were estimated for Italy (up to 1 Gg yr1 in 2007). However, a strong decline in these emissions after 2011 was established here. Similar values for Italian HFC-

376 P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a

Figure 8. Emission (Gg yr1) estimates for HFC-152a from northwestern Europe. The blue uncertainty bars represent the 5th and 95th percentiles of the InTEM estimates (rolling 3-yr averages). The orange bars and associated uncertainty are the UNFCCC inventory estimates for the NWEU region. (25 % uncertainty is estimated by the UK in their National Inventory Report (NIR) submission to the UNFCCC, the same uncertainty was assumed for northwestern Europe given the lack of additional information).

estimate North American emissions primarily from the eastern seaboard region. The average 2008 HFC-152a emission estimate was 31.3 5.9 Gg (Simmonds et al., 2015); in very

close agreement with the estimate from Barletta et al. (2011). HFC-152a emission estimates for 2005 (10.1 Gg) and 2006(12.5 Gg) reported by Stohl et al. (2009) are close to the (recalculated) UNFCCC estimates in those years.

If the sources of emissions from the US were solely technical aerosols and construction foam, emissions would be expected to be far lower. These were the historic uses in Europe and Japan and resulted in emissions 10 times less than those estimated for the US. However, in the US, do-it-yourself (DIY) relling of car air conditioners is not only permitted but thriving (Zhan et al., 2014), with an estimated 24 million DIY relling operations attempted each year. The practice is banned in Europe (OJ, 2014).

Furthermore, there is ample evidence online that HFC-152a is extensively used in DIY relling on account of its lower cost. It is a technically suitable replacement for HFC-134a, although there are safety concerns of importance to vehicle manufacturers (Hill, 2003). If the quantities estimated by Zhan et al. (2014) were met using HFC-152a diverted from the retail trade in technical aerosols, some 10 to 20 Gg yr1 of HFC-152a could be released into the atmosphere from this source alone.

5.2.4 East Asian emissions

Emissions of HFC-152a from China were estimated to be4.3 2.3 Gg yr1 in 20042005 (Yokouchi et al., 2006),

3.4 0.5 Gg yr1 in 2008 (Stohl et al., 2010) and 5.7 (4.3

7.6) Gg yr1 in 2008 (Kim et al., 2010). Li et al. (2011) using an interspecies correlation method also reported emission estimates for East Asia (China, South Korea, and Taiwan, with HCFC-22 as the reference tracer) and Japan (reference tracer

HFC-134a) for the period between November 2007 and December 2008. For China, emissions were estimated to be 5.4 (47.4) Gg yr1. In contrast, the Taiwan region, Korea, and

Japan had lower estimated emissions totalling 1.39 Gg yr1.

These estimates are within the uncertainties of our East Asia emissions reported in Sect. 5.2 and Table 3.

Yao et al. (2012), using the interspecies correlation method with carbon monoxide as the reference tracer, reported more recent Chinese emissions of 2 1.8 Gg yr1 in 20102011.

This would imply some reduction in Chinese emissions compared with earlier years.

5.2.5 Australian HFC-152a emissions from Cape Grim data

SE Australian emissions of HFC-152a are estimated using the positive enhancements above baseline or background concentrations observed at Cape Grim using interspecies correlation with CO as the reference species (ISC: Dunse et al., 2005; Greally et al., 2007) and inverse modeling (In-TEM: Manning et al., 2003, 2011). Figure 2a (CGO) shows an overall increase in the magnitude of HFC-152a pollution episodes, presumably due to increasing regional emissions.Detailed analysis of these pollution episodes using air mass back trajectories shows clearly that the HFC-152a pollution seen at Cape Grim originates largely from Melbourne and the surrounding Port Phillip region.

Australian HFC-152a emissions of 510 Mg yr1 via interspecies correlation (ISC) have been reported for the period 19982004, although it was noted that these emission estimates were near the detection limit of the ISC method (Gre-ally et al., 2007). Recently, signicant improvements have been made to this ISC method, including a revised (upward)CO emissions inventory for the Melbourne/Port Phillip region, exclusion of high CO events in the Cape Grim in situ CO record, resulting from CO emissions from biomass burning and coal combustion in the Latrobe Valley (east of Port Phillip) and a revised (upward) population-based scaling factor (5.4), used to convert Melbourne/Port Phillip emissions to Australian emissions (Fraser et al., 2014a, b). Each of these changes to the ISC method resulted in higher trace gas emission estimates. The revised (compared to Greally et al., 2007) Australian HFC-152a emission estimates from the ISC method are shown in the 2nd column of Table 4 and in Fig. 10 as 3-year running averages.

