MEMORANDUM

To: Margaret Sheppard, U.S. EPA

CC: Rachel Schwartz, Sally Hamlin, Rebecca von dem Hagen, Bella Maranion, Cindy Newberg, U.S. 

EPA

From: Kasey Knoell, Becky Ferenchiak, Jenny Tanphanich, Ed Carr, Mark Wagner, ICF International

Date: February 24, 2014

Re: Assessment of the Potential Impact of Hydrocarbon Refrigerants on Ground Level Ozone 

Concentrations (EPA Contract Number EP-W-10-031 Task Order 305, Task 01)

In response to technical direction received from EPA, ICF has prepared an assessment of the potential 

impact on ground level ozone concentrations caused by release of hydrocarbon refrigerants across 

refrigeration and air conditioning end uses. Refrigerant emissions were modeled from the U.S. EPA’s 

Vintaging Model (VM_IO file_V4.4_11.6.12) and their impacts on ground level ozone were modeled 

using the CMAQ model (version 4.7.1). Releases of isobutane and propylene were modeled in Los 

Angeles, Houston, and Atlanta under three emissions scenarios: 1) all refrigeration and air conditioning 

(ref/AC) end-uses, 2) ref/AC excluding motor vehicle air-conditioners (MVACs), and 3) ref/AC excluding 

MVACs and chillers. A final scenario was modeled, in which limitations on the properties of 

hydrocarbons (e.g., flammability) are considered to determine which end-uses were likely to transition

to hydrocarbon refrigerants. The remainder of this memorandum summarizes the results of this 

assessment. A quantitative analysis of the uncertainty levels can also be performed by undertaking 

additional CMAQ simulations if requested by the EPA TOCOR.

Please contact Mark Wagner at 202-862-1155 with any questions or comments.
2

Potential Impacts of Hydrocarbon Refrigerants 

on Ground-Level Ozone Concentrations

1 Executive Summary

Hydrocarbons are increasingly being considered for use in a number of refrigeration and air-conditioning 

(ref/AC) end-uses. Although hydrocarbons have a low global warming potential (GWP) and are thus 

desirable alternatives to commonly used hydrofluorocarbons (HFCs), there is concern that use of 

hydrocarbons could negatively impact local air quality due to their high maximum incremental reactivity 

(MIR) values. 

In order to evaluate their potential impact, three conservative hydrocarbon emission scenarios and one 

more realistic scenario were analyzed. The first three evaluate emissions of propylene and isobutane 

first from the entire refrigeration and air conditioning sector, second excluding motor vehicle air 

conditioning, and third excluding motor vehicle air conditioning and chillers. A fourth scenario, 

evaluating emissions of a mix of propylene, isobutane, and propane, was developed based on SNAP 

hydrocarbon applications and UL Standards and represents a more realistic transition to hydrocarbons. 

The hydrocarbon emissions from these scenarios were estimated based on U.S. EPA’s Vintaging Model, 

and their potential contributions to ozone concentrations were assessed using U.S. EPA’s Community 

Multiscale Air Quality (CMAQ) model.

CMAQ modeling was performed for April through the end of September, as these months presented the 

largest releases of hydrocarbon refrigerant as well as weather conditions favorable for ozone 

formation. The ozone concentrations were estimated for the Atlanta, Houston and Los Angeles regions, 

due to their distinctive geographic setting and chronic high levels of ground level ozone, and then scaled 

for national emission estimates. The results of the CMAQ modeling indicated that hydrocarbon 

refrigerants under the most conservative scenario could potentially increase ground level ozone by up to 

9% compared to the national ozone standard, but less than a 0.2% increase under the most realistic 

scenario.

The analysis performed is based on several assumptions and projections that cannot be known with 

certainty at this time. These limitations are associated with both the unknown market penetration of 

alternatives and climate conditions, among other factors. 

2 Introduction

Ozone-depleting substances (ODS) such as CFC-12, R-502, and HCFC-22 were historically used across the 

refrigeration and air-conditioning sector. Since identifying the damaging nature of CFCs and HCFCs, EPA 

moved swiftly to transition to alternatives. The phaseout of CFCs in 1996 and continued phasedown of 

HCFCs, has led the U.S. EPA Significant New Alternatives Policy (SNAP) program to review and approve a 

number of potential substitutes to these ODS. In particular, hydrocarbon refrigerants, which have zero 

ODP and low GWP, have already been recently approved for smaller refrigeration applications (e.g., 

domestic refrigeration, small retail food refrigeration, and vending machines) and have the potential to 

be used more broadly across the refrigeration and air-conditioning sector. Compared to the HFCs and 

HCFCs currently in use, which have high GWPs, hydrocarbons have the potential to significantly reduce 

climate impacts from the refrigeration and air conditioning sector.

