Significant New Alternatives Policy Program 
Refrigeration and Air Conditioning Sector
Risk Screen on Substitutes for CFC-11, CFC-12, HCFC-123, HCFC-22, and R-500 in Centrifugal, Reciprocating, Screw, and Scroll Chillers
                           Substitute: HFO-1234ze(E)
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This risk screen does not contain Clean Air Act (CAA) Confidential Business Information (CBI) and, therefore, may be disclosed to the public.
1. 	INTRODUCTION
Ozone-depleting substances (ODS) are being phased out of production in response to a series of diplomatic and legislative efforts that have taken place over the past two decades, including the Montreal Protocol and the Clean Air Act Amendments of 1990 (CAAA).  The U.S. Environmental Protection Agency (EPA), as authorized by Section 612 of the CAAA, administers the Significant New Alternatives Policy (SNAP) Program, which identifies acceptable and unacceptable substitutes for ODS in specific end-uses based on assessment of their health and environmental impacts.  
EPA's decision on the acceptability of a substitute is based on the findings of a screening assessment of potential human health and environmental risks posed by the substitute in specific applications.  EPA has already screened a large number of substitutes in many end-uses within all of the major ODS-using sectors, including refrigeration and air conditioning, solvent cleaning, foam blowing, aerosols, fire suppression, adhesives, coatings and inks, and sterilization. The results of these risk screens are presented in a series of Background Documents that are available in EPA's docket.
The purpose of this risk screen is to supplement EPA's Background Document on the refrigeration and air conditioning sector (EPA 1994) (hereinafter referred to as the Background Document). This risk screen evaluates the potential use of HBA-2 as a substitute for CFC-12 in new centrifugal and positive displacement (e.g., screw, scroll, and reciprocating) chillers. Table 1 presents information on the proposed substitute. 

                Table 1. Chemical Information of HFO-1234ze(E)
                              Proposed Substitute
                                 Chemical Name
                               Chemical Formula
                                  CAS Number
                                 HFO-1234ze(E)
                      trans-1,3,3,3-tetrafluoroprop-1-ene
                                    C3F4H2
                                   1645-83-6

The potential risks associated with use of substitutes in chillers have been examined at length in the Background Document.  The reader is referred to this reference for a detailed discussion of the methodologies used to conduct this risk screen.  This risk screen addresses flammability risk, and an occupational exposure assessment was performed to ensure that manufacture, installation, servicing, and disposal of the proposed substitute in the chiller end-use does not pose unacceptable risks to workers.  Lastly, general population exposure modeling was performed to ensure that the proposed substitute would not pose an unacceptable risk to the population at large.  
Section 2 summarizes the results of the risk screen for the proposed substitute listed in Table 1.  The remainder of the report is organized into the following sections:

         * Section 3: Atmospheric Assessment
         * Section 4: Flammability Assessment
         * Section 5: Potential Health Effects
         * Section 6: Occupational Exposure Assessment
         * Section 7: General Population Exposure Assessment
         * Section 8: Volatile Organic Compound Assessment
         * Section 9: References
            
2.	 SUMMARY OF RESULTS				
HFO-1234ze(E) is recommended for SNAP approval for the centrifugal chiller and reciprocating, screw and scroll chiller end-uses. EPA's risk screen indicates that the use of the proposed substitute will be less harmful to the atmosphere than the continued use of CFC-11, CFC-12, HCFC-123, HCFC-22, and R-500.  Given that appropriate safety and personal protective equipment (PPE) (OSHA Category C or higher) and engineering controls (e.g., exhaust ventilation, vapor-in-air detection systems) will be used during installation, maintenance, and disposal activities, no significant toxicity risks to workers or the general population are expected. In addition, based on the proper precautions listed in the MSDS for HFO-1234ze(E), the proposed substitute is not expected to present a significant flammability concern during manufacture or end-use (see Section 4).
3. 	ATMOSPHERIC ASSESSMENT
This section presents an assessment of the potential risks to atmospheric integrity posed by the use of HFO-1234ze(E) in the refrigeration and air-conditioning sector.  The ozone depletion potential (ODP), global warming potential (GWP), and atmospheric lifetime (ALT) of the proposed substitute are presented in Table 2. As compared to CFC-11, CFC-12, HCFC-123, HCFC-22, and R-500, HFO-1234ze(E) is substantially less harmful to the ozone layer, has less climate impact, and a shorter atmospheric lifetime. In addition, HFO-1234ze(E) also has lower climate impact and a shorter atmospheric lifetime than those predicted for other substitutes examined in the Background Document, as well as a more recent substitute, HFC-236fa. Thus, EPA believes that use of HFO-1234ze(E) would result in substantially less harm to the climate and ozone layer than the continued use of ODS. 

