### Title: Climate advantages of proposed short-lived c ompounds as replacements for longer-lived HCFCs and HFCs

Authors: Dong Wang, 1 Donald J. Wuebbles, 1 * Kenneth O. Patten, 1 Seth C. Olsen 1

Affiliations:

1 Department of Atmospheric Sciences, University of I llinois at Urbana-Champaign, Urbana, Illinois, USA.
*Correspondence to: wuebbles@illinois.edu

Abstract : Many of the halocarbons currently used as refrige rants, blowing agents, etc., are of environmental concern because they are sufficiently long-lived to affect climate. In this study, we not only examine the atmospheric lifetimes and G lobal Warming Potentials (GWPs) for six short-lived potential replacement hydrocarbons, but also evaluate the potential reduced effects on climate forcing from using such compounds. The atmo spheric lifetimes of these compounds are each less than one month and their 100-yr GWPs rang e from 0.9 to 4.7. The derived GWPs are substantially lower than those in prior published s tudies using simplified estimation techniques that overestimate the GWPs for these short lived co mpounds by 49% to 233%. If long-lived HFCs are completely replaced with these substitute compounds, radiative forcing of these shortlived compounds in 2050 will be 0.26 to 0.80 mW m -2 , substantially smaller than that of HFCs (0.25 to 0.40 W m -2 ).

Main Text: The halocarbons currently used as refrigerants, bl owing agents, etc., are not only greenhouse gases, but are sufficiently long-lived t o have meaningful effects on climate. For major applications ranging from refrigerants to foa m blowing agents, HFCs are currently the primary substitutes to chlorofluorocarbons (CFCs) a nd hydrochlorofluorocarbons (HCFCs), substances listed as Ozone-Depleting Substances (OD Ss) in the Montreal Protocol, mainly (IPCC/TEAP, 2005). Because they have atmospheric li fetimes of several years and absorb radiation in the window region at infrared waveleng ths, many HFCs are more than 100 times more powerful than CO 2 in terms of radiative forcing on a per-mass basis in the atmosphere (IPCC, 2007). The current radiative forcing of HFCs is small (0.012 Wm -2 in 2010), however, radiative forcing of HFCs can be as much as 0.25 to 0.40 Wm -2 in 2050, according to recently developed estimates for continued growth in the use of these compounds (Velders et al., 2009).
Recently, various unsaturated halogenated hydrocarb ons with very short atmospheric lifetimes have been proposed as substitutes for HCF Cs (now being phased out under the Montreal Protocol) and longer-lived HFCs. These sho rt-lived substances are expected to have small, essentially negligible, effects on atmospher ic ozone and on climate. The chemical compounds include trans -1-chloro-3,3,3-trifluoropropylene ( trans -CF 3 CH=CHCl, or tCFP for short), and five hydrofluoro-olefins (HFOs): 2,3,3, 3-tetrafluoropropene (CF 3 -CF=CH 2 , or HFO1234yf), trans -1,3,3,3-tetrafluoropropene ( trans -CHF=CH-CF 3 , or HFO-1234ze(E)), cis -1,3,3,3- tetrafluoropropene ( cis -CHF=CH-CF 3 , or HFO-1234ze(Z)), cis -1,2,3,3,3-pentafluoropropene ( cis -CF 3 -CF=CHF, or HFO-1225ye(Z)), and trans -1,2,3,3,3-pentafluoropropene ( trans -CF 3 - CF=CHF, or HFO-1225ye(E)). HFO-1234yf can serve as a mobile air conditioning refrigerant; HFO-1234ze(E) can work as a blowing agent and prope llant; tCFP can serve as a blowing agent; HFO-1234ze(Z) has been suggested as a refrigerant ( Brown et al., 2009); HFO-1225ye(Z) and - 1225ye(E) have been also proposed as refrigerants ( Hurley et al., 2007). The prospect of usingHFO-1234yf and HFO-1234ze(E) has been discussed in the literature (UNEP, 2011; Velders et

al., 2012).