The InTEM model (Manning et al., 2003, 2011) has been used to derive HFC-152a emissions from Victoria/Tasmania (Fraser et al., 2014a). Annual Australian emissions are calculated from Victoria/Tasmania emissions using a population based scale factor of 3.7 and are shown in Fig. 10 and the 3rd column of Table 4, interpolated from rolling 3-year emission estimates. Over the period 20022011, the average Australian HFC-152a emissions from ISC and InTEM agree to within 2 %. The method for estimating the InTEM uncertainties are discussed above. No additional uncertainty was

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P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a 377

Figure 9. HFC-152a emission estimates for different European regions using the Bayesian regional inversion (orange bars) and prior estimates as reported to UNFCCC (green bars). Error bars indicate 2 condence levels. Total prior uncertainties were set to 20 % of the total domain emissions, which may result in different levels of relative uncertainty for each country/region. Note that prior estimates for Italy were taken from EDGAR instead. Prior values for 2012 were repeated for each region after 2012.

Table 4. Australian HFC-152a emissions (Mg yr1, 3-year running averages) calculated from Cape Grim in situ observations via ISC (ADS and Medusa data, uncertainty: 1 SD) and inverse modeling using InTEM (Medusa data, range: 25th75th percentiles); ISC, NAME averages

weighted by uncertainties, ISC InTEM average for 2004 is based only on InTEM data.

YEAR ISC InTEM ISC and InTEM ISC / InTEM average ratio

1999 24 7

2000 25 8

2001 27 9

2002 28 10 32 (3134) 31 2

2003 28 10 32 (2933) 31 4 0.88

2004 29 10 31 (2933) 31 2 0.94

2005 32 10 31(3033) 31 4 1.03

2006 38 10 35 (3238) 35 6 1.09

2007 51 15 41 (3743) 42 6 1.24

2008 49 15 43 (4147) 44 5 1.14

2009 52 15 68 (6472) 65 8 0.76

2010 59 20 69 (6474) 67 10 0.86

2011 56 15 72 (6876) 69 7 0.78

2012 77 25

2013 69 24

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378 P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a

Figure 10. Australian HFC-152a emissions (Mg yr1) calculated from Cape Grim in situ observations via ISC, using ADS and

Medusa data, and inverse modeling using InTEM (Medusa data). Australian emissions are derived from SE Australian emissions, scaled by population (see text). Uncertainties are 25th75th percentiles (InTEM) and 1 SD (ISC).

applied to the estimates through the process of up-scaling from Victoria/Tasmania to Australian totals. The assumption was made that the use of HFC-152a per head of population was identical across Australia as we have no more detailed information.

Australian HFC-152a emissions have increased steadily from 25 Mg yr1 in the late-1990s to over 60 Mg yr1 in the late-2000s. The 2012 and 2013 emissions have been estimated from Cape Grim data by ISC at 77 and 69 Mg, respectively. Australian HFC-152a emissions (19982004) are 2530 Mg, signicantly higher than estimated (510 Mg yr1) in

Greally et al. (2007), resulting from improvements in the ISC method (see above).

Compared to the global values derived above, Australian emissions are 0.1 % of global emissions based on ISC/InTEM data. It is unusual for Australian emissions of an industrial chemical to be as low as 0.1 % of global emissions. For other HFCs, CFCs and HCFCs (for example HFC-134a, CFC-12, HCFC-22), Australian emissions as fraction of global emissions are typically 12 %, similar to Australias fraction of global gross domestic product (GDP, 1.9 %, 2014) but signicantly larger than Australias fraction of global population (0.33 %, 2014) (Fraser et al., 2014b).

The possible reasons for the low Australian HFC-152a emissions (relatively low use in Australia compared to rest of world) are being investigated. One suggestion (M. Bennett, Refrigerant Reclaim Australia, personal communication, 2013) is that a signicant major-volume use in other parts of the world for HFC-152a is as an aerosol propellant, a use not taken up to any signicant degree in Australia.

6 Conclusions

Atmospheric abundances and temporal trends of HFC-152a have been estimated from data collected at the network of 11

globally distributed monitoring sites. The longest continuous in situ record at Mace Head, Ireland covers a 20-year period from 19942014. Other stations within the network have observational records from 9 to 16 years, with only a short record (20102012) at Shangdianzi, China. From selected baseline in situ measurements and measurements of archived air samples dating back to 1978 the long-term growth rate of HFC-152a has been deduced. Analyzing the enhancements above baseline coupled with atmospheric transport models permitted us to estimate both regional and global HFC-152a emissions. However, it should be noted that the various models use different domains to obtain regional emissions estimates.