Table 2. Atmospheric Impacts of HFO-1234ze(E) Compared to Other Chiller Refrigerants
                                  Refrigerant
                                      ODP
                                      GWP
                                     ALT 
                                 HFO-1234ze(E)
                                     0[a]
                                     6[a]
                                 0.04 years[b]
                                    CFC-11
                                    1.0[c]
                                   4,750[d]
                                  45 years[d]
                                    CFC-12
                                    1.0[c]
                                   10,900[d]
                                 100 years[d]
                                   HCFC-123
                                    0.01[c]
                                     77[d]
                                 1.3 years[d]
                                    HCFC-22
                                    0.04[c]
                                   1,810[d]
                                     12[d]
                                   R-500[e]
                                    0.7[c]
                                   8,077[d]
                                      NA
                                   HFC-236fa
                                       0
                                   9,810[d]
                                    240[d]
a HFO-1234ze(E) SNAP Submission (Honeywell 2008).
b Submitter indicates that substitute's ALT is approximately 14 days (Honeywell 2008).
[c] WMO (2011)
d IPCC 4th Assessment Report (Forster et al. 2007)
[e] R-500 is a blend consisting of CFC-12 (73.8%) and HFC-152a (26.2%).
NA= not available. Atmospheric lifetimes are not given for blends because the components separate in the atmosphere. The ALT for CFC-12 is 100 years and HFC-152a is 1.4 years (IPCC 4th Assessment Report [Forster et al. 2007]).
4.	FLAMMABILITY ASSESSMENT
HFO-1234ze(E) is not flammable at ambient temperatures of 21°C (70°F), but is flammable at higher temperatures and can ignite when mixed with air under pressure and exposed to strong ignition sources. At 60°C (140°F), HFO-1234ze(E) is flammable when its concentration in air is in the range of 5.7% and 11.3% by volume (57,000 ppm to 113,000 ppm). However, HFO-1234ze(E) is difficult to ignite, requiring a minimum energy of ignition of 61,000 mJ at 54°C (129°F) and standard pressure. 

By keeping HFO-1234ze(E) away from all sources of ignition and high heat and adhering to the other  precautions listed in the proposed substitute's MSDS, HFO-1234ze(E) does not pose a significant flammability risk. 
5.	POTENTIAL HEALTH EFFECTS
To assess potential health risks from exposure to this proposed substitute, the AIHA (American Industrial Hygiene Association) WEEL (Workplace Environmental Exposure Level) Committee developed an acceptable exposure limit (AEL) of 800 ppm for HFO-1234ze(E). The AEL represents the maximum 8-hour time weighted average (TWA) at which a worker can be exposed regularly without adverse effects.  EPA also developed a short-term exposure limit (STEL) of 2,400 ppm over a 15-minute period, based on the Association's 800 ppm AEL value.

According to the MSDS, exposure to HFO-1234ze(E) may be harmful by ocular or dermal absorption or inhalation; ingestion is not expected as the proposed substance is a gas. Exposures of HFO-1234ze(E) to the eyes may cause eye irritation and may cause frostbite. In case of ocular exposure, the MSDS for HFO-1234ze(E) recommends that person(s) should immediately flush the eyes, including under the eyelids, with water for 15 minutes; should frostbite occur, the water should be lukewarm, not hot. Medical attention should be sought if irritation develops or persists. Exposures of HFO-1234ze(E) to the skin are not expected to cause irritation, but may cause frostbite in the event of rapid evaporation of the liquid. In the case of frostbite, the MSDS for HFO-1234ze(E) recommends that person(s) should bathe (not rub) the affected area with lukewarm, not hot, water. If water is not available, cover the affected area with a clean, soft cloth. Medical attention should be sought if irritation develops or persists. HFO-1234ze(E) vapors are heavier than air and can cause suffocation by reducing oxygen available for breathing, causing asphyxiation at high concentrations.  If person(s) are exposed to high concentrations, the person(s) will likely not realize that he/she is suffocating, but may experience central nervous system effects, such as drowsiness and dizziness. If HFO-1234ze(E) is inhaled, person(s) should be immediately removed and exposed to fresh air. The MSDS recommends that if breathing is difficult, person(s) should be given oxygen, provided a qualified operator is present, and medical attention be sought.  EPA's review of the human health impacts of this proposed substitute is contained in the public docket for this decision. 