Although the use of hydrocarbons could potentially mitigate greenhouse gas (GHG) emissions, it could 

also potentially contribute to increased levels of ground level ozone. Hydrocarbons have high MIR 
3

values, and may influence local air quality if emitted in sufficiently high quantities. Table 1 provides a 

summary of the propensity of these refrigerants to form tropospheric ozone. 1

Table 1: Propensity of Refrigerants to Form Tropospheric Ozone

Compound

North America Maximum Incremental 

Reactivity (MIR) Scale 

(g-O3/g-substance)

Europe POCP Scalea

(Relative Units)

HCFC-22 <0.1 0.1

HFC-134a <0.1 0.1

Propylene 11.57 112

Ethanea 0.28 12

Isobutane 1.34 31

Propane 0.56 18

Ethylene 9.07 100

Source: IPCC 2005

aThe POCP value is the ozone creating potential of a compound relative to ethylene (ethene), expressed as 

an index where ethylene = 100.

bEPA uses the reactivity of ethane as the threshold for determining whether a compound has negligible 

reactivity. Compounds that are less reactive than, or equally reactive to, ethane under certain assumed 

conditions may be deemed negligibly reactive and therefore suitable for exemption from the definition of 

a VOC (EPA 2012).

It should be noted that EPA is currently considering exempting certain hydrocarbons in certain end-uses 

from the ban on venting of refrigerant under §608 of the Clean Air Act. In order to evaluate the potential 

impact of the release of hydrocarbons during disposal, ICF modeled conservative and realistic 

hydrocarbon emission scenarios from the refrigeration/AC sector and assessed their impacts on groundlevel ozone, taking into consideration this revision to the end-of-life management requirement. 

The remainder of this report summarizes the methodology used and results in the following order:

 Section 3: Hydrocarbon Emission Scenarios

 Section 4: Impact on Ground-Level Ozone Concentrations

 Section 5: Analysis Limitations 

 Section 6: Summary of Findings

 Section 7: References

 

1 Reactivity scales rank the propensity of organic compounds to form ozone. The MIR scale assesses their

contributions to the photochemical ozone formation in urban plumes and is reflective of its use in North America, 

where ground-level ozone formation is considered an urban issue. In contrast, the POCP scale addresses longrange trans-boundary formation and transport of ozone and was developed for use in Europe, where ground-level 

ozone formation occurs on the regional scale in multi-day episodes (IPCC 2005). As the MIR scale is more 

applicable to conditions within the United States, the remainder of this report references the MIR scale. 
4

3 Hydrocarbon Emission Scenarios

Under this analysis, isobutane and propylene were each modeled under three conservative emissions 

scenarios for the refrigeration and air conditioning sector. A fourth, more realistic emissions scenario 

was also modeled, which considered more likely transitions to a variety of hydrocarbon refrigerants (i.e., 

isobutane, propylene, and propane) by end-use in the sector. The four emission scenarios considered 

are as follows: 

1) Scenario 1: the entire ref/AC sector: chillers, motor vehicle air conditioners (MVACs) (i.e., lightduty trucks and vehicles, trains, transit buses, school buses, and tour buses), residential and light 

commercial AC and heat pumps,

2

retail food,3cold storage warehouses, industrial process

refrigeration, refrigerated transport, and household refrigeration and freezers;

2) Scenario 2: the ref/AC sector excluding MVACs;

3) Scenario 3: the ref/AC sector excluding MVACs and chillers;

4 and

4) Scenario 4: the ref/AC sector for end-use applications in which a SNAP submission has been 

received for a hydrocarbon or in which a UL Standard addressing flammable refrigerant is in 

place. See Appendix B for more information on the end uses included in this scenario.

In order to estimate their potential impact on ground-level ozone concentration, the national scale 

hydrocarbon emissions from each scenario were estimated using EPA’s Vintaging Model (VM).

5 These 

national emissions were then proportionally scaled to each urban area by population by county. For Los 

Angeles, three counties were used representing about 4.4% of the U.S. population; for Houston, eight 

counties representing about 2.0% of the U.S. population were used; and for Atlanta, ten counties 

representing about 1.4% of the U.S. population were used. These county level emissions were then 

spatially allocated to each emission grid cell based on population density. The remainder of this section 

provides a summary of the methodology used to estimate the national inventory of hydrocarbon 

emissions, as well as a description of each scenario considered. 

3.1 Methodology for Emissions Estimates 

The year 2030 was chosen as the analysis year for all scenarios, in order to conservatively estimate the 

impact of hydrocarbons. The majority of non-hydrocarbon equipment will have been retired by the year, 

due to the lifetimes of equipment (i.e., 5 to 27 years). In addition, a 2030 nationwide emission inventory 

was available for other emission sources and was used to estimate the baseline ozone concentration 

(without hydrocarbon refrigerants). It was assumed that the hydrocarbon refrigerants would enter the 

market in 2012, and reach 100% market penetration in 2030. Output from the VM is provided on an 

annual basis; for the purposes of this analysis, the annual data was weighted on a monthly basis, and 

 

2 The residential and light commercial AC and heat pumps end-use includes the following applications: window AC 

units, residential unitary AC, small commercial unitary AC, large commercial unitary AC, packaged terminal AC and 

heat pumps (PTAC/PTHP), and water and ground source heat pumps. 