It is well established that the C=C bonds (Carbon-C arbon double bonds) are more prone than C-C or C-H bonds to react with the hydroxyl ra dical (OH) in the atmosphere (e.g., Atkinson et al., 2005). Therefore, these six compounds, all of which contain C=C bonds, typically have shorter lifetimes than those saturated compounds wi th similar molecular formulae. Laboratory chemical kinetics studies suggest that the reaction with OH is virtually the single degradation pathway of these compounds. The atmospheric lifetim es of all of these compounds are estimated to be less than one month (Table 1; Eq. S1) tCFP (S ulbaek Anderson et al., 2008), HFO-1234yf (Nielsen et al., 2007), HFO-1234ze(E) (Sondergaard et al., 2007), HFO-1234ze(Z) (Nilsson et al., 2009), HFO-1225ye(Z) (Hurley et al., 2007), an d HFO-1225ye(E) (Hurley et al., 2007). Assuming constant mixing ratio profiles throughout the atmosphere (troposphere and stratosphere), Global Warming Potentials (GWPs) ove r 20- and 100-year horizons were derived in these papers following the simple estimation app roach of Pinnock et al. (1995) (supporting material). The 100-yr GWPs derived this way are all less than 10 (Table 2). Patten and Wuebbles (2010) suggested that tCFP, the only compound conta ining Cl atoms among these six compounds, has negligible effect on ozone in the at mosphere (Ozone Depletion Potential (ODP) less than 0.001). The other five compounds, compris ing of carbon, hydrogen, and fluorine atoms, have essentially no impact on atmospheric ozone. Wi th such low GWPs and ODPs, these six unsaturated halogenated hydrocarbons have the poten tial to constitute a new generation of compounds that are friendly both to the ozone layer and to climate.
However, previous studies (Hurley et al., 2007; Nie lsen et al., 2007; Nilsson et al., 2009; Sondergaard et al., 2007; Sulbaek Anderson et al., 2008) use inaccurate approximations in deriving the lifetimes and radiative forcing for th ese short-lived substances. In those studies, atmospheric lifetimes are estimated using essential ly a box model, either assuming a global weighted-average OH concentration, or scaling to me thyl chloroform lifetime. As discussed below, neither method is adequate for very short-li ved compounds. In addition, the radiative forcing for the compounds were calculated assuming constant mixing ratios throughout the atmosphere. While these methods have been common pr actice and proved to be reasonable for long-lived halocarbons, they are not accurate for s hort-lived compounds.
Atmospheric mixing ratios for short-lived compounds are known to vary significantly with latitude and altitude (e.g., Olsen et al., 200 0; Wuebbles et al., 2001; Wuebbles et al., 2009). Previous studies have shown that vertically depende nt profiles of gases are important in evaluating their potential radiative forcing (e.g., Jain et al., 2000). In this study, temporally and spatially varying concentrations of the six species are obtained assuming a regional-dependent emission pattern (based on current primary and like ly application needs) using the WACCM (Whole Atmosphere Community Climate Model ) three-d imensional global climate-chemistry model, in order to derive their atmospheric lifetim es. The simulated mixing ratio profiles are
used in a narrow band radiative transfer model to c alculate their radiative forcing. Global Warming Potentials (GWPs) are then derived for each compound.
WACCM is a three-dimensional global atmospheric cli mate-chemistry model developed by the National Center for Atmospheric Research (NC AR) (e.g., Calvo et al., 2011; Garcia et al., 2011; Patten et al., 2011). WACCM has been evaluate d and used in a number of scientific studies, for example, in the Stratospheric Processe s And their Role in Climate (SPARC) 2010 Report (Eyring et al., 2010). The model performs we ll in simulating the stratosphere and theupper troposphere and lower stratosphere as shown i n the Chemistry-Climate Model Validation