The potential health effects of HFO-1234ze(E) are unlikely to occur when following the exposure guidelines and the ventilation and PPE recommendations outlined in the MSDS for HFO-1234ze(E) and this risk screen. 
6.  	OCCUPATIONAL EXPOSURE ASSESSMENT
To ensure that use of the proposed substitute in chillers does not pose an unacceptable risk to workers during installation, servicing, and disposal, occupational exposure modeling at installation was performed for the proposed substitute.  Although modeling is only performed for installation, it is assumed that exposure during servicing and disposal would not exceed exposure during installation. All installation, maintenance, and disposal activities for HFO-1234ze(E) chillers are anticipated to occur on-site. Servicing of the chiller is assumed to not exceed exposures during installation and disposal. Installation and disposal are anticipated to have similar exposures, since both require the connection of pipes to transfer the refrigerant; as it is likely that less refrigerant remains in the unit at disposal, it is assumed that installation of a chiller would require transfer of a greater amount of refrigerant, and thus result in greater worker exposure. Thus, occupational exposure was conservatively modeled based on installation.

Installation of chillers is expected to occur with limited frequency (up to approximately one event per day) and with limited duration of exposure to the refrigerant charge. Potential exposures to the refrigerant are expected to occur during activities related to charging the refrigerant from the storage tanks into the chiller (e.g., connecting of pipes). Such activities and related exposure is anticipated to occur within 15-30 minutes (per event/day). To evaluate the risk of exposure at installation, the maximum modeled exposure concentration was compared to 2,400 ppm, the HFO-1234ze(E) STEL developed by EPA based on AIHA's 800 ppm AEL value.

The methodology used for this screening assessment is described in Chapter 5 of the Background Document. A box-model approach was used to evaluate potential worker exposure to alternative refrigerants. This approach has been widely used for many years to estimate probable exposures of workers to hazardous airborne materials, and has been described in detail by the National Institute for Occupational Safety and Health (NIOSH).  This model takes into consideration the duration and magnitude of the resulting exposure which is influenced by 1) duration and intensity of the release, 2) rate at which contaminated air is diluted with uncontaminated air, 3) proximity of the worker to the source of the release, and 4) the length of time the worker remains in the affected space.  

The release per event was conservatively assumed to be 1 percent of the equipment charge during installation and then was multiplied by the number of installations estimated to occur over a workday.  One installation per workday was assumed based on data obtained from the Vintaging Model. The analysis models two refrigerant charge sizes: a typical maximum refrigerant charge size, and an atypical maximum charge size.  

Based on the assumptions described above, the modeling indicates that short-term (15-minute) worker exposure concentrations could reach 3,080 ppm during installation of the atypical maximum charge size chiller, and 1,160 ppm during installation of the typical maximum charge size chiller. Although the modeled exposure concentration for installation of the larger chiller is greater than the STEL (2,400 ppm),that charge size is representative of a very large and uncommon size for chillers, for which additional precautions would be taken. As indicated by the submitter, industry field experience has shown that vapor-in-air concentrations for HCFC-123 to be less than 50 ppm in all installations, and typically less than 0.6 ppm during routine maintenance and refrigerant transfers.  Further, the modeled exposure concentration for installation of a chiller containing the typical maximum charge size did not exceed the STEL.