3 The retail food end-use includes the following applications: ice makers, small retail food systems, medium retail 

food systems, and large retail food systems.

4 Chillers are large-scale air conditioning systems for large commercial and institutional buildings, such as for office 

buildings, hotels, hospitals, and the like.

5 EPA’s Vintaging Model estimates the annual chemical emissions from industry sectors that have historically used 

ODS, including AC, refrigeration, foams, solvents, aerosols, and fire protection. Within these industry sectors, there 

are over 60 independently modeled end-uses. The model uses information on the market size and growth for each 

end-use, as well as a history and projections of the market transition from ODS to alternatives. The model version 

used in this analysis is VM_IO file_V4.4_11.6.12.
5

assumed that 11.9% of annual emissions occurred during each month from April to August. Emissions of 

refrigerant are reasonably anticipated to occur primarily during the warmer spring and summer months, 

when air conditioning equipment is increasingly used. Emissions due to servicing MVACs are also 

expected to be higher during the spring and summer, with up to 80% of emissions due to repairs 

occurring April through August (MACS 2009). Furthermore, refrigerant emissions from servicing motor 

vehicle air conditioning systems were estimated at 47% of annual refrigerant emissions from motor 

vehicles (EPA 2008). This assumption was also applied for all scenarios to all stationary AC end-uses 

based on the assumption that seasonal emissions would follow similar patterns. Emissions of refrigerant 

from refrigeration equipment were assumed to occur equally year-round. 

Smaller end-uses (i.e., end-uses with a charge size less than 200 kg) were assumed to have a 100% 

release of refrigerant upon disposal. Disposal release rates for larger end-uses (i.e., chillers, industrial 

process refrigeration systems, and large retail food systems) and servicing release rates for all end-uses 

were assumed to remain consistent with current VM assumptions, due to the assumed regulations and 

financial incentives for the end-user.

Each state implements individual emission and VOC regulations that inhibit venting. For example, 

Regulation 7:27-16.1a for New Jersey (which is a nonattainment area) requires the use of reasonably 

available control technology for equipment. Venting would violate this regulation. Maryland implements 

a regulation that states that a person may not cause or permit the discharge of VOC from any 

installation constructed on or after November 15, 1992 in excess of 20 pounds (9.07 kilograms) per day 

unless the discharge is reduced by 85 percent or more overall. In addition to state regulations, industry 

standards also restrict venting. Under OSHA regulations, facilities are required to demonstrate that 25 

percent of the lower flammability limit (LFL) would not be exceeded at any point during venting of 

hydrocarbon refrigerants. Chillers and other large charge size commercial refrigeration systems do not 

currently implement the proper engineering controls that would be needed to allow for safe venting. 

The use of flares, stacks, or rapid emission controls may likely be required to meet these industry 

standards. Equipment would have to be modified to incorporate these controls, or venting would not be 

allowed. Furthermore, assuming 100% is vented during servicing of equipment is unrealistic from an 

economic perspective. Even with a low cost refrigerant, such as propane or isobutane, venting a charge 

size of over 1,000 grams still represents a significant cost and provides an incentive to collect refrigerant, 

which also is in keeping with good maintenance and technician practices.

The reader is referred to Appendix A for a summary of detailed assumptions used in this analysis.

3.2 Conservative Emissions Scenarios

Hydrocarbons are currently being proposed for use in a wide range of ref/AC end-uses. To estimate the 

most conservative impact that releases of hydrocarbons could have on ground-level ozone, the first 

scenario considered under this analysis assumes release from the entire ref/AC sector. However, due to 

the characteristics associated with hydrocarbons (e.g., flammability), this may not be realistic. To assess 

more realistic, but still conservative scenarios, two additional scenarios were considered. All three

scenarios assume that the end-uses would be replaced with either propylene or isobutane. Propylene 

exhibits the highest MIR of hydrocarbons, and therefore represents the most conservative estimates. 

Propylene is also a proxy for the hydrocarbon blend R-443A, a substitute refrigerant under evaluation 

for use in residential air conditioning, because that blend contains more than 50% propylene. Isobutane 

has the highest MIR of the saturated hydrocarbons under evaluation and was used as a conservative 

proxy for isobutane, propane, and the hydrocarbon blend R-441A. R-441A contains more than 50%

propane, as well as isobutane and other saturated hydrocarbons. 
6

Table 2 presents the monthly emissions in 2030 which were assumed under Scenarios 1, 2, and 3. Under 

each scenario, 100% of the emissions were assumed to occur from either isobutane or propylene. In 

other words, the impact on local ozone concentrations were evaluated for six total CMAQ modeling 

scenarios. 