Activity (CCMVal) (Eyring et al., 2010; Gettelman e t al., 2010). WACCM version 3.5.48 is used in this study to derive the steady-state concentrat ion profiles and atmospheric lifetimes for the very short-lived species. The model numerically sim ulates the atmosphere from the surface up to the 4.5 ×10 -4 Pa level (~145km). The model is run in offline mod e, that is, the meteorological fields are driven by a meteorological dataset obtai ned from a previous WACCM run approximating the current atmosphere; after one yea r’s simulation is complete, and the meteorological dataset is repeated again. The model simulates 125 chemical species and uses the
JPL06 recommendations on chemical reactions in the atmosphere (Sander et al., 2006) including representation of non-methane hydrocarbon chemistry , which is important to a good representation of tropospheric oxidative capacity. Surface concentrations of long-lived gases (e.g., CO 2 , CH 4 , and N 2 O) are all fixed for the year 2004.
For each of the compounds, the rate constants used for reaction with OH are summarized in Table 1. To have a realistic, statistically-sign ificant representation of the geographical and altitude distribution of these compounds, we assume a global emission of 15.6 Tg/yr, distributed proportional to blowing agents consumption in the y ear 2010 (Rajiv Singh, private communication, 2011). The annual emissions of each compound are 3.4, 0.6, 4.8, 0.9, 0.5, 0.9, 1.6, and 2.7 Tg in the regions of the European Unio n, Japan, China, India, Turkey, the Middle East and North Africa, Latin America, and North Ame rica, respectively. The basis of doing this is that consumption is proportional to regional pop ulation and GDP (IPCC, 2000). We assume that emissions are uniform within each region, and constant throughout the year. The WACCM model is run for five years until the key species r each steady state. Concentrations profiles and lifetime evaluation are based on analysis of the fi fth year’s WACCM output.
The simulated monthly averaged three-dimensional mi xing ratio profiles are input into the UIUC Radiative Transfer Model (UIUC RTM; Jain e t al., 2000; Naik et al., 2000; Youn et al., 2009; Patten et al., 2011), in order to evalua te the radiative forcing of these proposed compounds. Shortwave (solar) and longwave (terrestr ial) radiation across atmospheric layers is calculated in the UIUC RTM. The shortwave spectrum is divided into 18 bins between 0.2 and 5.0 microns and takes into account the absorption e ffects of H 2 O, O 3 , O 2 , CO 2 , clouds and the surface, as well as scattering processes of clouds, gas-phase molecules, and the surface. Within the UIUC RTM, a narrow band terrestrial radiation ( wave number between 0 and 3000 cm -1 ) model calculates absorptivity and emissivity for O 3 , CH 4 CO 2 , and N 2 O at 10 cm -1 resolution, and for H 2 O, CFC-11 and CFC-12 at 5 cm -1 resolution. Infrared spectra for these gases are f rom the HITRAN 2004 database, described in Rothman et a l. (2005). Clouds, surface albedo, and surface emissivity are based on observation from th e International Satellite Cloud Climatology Project. Infrared spectra data for tCFP, HFO-1234yf, HFO-1234ze(E), HFO-1234ze(Z), HFO1225ye(Z) and HFO-1225ye(E) are reported respective ly in Sulbaek Anderson et al. (2008), Nielsen et al. (2007), Sondergaard et al. (2007), N ilsson et al. (2009), Hurley et al. (2007) and Hurley et al. (2007) are regridded to 10 cm -1 resolution and used in this study.
Table 1 lists the lifetimes for these compounds cal culated from the WACCM model results using Eq. S4. Among these six unsaturated h alogenated hydrocarbons, HFO-1225ye(E) has the shortest lifetime of 9.2 days, while tCFP h as the longest lifetime of 30.5 days. With lifetimes much shorter than six months, these compo unds fall well into the category of very short-lived substances (VSLS) (WMO, 2007). Most are degraded before being transported to remote regions resulting in very inhomogeneous dist ributions. Fig. 1 shows the WACCMsimulated steady-state annual mean zonal average mi xing ratios. For each compound, the largest