Chillers are typically installed, maintained, and disposed by trained personnel using proper industrial hygiene techniques. When installing, servicing, or disposing of large chillers, these techniques should be strictly followed.  Adherence to the proposed substitute's MSDS and use of proper engineering controls and PPE make it unlikely that exposure to HFO-1234ze(E) would occur. Adequate ventilation should always be established during any use, handling, or storage of HFO-1234ze(E). Engineering controls should include vapor-in-air detection systems and local exhaust ventilation during use of HFO-1234ze(E); filling operations are to occur only at stations with exhaust ventilation facilities. In addition, an eye wash and safety shower should be near the manufacturing facility and ready for use. In general, use of OSHA Category C or higher PPE is recommended, such as respiratory protection (a NIOSH-approved positive-pressure supplied-air respirator is recommended where there is insufficient ventilation), chemical splash goggles or face shield, impervious gloves, and chemical-resistant boots and clothing (OSHA 1994). Because use of exposure controls and adherence to the appropriate occupational safety guidelines would be followed under good manufacturing practices, as mentioned above, EPA believes that HFO-1234ze(E) does not pose a significant risk to workers during installation, maintenance, and disposal activities.

In addition, there is a risk of generation of toxic decomposition products such as hydrogen fluoride and fluorocarbons, if HFO-1234ze(E) is exposed to fire. Containers of HFO-1234ze(E) should not be exposed to sunlight or temperatures exceeding 50°C. EPA believes that when the proper handling and disposal guidelines are followed in accordance with good industrial hygiene and manufacturing practices and the MSDS for HFO-1234ze(E), there is no significant risk to workers during the installation, servicing, or disposal of HFO-1234ze(E).

7. 	GENERAL POPULATION EXPOSURE ASSESSMENT
HFO-1234ze(E) is not expected to cause a significant impact on human health in the general population when used as a refrigerant in chillers. The compound is proposed for use in closed systems that are typically operated near atmospheric pressure. General population exposures to ambient air and surface water releases are not expected from this end use. 
8. 	VOLATILE ORGANIC COMPOUND (VOC) ASSESSMENT
HFO-1234ze(E) is exempt from the definition of a volatile organic compound (VOC) under CAA regulations (40 CFR §51.100(s)).  Therefore, environmental impacts from the release of HFO-1234ze(E) as a VOC are not a concern.

9.  	REFERENCES
Honeywell, Inc. 2008. Significant New Alternatives Policy Program Submission to the United States Environmental Protection Agency for HFO-1234ze(E). July 2008. 
Honeywell, Inc. 2011. Chiller Risk Assessment for HFO-1234ze(E) Use in Chillers. August 1, 2011.
Honeywell, Inc. 2012a. Response to Incomplete SNAP Submission for HFO-1234ze(E) Use in Chillers. March 1, 2012.
Honeywell, Inc. 2012b. Response to Incomplete SNAP Submission for HFO-1234ze(E) Use in Chillers. May 8, 2012.
EPA 2011. National Emissions Inventory Air Pollutant Emissions Trends Data. Accessed 23 December 2011. Updated October 2011. Available at: <http://www.epa.gov/ttn/chief/trends/index.html#tables>
EPA 1994.  Significant New Alternatives Policy Technical Background Document:  Risk Screen on the Use of Substitutes for Class I Ozone-depleting Substances: Refrigeration and Air Conditioning.  Stratospheric Protection Division.  March, 1994.

EIA 2011. Annual Energy Outlook 2011 with Projections to 2035. Independent Statistics & Analysis Table Browser. Accessed September 2011. Available online at <http://www.eia.gov/oiaf/aeo/tablebrowser/>.
Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland. 2007.  Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007:The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
ICF International.  2009a.  Revised Final Draft Assessment of the Potential Impacts of HFO-1234yf and the Associated Production of TFA on Aquatic Communities and Local Air Quality.  Prepared for the U.S. Environmental Protection Agency.  August 4, 2009.
 ICF International.  2009b.  Atmospheric Oxidation of HFO-1234ze.  Prepared by Dr. Don Wuebbles for ICF International.  Deliverable under EPA Contract EP-W-06-008, TO 038, Task 5.  September 2009

OSHA. 1994. General description and discussion of the levels of protection and protective gear. 1910.120 App B. Last updated August 22, 1994. Available at: http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9767