Table 2: 2030 National Refrigerant Emissions under Scenarios 1, 2, and 3

Month 

Scenario1 Emissions (MT)

(All Ref/AC)

Scenario 2 Emissions (MT)

(No MVAC)

Scenario 3 Emissions (MT)

(No MVAC or Chillers)

Isobutane or Propylene Isobutane or Propylene Isobutane or Propylene

January 8,065 5,894 5,793

February 8,065 5,894 5,793

March 8,065 5,894 5,793

April 13,404 9,356 9,140

May 13,404 9,356 9,140

June 13,404 9,356 9,140

July 13,404 9,356 9,140

August 13,404 9,356 9,140

September 8,065 5,894 5,793

October 8,065 5,894 5,793

November 8,065 5,894 5,793

December 8,065 5,894 5,793

Note: Emissions of isobutane and propylene are equivalent; shaded months indicate those used in the analysis.
7

Figure 1 presents the percent contribution of isobutane and propylene emissions by end-use under 

Scenarios 1 through 3. As illustrated in these charts, residential AC end-uses presented the most 

significant source of emissions across all three scenarios. 

Figure 1. Percent Contribution of End-Use to Total Emissions under Scenarios 1-3

Note: Emissions under the residential and light commercial AC and 

heat pumps end-use (Res and Comm AC) consists of 43% from 

residential unitary AC, 4% from window AC, 2% from small 

commercial unitary AC, 0.2% from large commercial unitary AC, 0.2% 

from PTACs, and 0.1% from water and ground source HPs (relative to 

total emissions in Scenario 1). Emissions under the MVAC end-use 

consists of 15% from light-duty vehicles, 14% from light-duty trucks, 

0.3% from school and tour buses, 0.1% from transit buses, and 0.1% 

from trains (relative to total emissions in Scenario 1).

Note: Emissions under the residential and light commercial AC and 

heat pumps end-use (Res and Comm AC) consists of 61% from 

residential unitary AC, 6% from window AC, 3% from small 

commercial unitary AC, 0.3% from large commercial unitary AC, 0.3% 

from PTACs, and 0.2% from water and ground source HPs (relative to 

total emissions in Scenario 2).

Retail Ref

4%

Household 

Ref

1%

IPR

4%

Transport 

Ref

9%

Dehumidif

ers

0%

MVAC

Chillers 30%

2%

Res and 

Comm AC

50%

Cold 

Storage

0%

Scenario 1

Retail Ref

5%

Household 

Ref

1%

IPR

6%

Transport 

Ref

13%

Dehumidif

ers

0%

Chillers

2%

Res and 

Comm AC

72%

Cold 

Storage

1%

Scenario 2
8

Note: Emissions under the residential and light commercial AC 

and heat pumps end-use (Res and Comm AC) consists of 63% 

from residential unitary AC, 6% from window AC, 3% from small 

commercial unitary AC, 0.4% from large commercial unitary AC, 

0.3% from PTACs, and 0.2% from water and ground source HPs

(relative to total emissions in Scenario 3).

 

3.3 Realistic Emissions Scenario

In addition to the three end-use scenarios, a fourth more realistic scenario (i.e., Scenario 4) was 

developed in which hydrocarbons were only modeled in refrigeration and air conditioning equipment 

for which a SNAP submission was received or for which there is a UL Standard in place regulating 

flammable refrigerants. Furthermore, the type of hydrocarbon (i.e., isobutane, propylene, or propane) 

or blend likely to be used in each end-use was also considered. The reader is referred to Appendix B for 

additional information regarding the assumptions used under this scenario. Estimated emissions for the 

most realistic scenario are shown in Table 3.

 Table 3: 2030 National Refrigerant Emissions of under Scenario 4

Month

Scenario 4 Emissions (MT)

(“Most Realistic Scenario”)

Propane Isobutane Propylene

January 1,266 78 183

February 1,266 78 183

March 1,266 78 183

April 1,373 84 207

May 1,373 84 207

June 1,373 84 207

July 1,373 84 207

August 1,373 84 207

September 1,266 78 183

October 1,266 78 183

November 1,266 78 183

December 1,266 78 183

Note: Shaded months indicate those used in the analysis

Retail Ref

5%

Household 

Ref

IPR 1%

7%

Transport 

Ref

13%

Dehumidif

ers

0%

Res and 

Comm AC

73%

Cold 

Storage

1%

Scenario 3
9

Figure 2 presents the percent contributions of each refrigerant and each end-use to the total emissions 

under the Scenario 4.

Figure 2. Percent Contribution to Total Emissions under Scenario 4

Note: Emissions under the residential and light commercial AC and 

heat pumps end-use (Res and Comm AC) consists of 67% from

window AC and 3% from PTACs (relative to total emissions in the 

Scenario 4).

4 Impact on Ground Level Ozone Concentrations

Using EPA’s CMAQ photochemical model version 4.7.1, a simulation of the potential ground-level ozone 

impacts under the hydrocarbon emission scenarios summarized in the previous section was performed. 

The remainder of this section describes the methodology and results of the CMAQ modeling, followed 

by a comparison of the results to the national standards of ambient air quality.

4.1 Methodology

The projected emissions of isobutane and propylene under Scenarios 1, 2, and 3, and the mix of 

isobutane, propylene, and propane under Scenario 4 described in Section 3 were input into the CMAQ 

to predict impacts on the local air quality. Concentrations were only modeled from April through the 

end of September, as these months presented the largest releases of hydrocarbon refrigerant (see Table 

2 and Table 3), as well as warmer air temperature and longer period of sunlight which increase the 

potential for ozone creation.