The annual average NH (Mace Head + Trinidad Head)

baseline mole fraction in 1994 was 1.2 ppt reaching an annual average mole fraction of 10.1 ppt in 2014. In the SH (Cape Grim) the annual average mole fraction increased from0.84 ppt in 1998 to 4.5 ppt in 2014. Using the global average mole fraction obtained from the AGAGE 12-box model we estimate that the HFC-152a contribution to radiative forcing was 0.61 0.02 mW m2 in 2014. Since the rst in situ

measurements in 1994 the global annual growth rate of HFC-152a has increased to a maximum annual growth rate in 2006 of 0.84 0.05 ppt yr1. More recently the average annual

growth rate has slowed to 0.38 0.04 ppt yr1 in 2010, and

become negative, with a growth rate in 20132014 of minus0.06 0.05 ppt yr1.

Global HFC-152a emissions increased from7.3 5.6 Gg yr1 in 1994 to 52.5 20.15 Gg yr1 in

2014. Global emissions are dominated by emissions from North America with this region being responsible for 67 %

of global emissions in our estimates. Estimates of northwest European emissions of 0.9 Gg yr1, (20102012 average)

agree within the uncertainties for the two regional models (see Sect. 3.3 and 3.4) and overlap with the UNFCCC inventory. For the combined Eulerian and Lagrangian models (see Sect. 3.2 and Table 3) that encompass all European countries, we derive a 20102012 average emission of5.2 Gg yr1. East Asian countries contribute 1 Gg yr1 (Annex 1) and 6 Gg yr1 (Non-Annex 1) to the global total (20102012 averages). All of the models studies indicate a current declining trend in European and Asian emissions.

Substantial differences in emission estimates of HFC-152a were found between this study and those reported to the UNFCCC which we suggest arises from underestimated North American emissions and undeclared Asian emissions; reecting the incomplete global reporting of GHG emissions to the UNFCCC and/or biases in the accounting methodology. Ongoing, continuous, and accurate globally and regionally distributed atmospheric measurements of GHGs, such as HFC-152a, are required for top-down quantication of global and regional emissions of these gases, thereby enabling improvements in national emissions inventories, or bottom-up emissions data collected and reported to the UNFCCC (Weiss and Prinn, 2011).

Atmos. Chem. Phys., 16, 365382, 2016 www.atmos-chem-phys.net/16/365/2016/

P. G. Simmonds et al.: Global and regional emissions estimates of HFC-152a 379

Data availability

The entire ALE/GAGE/AGAGE data base comprising every calibrated measurement including pollution events is archived on the Carbon Dioxide Information and Analysis Center (CDIAC) at the US Department of Energy, Oak Ridge National Laboratory.

Acknowledgements. We specically acknowledge the cooperation and efforts of the station operators (G. Spain, MHD; R. Dickau, THD; P. Sealy, RPB; NOAA ofcer-in-charge, SMO) at the AGAGE stations and all other station managers and support staff at the different monitoring sites used in this study. We particularly thank NOAA and NILU for supplying some of the archived air samples shown, allowing us to ll important gaps. The operation of the AGAGE stations was supported by the National Aeronautic and Space Administration (NASA, USA) (grants NAG5-12669, NNX07AE89G and NNX11AF17G to MIT; grants NAG5-4023, NNX07AE87G, NNX07AF09G, NNX11AF15G and NNX11AF16G to SIO); the Department of the Energy and Climate Change (DECC, UK) (contract GA0201 to the University of Bristol); the National Oceanic and Atmospheric Administration (NOAA, USA) (contract RA133R09CN0062 in addition to the operations of American Samoa station); and the Commonwealth Scientic and Industrial Research Organisation (CSIRO, Australia), Bureau of Meteorology (Australia). Financial support for the Jungfraujoch measurements is acknowledged from the Swiss national programme HALCLIM (Swiss Federal Ofce for the Environment (FOEN)). Support for the Jungfraujoch station was provided by International Foundation High Altitude Research Stations Jungfraujoch and Gornergrat (HFSJG). The measurements at Gosan, South Korea were supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014R1A1A3051944). Financial support for the Zeppelin measurements is acknowledged from the Norwegian Environment Agency. Financial support for the Shangdianzi measurements is acknowledged from the National Nature Science Foundation of China (41030107, 41205094). The CSIRO and the Australian Government Bureau of Meteorology are thanked for their ongoing long-term support of the Cape Grim station and the Cape Grim science program. M. Rigby is supported by a NERC Advanced Fellowship NE/I021365/1.

Edited by: E. Harris

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