                                 Appendix A: 
                  Determination of an AEL for HFO-1234ze(E) 
                         (Occupational Exposure Limit)
                                       

Recommended AEL: 		1000 ppm (8-hour Time Weighted Average)				
	
Basis and
Endpoints: 	Hematology and clinical chemistry changes, mononuclear infiltration of heart tissue

Study: 	Sub-chronic (13-week) inhalation toxicity study with HFO-1234ze(E) in rats (Muijser 2008)

Protocol:	Nose-only inhalation, 6 hours/day, 5 days/week for 13 weeks 

Concentrations:			0; 1,500; 5,000 and 15,000 ppm

NOAEC:			5,000 ppm

LOAEC:			15,000 ppm

[HEC]: 				3750 (5000 x 6/8) 

Uncertainty Factors:	3 (interspecies extrapolation)



HFO-1234ze(E) is a hydrofluorocarbon refrigerant/heat transfer solvent that is structurally similar to HFO-1234yf.  The database of toxicity studies for both compounds is very thorough and includes genotoxicity assays (bacterial reversion assay, in vitro chromosomal aberration study and an in vivo unscheduled DNA synthesis assay and micronucleus assay in mice); acute (4-hour), 28-day, and 14-week inhalation toxicity studies in rats of both sexes; an acute cardiac sensitization study in male beagle dogs; a developmental study in rabbits; ecotoxicity studies; and a toxicogenomic study. 

Due to additional data available on developmental toxicity, we are able to use a lower uncertainty factor and are now recommending a higher AEL compared to our preliminary AEL analysis (where an AEL of 375 ppm was recommended). This AEL is developed based on the findings in the 90-day study inhalation toxicity study in rats (Muijser 2008).  In this subchronic study, 10 each male and female Sprague-Dawley rats were exposed nose-only to HFO-1234ze(E) at concentrations of 1,500, 5,000, and 15,000 ppm for 6 hours/day, 5 days/week for 91-92 days.  Exposure to the test compound did not have any effect on the following measured parameters:  mortality, clinical signs, ophthalmoscopy, body weights, food consumption, food conversion efficiency, organ weights, or gross pathology.  Decreased absolute and relative uterus weight in females at 15,000 ppm was considered to be the result of estrus cycle and not related to treatment.  

Some changes in hematology and clinical chemistry parameters were noted in rats exposed to 15,000 ppm HFO-1234ze(E) that might be related to compound exposure.  These are the following:  increase in thrombocytes and monocytes in males and an increase in hemoglobin, packed cell volume, and monocytes in females; an increase in ASAT and urea in plasma of males, and an increase in glucose, urea, inorganic phosphate, and potassium in females.  Histopathology was unaffected in most organs.  As no microscopic effects were noted in the liver tissue, the clinical chemistry changes in males and females were not considered clinically relevant.  Exposure at the highest concentration resulted in predominantly slight multifocal mononuclear cell infiltrates in the hearts of rats of both sexes (in the absence of fibrosis; see Table 1 below).  Based on this result, the NOAEC was identified as 5,000 ppm.  

Table 1.  Incidences of Heart Histopathology in Male and Female Rats Exposed to HFO-1234ze(E) for 13 Weeks

                                 Males# (ppm)
                                 Females (ppm)
Heart Histopathology Multifocal mononuclear cell infiltrate (grade)
                                       0
                                     1500
                                     5000
                                    15,000
                                       0
                                      500
                                     5000
                                    15,000
                                                                    Very slight
                                       0
                                       0
                                       0
                                       1
                                       0
                                       0
                                       0
                                       1
                                                                         Slight
                                       0
                                       0
                                       0
                                       7
                                       0
                                       0
                                       0
                                       4
                                                                       Moderate
                                       0
                                       0
                                       0
                                       1
                                       0
                                       0
                                       0
                                       0
                                                                Expanded Totals
                                       0
                                       0
                                       0
                                      9**
                                       0
                                       0
                                       0
                                      5*
Data taken from Muijser 2008; pg 77.
#Animals evaluated per concentration group: 10
*, Statistically significant at p<0.05; **, p<0.001

Other toxicity studies were negative, increasing our overall confidence in the developed AEL.  For example, the results of the cardiac sensitization assay in male beagle dogs indicates that 1234ze does not cause cardiac sensitization at concentrations up to 120,000 ppm (Weinberg 2004). In a range-finding developmental study in New Zealand white rabbits (via whole-body inhalation), exposures up to 15,000 ppm during organogenesis (gestational day 6-28) did not cause any maternal or developmental toxicity (Fleeman 2008).  In addition, all in vitro and in vivo genotoxicity studies were negative, indicating that the compound is not genotoxic.  