The maximum increase in 8-hr ozone concentrations were estimated for the Atlanta, Houston and Los 

Angeles regions. These three cities were chosen based on their distinctive geographic setting and 

continuing and chronic problem with high levels of ground level ozone. Local refrigerant emissions were 

scaled from the national emission estimates to estimate reasonable conservative impacts.

The simulation was performed using 2005 hourly meteorological data at 36-km horizontal resolution 

with 14 vertical layers to model the maximum increase in 8-hour ground level ozone concentrations. The 

model used the carbon bond 05 chemical mechanism which explicitly includes 156 chemical reactions 

(EPA, 2010). The 2030 nationwide emission inventory (NHTSA, 2012), as previously mentioned, was 

readily available and used to estimate the baseline ozone concentration (i.e., without hydrocarbon 

refrigerant). 

Retail 

Food

6%

Transport 

Ref

22%

Househol

d Ref

2%

Res and 

Comm AC

70%

By End-Use

Propane

83%

Isobutan

e

5%

Propylen

e

12%

By Refrigerant
10

Split factors for use in the carbon bond 05 chemical mechanism for isobutane, propylene, and propane 

were determined from the Emit Worksheet in the emitdb.xls workbook form.

6 The split factors and 

related parameters used in processing the refrigerant emissions are presented in Table 4 below.

Table 4. Mass Fractions of CB05 Species per gram of Emitted Compound (g/g)

Compound Paraffin

(PAR)

Molecular 

Weight (g/mole)

Olefins

(OLE)

Molecular 

Weight 

(g/mole)

Unreactive

(UNR)

Molecular 

Weight 

(g/mole)

Isobutane 1.0 14.54 0.0 -- 0.0 --

Propylene 0.34 14.03 0.668 28.06 0.0 --

Propane 0.5 14.70 0.0 -- 0.5 14.7

Note: Paraffin compounds are saturated hydrocarbons. Paraffinic compounds consist only of hydrogen and carbon 

atoms; all bonds are single bonds, and the carbon atoms are not joined in cyclic structures, but instead form an 

open chain. Olefins are unsaturated chemical compounds containing at least one carbon-carbon double bond. 

4.2 CMAQ Results

Table summarizes the maximum increase in 8-hour ground-level ozone from the four scenarios for the 

Los Angeles, Houston, and Atlanta regions. 

Table 5. Maximum Increase in the Daily 8-hour Ground Level Ozone from Hydrocarbon Refrigerants in 

2030 for Three US cities

Refrigerant Type and Use

Scenario

Cities

Los Angeles 

8-hr O3 Maximum 

Change 

(ppb)

Houston 

8-hr O3 Maximum 

Change 

(ppb)

Atlanta 

8-hr O3 Maximum 

Change 

(ppb)

Isobutane

Scenario 1 0.72 0.15 0.05

Scenario 2 0.50 0.10 0.03

Scenario 3 0.49 0.10 0.03

Propylene

Scenario 1 6.61 0.96 0.56

Scenario 2 4.47 0.67 0.39

Scenario 3 4.37 0.66 0.38

Combination of Isobutane, Propylene and Propane (Scenario 4)

Scenario 4 0.15 0.03 0.01

ppb = parts per billion; O3 = ozone

In Los Angeles, isobutane under Scenario 1 resulted in a maximum increase in the 8-hr ozone 

concentration of 0.72 ppb. Propylene, as anticipated due to its high MIR, resulted in the most significant 

increases of ground-ozone concentrations; under Scenario 1, results for the Los Angeles region show 

 

6 The Emit Worksheet in the emitdb.xls form is available at: http://www.engr.ucr.edu/~carter/emitdb/.
11

that the maximum increase in the 8-hr ozone concentration is 6.61 ppb. Releases of isobutane and 

propylene under Scenarios 2 and 3 each show similar patterns to Scenario 1, but with smaller increases. 

Scenario 4 (i.e., the “most realistic” scenario) resulted in a peak 8-hr ozone increase of just 0.15 ppb for 

Los Angeles. These relatively small increases in ozone are about 75% associated with the use of 

propylene as a refrigerant and 21% from propane under this scenario (this is true of Scenario 4 under all 

locations). For all four scenarios, Los Angeles showed the largest ground-ozone concentration increases 

of all three cities. Figure 3 shows the maximum increases and spatial extent of the 8-hour ozone 

concentration increase for isobutane Scenario 1, propylene Scenario 1, and Scenario 4 for Los Angeles. 

In these figures, yellow, orange, and red shading represent modeled changes in ozone concentrations.