mixing ratios are near surface in the northern mid- latitudes, where human activity is most intensive and emissions are strongest. Mixing ratio s decrease rapidly with distance away from location of emissions because of their short lifeti me. The longer its lifetime, the further the compound can be transported to remote regions. For the assumed emissions scenario, mixing ratios of tCFP can reach more than 100 ppt in the n orthern mid- and high-latitudes below 8 km altitude, whereas 100 ppt HFO-1225ye(E) mixing rati o contour does not exceed 3 km altitude.
Table 1 also gives the lifetimes for these VSLS rep orted in the literature, calculated using the estimation approach by substituting the rate co nstant of the reaction X + OH obtained in laboratories and assuming a constant global weighte d-average OH concentration into Eq. S1. As OH concentrations in the atmosphere are highly vari able, it does not make sense to use a globally-averaged OH concentration for such short-l ived compounds; as will be shown in the coming discussion, the appropriate average would be species and emissions scenario dependent. A global weighted-average OH concentration of 1.0×1 0 6 molecules cm -3 were used for HFO1234yf, HFO-1234ze(E), HFO-1234ze(Z), and tCFP (Nie lsen et al., 2007; Nilsson et al., 2009; Sondergaard et al., 2007; Sulbaek Anderson et al., 2008). Whereas for HFO-1225ye(E) and HFO-1225ye(Z), their lifetimes were scaled with tha t of CH 3 CCl 3 , whose lifetime is 6.1 years and the rate constant for its reaction with OH is 1 .0×10 -14 cm 3 molecules -1 s -1 (Hurley et al., 2007). This scaling is equivalent to assuming a glo bal weighted-average OH concentration of 5.2×10 5 molecules cm -3 . Compared with the lifetimes derived from the 3-D atmospheric model simulations in this study, using the estimation app roach with a global weighted-average OH concentration of 1.0×10 6 molecules cm -3 leads to an underestimation of atmospheric lifetim es by 30%, 19%, 30%, and 15% for HFO-1234yf, HFO-1234ze(E ), HFO-1234ze(Z), and tCFP, respectively; using a global weighted-average OH co ncentration of 5.2×10 5 molecules cm -3 , the estimation approach results in an overestimation of atmospheric lifetimes by 29% and 9% for HFO-1225ye(Z) and HFO-1225(E), respectively (Table 1).
For the model derived OH distribution, the appropri ate average OH concentrations, using the estimation approach, to derive the lifetimes th e same as those derived from 3-D model simulations are also listed in Table 1; these value s are compound-dependent and range from 5.9 to 8.6 ×10 5 molecules cm -3 . The longer the atmospheric lifetime of a compound , the larger the corresponding global weighted-average OH concentrat ion. This is because a larger fraction of a compound with longer lifetime can be transported to the regions rich in OH, i.e. low latitudes, from mid-latitudes where majority of the compound i s emitted. The discussion suggests that a universal global weighted-average OH concentration for VSLS does not exist, hence assuming a global average OH concentration, as in the estimati on approach, is not appropriate.
The Global Warming Potential (GWP) is defined as th e ratio of time-integrated radiative forcing over a select time horizon after a pulse em ission of gas X to that after a pulse emission of CO 2 of the same mass (IPCC, 1990). The time-integrated forcing for each gas is also called absolute GWP, or AGWP. For a VSLS, both the AGWP an d radiative forcing caused by sustained emission of the strength one unit mass pe r unit time can be approximated to the product of F and its atmospheric lifetime (see supplementary ma terial for derivation), which is readily calculated in the radiative transfer model and are listed in Table 2. GWPs over 20-, 100-, and 500-yr horizons for the six unsaturated halogen ated hydrocarbons derived from the 3-D model method are listed in Table 2. Previous estima tes of the 100-yr GWPs based on simplified assumptions overestimate the full calculations by 4 9% to 233%.For HFO-1225ye(Z) and HFO-1225(E), the 233% overest imation of their 100-yr GWPs

using the estimation approach is a combined effect of their overestimated lifetimes and overestimated radiative efficiency assuming constan t concentration profiles; for the other four compounds, even though the overestimation in radiat ive efficiency is partly offset by the underestimation of their lifetime, the overall effe ct is still an overestimation of 100-yr GWPs by more than 49%.

In order to estimate how much of the GWP difference s come from the different radiative transfer models used, we carry out experimental RTM simulations in which the WACCM calculated VSLS burden in the atmosphere is homogen eously distributed in the atmosphere (i.e., constant mixing ratios in the troposphere and strat osphere). The resulting radiative forcing changes on a per ppb basis are listed in Table S1, and compared with those reported in the previous studies. The differences are within 11%, c onfirming that the differences in derived GWPs come primarily from differences in assumed mix ing ratio profiles and atmospheric lifetimes.
This study suggests that the 100-yr GWPs for these very short-lived unsaturated halogenated hydrocarbons are all less than 5. Scena rios of replacing HFCs with the low-GWP alternative are developed based on the needs for su ch compounds put together by Velders et al. (2009) with the assumptions that (i) productions an d consumptions of HFCs decrease linearly starting in 2015; (ii) productions and consumptions of HFC-32, -125, -143a, -245fa, and -365mfc decrease to zero in 2035; (iii) productions and con sumptions of HFC-134a decrease to zero in 2025; (iv) the demands for refrigerants from use of HFC-32, -125, -134a, and -143a are replaced by HFO-1234yf, -1234ze(Z), -1225ye(Z), or -1225ye(E ); (v) the demands for blowing agents from use of HFC-152a, -245fa, and -365mfc are repla ced by HFO-1234ze(E), or tCFP. With these assumptions, the radiative forcing of the low -GWP alternatives in 2050 is between 0.26 and 0.80 mW m -2