References

Fleeman TL. 2008.  An inhalation range-finding prenatal developmental toxicity study of HFO-1234ze (1,1,1,3-tetrafluoropropene) in rabbits.  WIL Research Laboratories.  30 January 2008.
Muijser H.  2008.  Sub-chronic (13-week) inhalation toxicity study with HFO-1234ze in rats.  TNO Laboratories.  28 March 2008.
Muijser H and K Junker.  2006. Sub-acute (4-week) inhalation toxicity study, including Unscheduled DNA Syntesis and Micronucleus test, with HFP-1234ze in rats.  TNO Laboratories.  11 December 2006.
Weinberg JT. 2004.  Acute cardiac sensitization of HFO 1234ze and HFO 1234yf in dogs.  WIL Research Laboratories.  23 August 2006.
                                       

                                 Appendix B: 
                      Atmospheric Oxidation of HFO-1234ze
HFO-1234yf (2,3,3,3-tetrafluoropropene) has been proposed as a replacement for the ozone-depleting substance (ODS) CFC-12 and for HFC-134a in vehicle air conditioning systems, but analyses (e.g., Luecken et al., 2009) suggest that, while its 100-year Global Warming Potential (GWP) is quite low, atmospheric oxidation will mostly produce TFA (trifluoroacetic acid, CF3C(O)OH). TFA is of concern because it is a strong organic acid that has local air quality impacts and can accumulate in waterways and aquatic ecosystems where it is potentially hazardous to various lifeforms.
An isomer of HFO-1234yf, HFO-1234ze (1,3,3,3-tetrafluoropropene), is also proposed for use as an ODS replacement (specifically, in the foam blowing, aerosols and heat transfer end-uses). This memorandum examines the most likely tropospheric oxidation pathways for HFO-1234ze, particularly to evaluate its potential for production of TFA.
The most likely route for atmospheric oxidation of either cis- or trans- isomers of HFO-1234ze starts with OH addition to the C=C double bond. In the absence of measurements, we are uncertain whether OH addition to either carbon atom will be preferred. Such OH adducts then add O2 in a three-body reaction, yielding the peroxy radicals HOCHFCH(O2)CF3 and O2CHFCH(OH)CF3 as shown by reactions (1) and (5) below. Each peroxy radical is converted by NO reaction to an alkoxy radical (reactions (2) and (6)) that then fragments according to reactions (3) and (7). OH addition to the 1-carbon of HFO-1234ze results in trifluoroacetaldehyde (CF3CHO) plus a radical fragment which reacts with O2 to yield formyl fluoride (CHFO) in reaction (4), and OH addition to the 2-carbon produces CHFO plus a radical fragment which reacts with O2 to yield CF3CHO in reaction (8). Thus, the first result of HFO-1234ze oxidation is expected to be CHFO + CF3CHO.
CHF=CHCF3 + OH --> + O2 --> HOCHFCH(O2)CF3	(1)
HOCHFCH(O2)CF3 + NO --> HOCHFCH(O)CF3 + NO2	(2)
HOCHFCH(O)CF3 --> CF3CHO + HOCHF	(3)
HOCHF + O2 --> CHFO + HO2	(4)

CHF=CHCF3 + OH --> + O2 --> O2CHFCH(OH)CF3	(5)
O2CHFCH(OH)CF3 + NO --> OCHFCH(OH)CF3 + NO2	(6)
OCHFCH(OH)CF3 --> CHFO + HOCHCF3	(7)
HOCHCF3 + O2 --> CF3CHO + HO2	(8)