Figure 3. Maximum 8-hour Ozone Increase for Los Angeles in 2030 

(Clock-wise from Left: Scenario 1 Isobutane, Scenario 1 Propylene, and Scenario 4) 
12

In Houston, isobutane under Scenario 1 resulted in a maximum increase in the 8-hr ozone 

concentration of 0.15 ppb. For the propylene Scenario 1, results for the Houston region show that the

maximum increase in the 8-hr ozone concentration is 0.96 ppb. Releases of isobutane and propylene 

under Scenarios 2 and 3 each show similar patterns to Scenario 1, but with smaller increases. Scenario 

4 (i.e., the “most realistic” scenario) resulted in a peak 8-hr ozone increase of just 0.03 ppb for 

Houston.

Figure 4 shows the maximum increases and spatial extent of the 8-hour ozone concentration increase 

for the isobutane Scenario 1, the propylene Scenario 1, and Scenario 4 for Houston. In these figures,

yellow, orange, and red shading represent modeled changes in ozone concentrations. 

Figure 4. Maximum 8-hour Ozone Increase for Houston in 2030 

(Clock-wise from Left: Scenario 1 Isobutane, Scenario 1 Propylene, and Scenario 4)
13

In Atlanta, isobutane under Scenario 1 resulted in a maximum increase in the 8-hr ozone 

concentration of 0.05 ppb. For the propylene Scenario 1, results for the Atlanta region show that the

maximum increase in the 8-hr ozone concentration is 0.56 ppb. Releases of isobutane and propylene 

under Scenarios 2 and 3 each show similar patterns to Scenario 1, but with smaller increases. Scenario 

4 (i.e., the “most realistic” scenario) resulted in a peak 8-hr ozone increase of just 0.01 ppb for 

Atlanta.

Figure 5 shows the maximum increases and spatial extent of the 8-hour ozone concentration increase 

for the isobutane Scenario 1, the propylene Scenario 1, and Scenario 4 for Los Angeles. In these figures, 

yellow, orange, and red shading represent modeled changes in ozone concentrations.

Figure 5. Maximum 8-hour Ozone Increase for Atlanta in 2030 

(Clock-wise from Left: Scenario 1 Isobutane, Scenario 1 Propylene, and Scenario 4) 

4.3 Comparison to National Ambient Air Quality Ozone Standard

The current national ambient air quality 8-hour average ozone standard (NAAQS) is 75 ppb. The 

maximum change in ozone concentrations caused by each refrigerant across all scenarios and locations, 

and comparison to the national standard, is summarized in Table 6. 
14

Table 6. Comparison of Maximum Increase in 8-hour Ozone Concentration Relative to the NAAQS

Refrigerant Type (Scenario) Maximum

8-hr O3 Increase (ppb) 

Percent Increase over 

75 ppb 8-hr National Ambient Air 

Quality Ozone Standard

Isobutane (Scenario 1) 0.72 1%

Propylene (Scenario 1) 6.61 9%

Most Realistic Mix (Scenario 4) 0.15 0.2%

ppb = parts per billion; O3 = ozone

With the exception of Los Angeles, all of the three regions are forecast to be in attainment of the 

current 8-hr 75 ppb ozone standard by 2030. Currently, the South Coast is demonstrating attainment by 

2023 for the 1997 8-hr standard of 85 ppb. The deadline for demonstrating attainment of the 75 ppb 

standard is December 2015 and regions must show compliance by 2032.

5 Limitations of this Analysis

The analysis described above is based on numerous assumptions and projections that cannot be known 

with certainty at this time. The following discussion recognizes these uncertainties and indicates various 

factors that have potential to impact and change the results of this analysis. 

5.1 Market Modelling

 The market penetration assumptions used for this analysis assume hydrocarbons will be the sole 

replacement for HFCs across the refrigeration and air-conditioning sector. However, the 

introduction of additional substitutes could potentially lower the penetration of hydrocarbons

into the market, thus decreasing the emissions and ground level ozone impacts of 

hydrocarbons.

 EPA’s Vintaging Model provides emissions estimates up to 2050. However, projected estimates 

are based on assumptions developed using current data and expert opinion regarding the near 

future, and cannot be reasonably expected to reflect future emissions with 100% accuracy. 

Changes that may occur in the distant future (i.e., post-2020), such as regulatory measures and

market events, will likely impact future emissions. 

 Scenarios 1 through 3 in EPA’s Vintaging Model does not account for reductions in emissions 

that may occur as a result of using hydrocarbons as a replacement for HFCs. Factors such as 

refrigerant price, charge size restrictions, and advancements in leak-tightness of ref/AC systems 

are not adjusted for in the modeling. Consequently, hydrocarbon emission projections are 

considered to be conservative.

 Although 2030 was chosen at the analysis year for this assessment because the hydrocarbon 

refrigerants were assumed to reach maximum market penetration by this time, some larger 

pieces of equipment with long lifetimes (i.e., lifetimes larger than 18 years) would not yet have 

reached end-of-life by 2030. Therefore, the effects of the disposal emissions from these pieces 

of equipment are not factored into this analysis. It should also be noted, however, that the 

remaining refrigerant charge in these large pieces of equipment are also more likely to be 

recovered given its economic value.