(Fig. 2). As a result, radiative forcing from futu re needs for refrigerants, etc. is significantly reduced. In contrast to the monotonic ally increasing radiative forcing of long-lived HFCs that reaches 0.25 to 0.40 W m -2 in 2050, radiative forcing of long-lived HFCs peak s at 46.3 to 53.5 mW m -2 near 2030 and decreases to 26.1 to 31.3 mW m -2 in 2050. The resulting saving in radiative forcing on climate in 2030 is b etween 0.04 and 0.07 W m -2 , comparable to the savings of the regulatory black carbon reduction me asures, or a sizable fraction of the savings of the technological black carbon reduction measures a nd CH 4 reduction measures recently suggested by Shindell et al. (2012).

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unsaturated halogenated hydrocarbons calculated by the WACCM model.Fig. 2. Range of radiative forcing of HFCs in the Velders scenario (red), and radiative forcing of

low-GWP alternatives (green) and HFCs (blue) in the low-GWP replacement scenario as
described in the main text.Table 1.Rate constants for the reactions of the six unsaturated halogenated hydrocarbons and hydroxyl radical(OH). Temperaturedependence of the reaction rate is not considered in this study based on these laboratory studies. Atmospheric lifetimes for the sixunsaturated halogenated hydrocarbons derived from the 3-D model method, and from the estimation approach and percent differenceof the estimation approach. OH concentration in order to derive the same lifetimes as do the 3-D method for each gas.compoundrate constant(cm3molecules-1s-1)reference3-D model methodthe estimationapproachNOHto derive same lifetime asthose from the 3-D modelmethod (105molecules cm-3)lifetime (days)lifetime(days)%diff.HFO-1234yf(1.05±0.17)×10-12Nielsen et al.(2007)15.711 -30% 7.0HFO-1234ze(E)(9.25±1.72)×10-13Sondergaard etal. (2007)17.314 -19% 7.2HFO-1234ze(Z)(1.20±0.14)×10-12Nilsson et al.(2009)14.210 -30% 6.8HFO-1225ye(Z)(1.22±0.14)×10-12Hurley et al.(2007)14.018 +29% 6.8HFO-1225ye(E)(2.15±0.23)×10-12Hurley et al.(2007)9.210 +9% 5.9tCFP(4.40±0.38)×10-13SulbaekAndersen et al.(2008)30.526 -15% 8.6Table 2.Steady state radiative forcing due to sustained 15.6 Tg/yr emission for each of the six unsaturated halogenated hydrocarbonsand their AGWPs. GWPs over 20-, 100-, and 500-yr horizons for the six unsaturated halogenated hydrocarbons derived from the 3-Dmodel method used in this study, and GWPs over 20-and 100-yr horizons from the estimation approach and errors of the estimationapproach.compound3-D model methodThe estimation approachRF(mW m-2)AGWP(10-13W yr m-2kg-1)20-yr100-yr500-yr20-yr20-yr % diff.100-yr100-yr % diff.HFO-1234yf 2.72 1.75 7.4 2.1 0.612 +62% 4 +90%HFO-1234ze(E)3.94 2.53 10.2 2.9 0.919 +86% 6 +107%HFO-1234ze(Z)2.27 1.46 5.9 1.7 0.510 +69% 3 +76%HFO-1225ye(Z)2.40 1.54 6.2 1.8 0.521 +239% 6 +233%HFO-1225ye(E)1.28 0.82 3.3 0.9 0.311 +233% 3 +233%tCFP 6.27 4.03 16.3 4.7 1.426 +60% 7 +49%Supplementary Materials:

Materials and Methods:

Lifetime estimation approach

τ  = 

 (S1)

where N OH is global weighted-average OH concentration.