CHFO is likely to be removed from the atmosphere mostly by wet deposition with possible hydrolysis; its highest possible OH reaction rate constant consistent with the existing measurement is a slow 1.0 x 10[-14] cm[3] molecule[−][1] s[−][1] at 298 K (Wallington and Hurley, 1993; IUPAC Data Sheet oFOx28, 2005), and absorption cross sections at near-ultraviolet wavelengths of 300 to 400 nm are unlikely to support significant photolysis (Sander et al., 2006, page 4-82). The most likely process for CF3CHO is reaction with OH followed by addition of O2 as shown in reaction (9) below: while absorption cross sections are significant for wavelengths above 300 nm, the quantum yield for photodissociation is no more than 2% (Sander et al., 2006, page 4-84). CF3C(O)O2 produced in reaction (9) could react with HO2 to produce TFA (CF3C(O)OH) by reaction (10) or CF3C(O)OOH by reaction (11). While the actual rate constant and branching ratio for CF3C(O)O2 + HO2 is not yet measured, the analogous CH3C(O)O2 + HO2 reaction has a branching ratio for CH3C(O)OOH to CH3C(O)OH of 37 x exp(−660/T) (Sander et al., 2006, page 1-59) which equals 4.1 for T = 300 K, 2.6 for T = 250 K, and 1.4 for T = 200 K. CF3C(O)OOH should either react with OH to regenerate the CF3C(O)O2 radical (reaction (12)) or photolyze to an unstable radical which dissociates to CF3 + CO2 (reaction (13)). It must be noted that the CH3C(O)O2 + HO2 reaction has a rate constant expression of 4.3 x 10[−][13] exp( (1040 K)/T ) (Sander et al., 2006, page 1-12), and colder temperatures at altitude would favor TFA formation if the analogy of CF3C(O)O2 to CH3C(O)O2 for reaction with HO2 is valid. Reaction (14), the NO reaction with CF3C(O)O2, produces the same unstable radical that dissociates to CF3 + CO2 as does CF3C(O)OOH reaction with OH (reaction (12)). Its analog, CH3C(O)O2 + NO, has a rate constant expression of 8.1 x 10[−12] exp( (270 K)/T ) (Sander et al., 2006, page 1-13) which suggests that reaction (14) could be the primary channel for CF3C(O)O2 reaction where NO concentrations are high. Measurement of the rate constant expressions for reactions (10), (11), and (14) would be useful in determining whether significant TFA is produced from trifluoroacetaldehyde (CF3CHO) in the atmosphere.

CF3CHO + OH --> + O2 --> CF3C(O)O2	(9)
CF3C(O)O2 + HO2 --> CF3C(O)OH + O3	(10)
CF3C(O)O2 + HO2 --> CF3C(O)OOH + O2	(11)
CF3C(O)OOH + OH --> CF3C(O)O2 + H2O	(12)
CF3C(O)OOH + hν --> CF3 + CO2 + OH	(13)
CF3C(O)O2 + NO --> CF3 + CO2 + NO2	(14)

Similarly to reactions (1), (5), and (9), CF3 adds O2 by reaction (15), and similarly to reactions (2) and (6), NO reacts with CF3O2 in reaction (16) to produce CF3O radical. This radical fragments much more slowly than analogous alkoxy radicals (Sander et al., 2006, page 2-13) and is more likely to react with NO to produce CF2O + FNO (reaction (17)), NO2 to produce CF3ONO2 (reaction (18)), or with any hydrocarbon RH to produce CF3OH (reaction (19)). FNO could be removed by wet deposition, but is also likely to photolyze in the troposphere according to reaction (20) (Sander et al., 2006, page 4-80), and the resulting F atom will quickly produce HF by reaction with any hydrocarbon RH. The production of CF3OH, CF2O + HF, and CF3ONO2 from HFO-1234ze depends on the air composition because the rate constant for reaction (17) at 300 K is five times that of reaction (18) and the rate constant for reaction (19) with RH = C2H6 at 300 K is 10 times lower than that of reaction (18), as shown in Table 3. (The rate constant for reaction (19) with RH = CH4 is lower still.) 