5.2 Air Quality Modelling
15

 The air quality modeling used in this study was at 36-km resolution, as used in the Corporate 

Average Fuel Economy Standards, Passenger Cars and Light Trucks, Model Years 2017–2025, 

Final Environmental Impact Statement (NHTSA, 2012). In Appendix E: Air Quality Modeling and 

Health Impacts Assessment for Proposed Corporate Average Fuel Economy Standards for 

Passenger Cars and Light Trucks: Model Years 2017–2025, the model performance evaluation is 

discussed stating that, "EPA evaluated CMAQ model performance for the 2005 base year as part 

of the air quality modeling analysis to support the rulemaking for light-duty vehicle greenhouse 

gas emissions standards for MYs 2012-2016 (EPA, 2010). Overall, the performance for the 2005 

modeling platform was similar to that for other national- and regional-scale applications, and 

the model was able to reproduce historical concentrations of ozone and PM2.5 with low bias 

and error results." All meteorological and emission inventory files were taken directly from the 

NHTSA study. The only modification was to include the emission changes due to refrigerants, as 

discussed in Section 2. Higher grid resolution (i.e., smaller grid cell) may potentially show higher 

ozone concentrations due to the mixing of air at the coarser resolution, however most 

refrigerant emissions are widely dispersed over a grid cell, so the effect of this limitation on the 

overall modeling results is likely minimal. 

 Further climate change impacts will result in region-specific changes that may impact ozone 

production rates. For example, increase (or decrease) in local cloud cover would lead to 

decrease (increase) local photolysis rates, resulting in reduced (increased) ozone production.

 It is possible that larger impacts of VOC concentrations could have been noted if additional VOClimited areas in the U.S. were modeled (e.g., Chicago). However, it is very unlikely that impacts 

for those areas would exceed that for Los Angeles. While the higher ozone concentrations 

observed in Los Angeles (8-hour ozone [2008] design value is 112 ppb; Chicago is 77 ppb) 

depends upon the local VOC-to-NOx ratio, it tends to be more sensitive to a given change in VOC. 

Therefore, while it is possible that a higher ozone value may have occurred in Chicago than Los 

Angeles on a given day, it is more likely that the Los Angeles area will have experienced a 

greater change in ozone due to increased VOCs over the course of the ozone season.

 The changes in standards for other pollutants such as air toxics or criteria pollutants could affect 

the ozone precursor emissions. For example, EPA recently lowered the 1-hour NO2 standard to 

100 ppb and could potentially lower this value again in the future to 80 ppb as initially proposed. 

In order to meet this standard, significant NOx emission reductions may be required, thus 

impacting the potential production of ground level ozone. The ozone air quality standard may 

be lowered in the future (historically the standard has been lowered a number of times) and 

would require additional reductions of ozone precursor emissions, which could increase the 

importance of hydrocarbon refrigerants to the production rate for ozone.

7

6 Summary of Findings

From the analysis summarized in the previous sections of this report, it is concluded that nonattainment resulting from hydrocarbon refrigerant emissions is not likely to be a major concern for local 

air quality. Hydrocarbon refrigerants could potentially increase ground level ozone by less than 1% 

 

7

In August 2012, EPA published their first external draft policy assessment for the review of the ozone standard 

which suggested some potential lowering of the ozone standard; however no final action has occurred on a new 

primary ozone standard.
16

under Scenario 4, but up to a 9% increase on a given day for the most reactive hydrocarbons (propylene) 

in the most extreme case. However, in most cases this upper bound level of increase is not likely, as 

most ozone nonattainment areas are not VOC-limited (i.e., the formation of ozone in these areas are 

not by limited by VOC emissions, but by other compounds such as nitrogen oxides [NOx]). In fact,

Scenario 4, the “most realistic” scenario for hydrocarbon refrigerant emission usage, showed a less than 

0.2% increase in ground level ozone for the most extreme case. Future changes that could alter these 

findings include: changes in cloud cover from global warming which could increase or decrease ground 

level ozone production; and future emission reduction measures that may be required to reduce ozone 

precursors which would could increase the relative importance of the hydrocarbon refrigerants on the 

production of ground level ozone. 

7 References

Intergovernmental Panel on Climate Change (IPCC). 2005. Safeguarding the Ozone Layer and the Global 

Climate System: Issues Related to Hydrofluorocarbons and Perfluorocarbons. Available at: 

http://www.ipcc.ch/publications_and_data/_safeguarding_the_ozone_layer.htm. 

MACS. 2009. Email communication between Elvis Hoffpauir, MACS and Karen Thundiyil, EPA. 10 March.

National Highway Traffic Safety Administration (NHTSA), 2012. Final Environmental Impact Statement, 

Corporate Average Fuel Economy Standards, Passenger Cars and Light Trucks, Model Years 2017–2025, 

Appendix E: Air Quality Modeling and Health Impacts Assessment. NHTSA, July 2012.

United States Environmental Protection Agency (EPA). 2008. Vintaging Model, Version VM IO File v4.2 

10-07-08.