GWP estimation approach in Pinnock et al. (1995)
In the previous studies the GWPs for these VSLS are estimated through the Halocarbon Global Warming Potential (HGWP), over time horizon t  defined as the ratio of the GWP for gas X to that for CFC-11

GWP   t   = HGWP

 t   × GWP

 t   (S2)

HGWP  t   for gas X is obtained by

HGWP  t   =  

    

   !"#	$  ⁄  !"#	$  

⁄   (S3)

where M  , M  , τ  , τ  , IF  , and IF  are the molecular weights, atmospheric lifetimes, and instantaneous forcings f or gas X and CFC-11, respectively. Note in the estimation approach the instantaneous forcings for the VSLS are calculated following Pinnock et al. (1995) assuming constant mixing rati os in the atmosphere.

Lifetime calculation from 3-D model output
The atmospheric lifetime ( τ  ) of a gas X is determined as the ratio of its burd en X in the atmosphere over the total sink of X. Lifetime can b e calculated by integrating its burden and sink throughout the whole atmosphere, as shown in Eq.S4

τ  = ∫∫ [] ,,- ,,- ./.$ ∫∫  [] ,,- [0] ,,- ,,- 1 ./.$ (S4)

Where k 30 is the rate constant of the reaction of X and OH, [X] /,$ , [OH] /,$ , and N /,$ the mixing ratios of X and OH and number of air mol ecules per unit volume, respectively, in the volume element of interest (V) at time t.
Eq.S1 is a simplified form of Eq.S4 if k 30 is not dependent on temperature, [X] is nearly constant throughout the atmosphere, and

7 89 = ∫∫ [0] ,,- ,,- 1 ./.$

∫∫ ,,- ./.$ (S5)

However, [X] is highly variable in the atmosphere f or very short-lived substances. Therefore, Eq.S1 is inadequate to calculate atmosph eric lifetimes for very short-lived.AGWP and radiative forcing caused by sustained emis sion

It is well established that atmospheric concentrati on of a gas, whose sink is dependent on its concentration to the first order, will decay ex ponentially after emission into the atmosphere (except for CO 2 , which is more complicated). Accordingly, for a ga s X whose efficiency in changing radiative forcing is proportional to its m ass in the atmosphere, the radiative forcing caused by a pulse emission of one unit mass X, deca ys exponentially following

RF #  t  = F exp	− $

  , (S6)

where F is the instantaneous radiative forcing at the time X is injected into the atmosphere, and τ  atmospheric lifetime of X. Hence AGWP, or time-int egrated radiative forcing, over time horizon t  for gas X can be derived as the following

AGWP = @ RF #  t 

$

dt = @ F exp	− $

  $

dt = F τ  B1 − exp	− $

 D . (S7)

The conventionally chosen time horizons for GWP com parisons are 20-, 100-, and 500- years (IPCC, 1990). The atmospheric lifetimes of th e VSLS considered here are all less than 1 month, i.e. t  ≪ τ  . Hence, for these VSLS

AGWP ≅ F τ  . (S8)

On the other hand, the UIUC RTM calculates the radi ative forcing change due to sustained emission of a gas. The following deductio n shows that the AGWP of X just equals the radiative forcing caused by sustained X emissions.
Considering a sustained emission such that one unit mass X is emitted in one unit time, the radiative forcing caused by such sustained emis sion at time t can be calculated

RF G t = @ RF #  u − t 

$

du = @ F exp	− $I

  $

du = F τ  B1 − exp	− $

 D . (S9)

If t is sufficiently longer than τ  , RF G ≅ F τ  . (S10)
The above derivation (Eqs. 3 and 5) shows that for a VSLS, both the AGWP and radiative forcing caused by sustained emission of t he strength one unit mass per unit time can be approximated to the product of F and its atmospheric lifetime.Table S1. Instantaneous radiative forcing (in W m -2 ppb -1 ) for each of the six unsaturated

halogenated hydrocarbons calculated following Pinno ck et al. (1995) and calculated in the UIUC RTM, both assuming constant mixing ratio profiles for t he gases of interest

.
gas IRF Pinnock IRF UIUC RTM HFO-1234yf 0.22 0.20 HFO-1234ze(E) 0.27 0.24 HFO-1234ze(Z) 0.20 0.19 HFO-1225ye(Z) 0.25 0.23 HFO-1225ye(E) 0.24 0.23
tCFP 0.214 0.20