CF3 + O2 + M --> CF3O2 + M	(15)
CF3O2 + NO --> CF3O + NO2	(16)
CF3O + NO --> CF2O + FNO	(17)
CF3O + NO2 + M --> CF3ONO2 + M	(18)
CF3O + RH --> CF3OH + R	(19)
FNO + hν --> F + NO	(20)

Table 3. Rate constant data for several CF3O reactions recommended by Sander et al. (2006).
                                   Reaction
                        A, cm[3] molecule[−1] s[−1]
                                    Ea/R, K
                                k at T = 300 K
                                k at T = 250 K
                                   CF3O + NO
                              3.7 x 10[−][11]
                                    −110
                              5.3 x 10[−][11]
                               5.7 x 10[−11]
                     CF3O + NO2 + M (high-pressure limit)
                        1.1 x 10[−11] (T/300)[−1]
                               1.1 x 10[−11]
                               1.3 x 10[−11]
                                  CF3O + CH4
                               2.6 x 10[−12]
                                     1420
                               2.3 x 10[−14]
                               8.9 x 10[−15]
                                  CF3O + C2H6
                               4.9 x 10[−12]
                                      400
                               1.3 x 10[−12]
                               9.9 x 10[−13]

CF2O is likely removed by wet deposition because it has little gas-phase reactivity and its photolytic absorption lies in the vacuum ultraviolet (Sander et al., 2006, page 4-82). CF3OH is removed from the atmosphere by hydrolysis and wet deposition (Lovejoy et al., 1995) because OH does not react, thermal decomposition is exceedingly slow (Huey et al., 1995) and photolysis is negligible (Sander et al., 2006, page 4-82). CF3ONO2 may be water-soluble, and could also be removed by photolysis which regenerates the CF3O radical. What CF3C(O)OH is produced will likely be removed by wet deposition; an OH reaction should be possible as well as photolysis, but we have not found measurements of either reaction in the literature.
TFA production from HFO-1234yf in the atmosphere arises because the F atom and CF3 group are on the same carbon atom, so that nearly any degradation pathway results in CF3CFO, a species that is effectively removed only by hydrolysis to TFA. For HFO-1234ze, the F atom and CF3 group are on different carbon atoms, so that CF3CHO and CHFO result from the first step of tropospheric oxidation. While CHFO is removed primarily by wet deposition, CF3CHO is subject to further oxidation. The production of CF3OH, CF2O + HF, CF3ONO2, or TFA from CF3CHO oxidation depends on the relative amounts of NO and HO2 to react with the CF3C(O)O2 radical and on reaction rate constants that have not yet been measured. Most atmospheric conditions favor the reaction with NO, which does not lead to TFA production, over any channel involving HO2 but because of a lack of laboratory data, significant uncertainty remains. In the absence of such data, it is probable that the oxidation of HFO-1234ze in the atmosphere will result in limited production of TFA.

References
Huey, L.G., D.R. Hanson, and E.R. Lovejoy, Atmospheric fate of CF3OH 1: Gas phase thermal decomposition, J. Geophys. Res. 100, 18771 - 18774, 1995.
International Union of Pure and Applied Chemistry (IUPAC) Data Sheet oFOx28, Subcommittee for Gas Kinetic Data Evaluation, Evaluated Kinetic Data, 2005. Available via http://www.iupac-kinetic.ch.cam.ac.uk.
Lovejoy, E.R., L.G. Huey, and D.R. Hanson, Atmospheric fate of CF3OH 2: Heterogeneous reaction, J. Geophys. Res. 100, 18775 - 18780, 1995.
Luecken, D., R.E. Waterland, S. Papasavva, K. Taddonio, H. Hutzell, J. Rugh, and S. Andersen, 2009: Ozone and TFA Impacts in North America from degradation of 2,3,3,3-tetrafluoropropene (HFO-1234yf), a potential greenhouse gas replacement. Environ. Sci. Technol., submitted.
Sander, S.P., et al., Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 15, Jet Propulsion Laboratory Publication 06-2, California Institute of Technology, Pasadena, California, 2006. Available via http://jpldataeval.jpl.nasa.gov.
Wallington, T.J., and M.D. Hurley, Atmospheric Chemistry of HC(O)F: Reaction with OH Radicals, Environ. Sci. Technol. 27, 1448 - 1452, 1993.