United States Environmental Protection Agency (EPA). 2010. Operational Guidance Document CMAQ 

4.7.1 Available at www.cmascenter.org.

United States Environmental Protection Agency (EPA). 2012. “Air Quality: Revision to Definition of 

Volatile Organic Compounds—Exclusion of trans-1,3,3,3-tetrafluoropropene.” Final Rule. 40 CFR Part 51. 

EPA-HQ-OAR-2010-0605; FRL-9679-2. June 22, 2012. Available online at: 

http://www.gpo.gov/fdsys/pkg/FR-2012-06-22/html/2012-15347.htm
17

Appendix A

Table A-1. Scenarios 1, 2, and 3 Isobutane and Propylene Modeling Assumptions

SNAP End-Use Vintaging Model End-Use Size Classification Annual Leak Rate Disposal Release 

Rate^

Motor Vehicle Air 

Conditioning (MVAC)

MVAC Light-duty Vehicle* Small 18% 100%

MVAC Light-duty Trucks* Small 18% 100%

School & Tour Buses* Small 10% 100%

Transit Buses* Small 10% 100%

Trains* Small 2% 100%

Chillers

Centrifugal Chillers** Large 2% 10%

Positive Displacement 

Chillers** Large 1.5% to 2.5% 10%

Screw Chillers** Large 1% 10%

Scroll Chillers** Large 0.5% 10%

Reciprocating Chillers** Large 1.5% 10%

Industrial Process 

Refrigeration Systems

Industrial Process 

Refrigeration Large 3% to 7.5% 10% to 20%

Cold Storage 

Warehouses Cold Storage Large 8% to 16% 10%

Retail Food 

Refrigeration

Large Retail Food Large 14% to 28% 10%

Medium Retail Food Small 8% to 15% 100%

Small Retail Food Small 1% 100%

Ice Makers Small 5% 100%

Refrigerated 

Transport Refrigerated Transport Small 15% to 20% 100%

Household 

Refrigerators and 

Freezers

Refrigerated Appliances Small 1% 100%

Residential 

Dehumidifiers Dehumidifiers Small 0.50% 100%

Residential and Light 

Commercial Air 

Conditioning and 

Heat Pumps

Window Units Small 0.65% 100%

Residential Unitary AC Small 11% 100%

Small Commercial Unitary 

AC Small 9% 100%

Large Commercial Unitary 

AC Small 8% 100%

Water & Ground Source 

HPs Small 4% 100%

Packaged Terminal 

AC/Packaged Terminal 

Heat Pumps (PTAC/PTHP)

Small 4% 100%

^The assumed disposal release rates are dependent upon the size of the equipment. Equipment that is considered small (i.e., 

with a charge size less than 200 kg) are assumed to have a 100% release of the charge upon disposal. Equipment that is 

considered large (i.e., a charge size larger than 200 kg) has a disposal release rate that is consistent with what is currently 

assumed in the Vintaging Model for HFC refrigerants.

*Excluded from Scenario 1 and Scenario 2

**Excluded from Scenario 2
Appendix B

Table B-1: Scenario 4 Modeling Assumptions

SNAP End-Use

Vintaging 

Model EndUseHC*

Market 

Penetration** Charge Size

Annual

Leak 

Rate

Disposal 

Release 

Rate

Retail Food 

Refrigeration

Medium 

Retail Food Propane 100% 1,500 to 12,500 g 8% to 15%

100%

Small Retail 

Food***

Propane 33% 150 g 1% 100%

R-441A 33% 150 g 1% 100%

Isobutane 33% 150 g 1% 100%

Ice Makers Propane 100% 50 g 5% 100%

Refrigerated 

Transport

Refrigerated 

Transport R-441A 100% 2,130 g 15% to 20%

100%

Household 

Refrigerators 

and Freezers

Refrigerated 

Appliances

Isobutane 33% 50 g 1% 100%

Propane 33% 50 g 1% 100%

R-441A 33% 50 g 1% 100%

Residential 

Dehumidifiers Dehumidifiers None None None 0.50% 100%

Residential and 

Light 

Commercial Air 

Conditioning and 

Heat Pumps

Window 

Units

R-441A 33% 1 kg 0.65% 100%

R-443A 33% 1 kg 0.65% 100%

Propane 33% 1 kg 0.65% 100%

Water & 

Ground 

Source HPs

R-443A 33% 1 kg 4% 100%

Propane 33% 1 kg 4% 100%

R-441A 33% 1 kg 4% 100%

PTAC/PTHP

R-443A 33% 1 kg 4% 100%

R-441A 33% 1 kg 4% 100%

Propane 33% 1 kg 4% 100%

*R-441A is a blend consisting of propane, n-butane, isobutane, and ethane; R-443A consists of propylene, propane, and 

isobutane. For purposes of this assessment, both R-441A and R-443A were considered to only contain the components included 

in this analysis, propane, isobutane, and propylene.

**In the absence of market penetration projections, it was assumed that the market was equally divided by the number of 

hydrocarbon substitutes available for each end-use.

***Water coolers and vending machines were assumed to be included under small retail food.