MEMORANDUM

To: Margaret Sheppard, U.S. EPA

CC: Chenise Farquharson, Rebecca von dem Hagen, U.S. EPA

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

Date: March 21, 2016

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

Ozone Concentrations (EPA Contract Number EP-W-10-031 Task Order 405, Task 01)

In response to technical direction received from EPA, ICF has prepared a follow-on to the February 2014 

Assessment of the Potential Impact of Hydrocarbon Refrigerants on Ground Level Ozone Concentrations

[EPA-HQ-OAR-2013-0748-0003]. The February 2014 analysis examined the impacts of four different 

scenarios modeling the use and emissions of hydrocarbon refrigerants in various refrigeration and air 

conditioning (ref/AC) end-uses. In this analysis, refrigerant emissions were modeled for two additional

hydrocarbon refrigerant emission scenarios from the U.S. EPA’s Vintaging Model (VM) (VM_IO 

file_v4.4_10.13.14) and their impacts on ground level ozone were modeled using EPA’s Community 

Multiscale Air Quality (CMAQ) model (version 5.0.2). Releases of hydrocarbon refrigerants were 

modeled in Los Angeles, Houston, and Atlanta. In addition, the most conservative and most realistic 

scenarios from the February 2014 analysis were re-run to reflect updated versions of the VM, CMAQ, 

meteorology, and, most importantly, a 2040 emissions baseline. 

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

As a follow-on to the February 2014 Assessment of the Potential Impact of Hydrocarbon Refrigerants on 

Ground Level Ozone Concentrations [EPA-HQ-OAR-2013-0748-0003], releases of hydrocarbon 

refrigerants were modeled to reflect additional U.S. EPA Significant New Alternatives Policy (SNAP) 

Program applications for the use of hydrocarbons in various refrigeration and air conditioning (ref/AC)

end-uses. Hydrocarbon refrigerants were modeled in Los Angeles, Houston, and Atlanta under two new 

emissions scenarios. Specifically, propylene was modeled under a more conservative emissions scenario

for cold storage and multiple air-conditioning end-uses (Scenario 5)., and a more realistic emissions 

scenario was also modeled, which considered more likely transitions to a variety of hydrocarbon 

refrigerants (i.e., propane, R-441A, and R-443A) by end-use within cold storage and multiple AC enduses (Scenario 6). 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 (Scenario 1) could potentially increase ground level 

ozone by up to 11% compared to the national ozone standard, with increases between 4% and 6% for 

the two new scenarios examined in this analysis (Scenarios 5 and 6). The most realistic scenario 

(Scenario 4) results in a potential increase in ground level ozone of up to 4% compared to the national 

ozone standard. 

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 future climate conditions, among other factors. 

2 Introduction

The February 2014 Assessment of the Potential Impact of Hydrocarbon Refrigerants on Ground Level 

Ozone Concentrations [EPA-HQ-OAR-2013-0748-0003] analyzed the potential air quality impacts from 

three conservative hydrocarbon emission scenarios and one more realistic scenario, 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 
3

commercial AC and heat pumps,1 retail food,2 cold 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;

3 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.

This analysis considers two additional scenarios that model the use of propylene and a mixture of 

hydrocarbon refrigerants and blends in various ref/AC applications (described below in Section 3.1). 

Furthermore, emission estimates from Scenario 1 and Scenario 4 were remodeled to account for 

updates to the CMAQ model, improved meteorology characterization and most importantly 2040 

emission projections and to enable comparison with previous emission scenarios.

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

3 Hydrocarbon Emission Scenarios

As a follow-on to the four scenarios modeled in the February 2014 analysis, releases of hydrocarbon 

refrigerants were modeled in Los Angeles, Houston, and Atlanta under two new emissions scenarios. 

Specifically, propylene was modeled under a more conservative emissions scenario for the ref/AC sector

(Scenario 5). A more realistic emissions scenario was also modeled, which considered more likely 

transitions to a variety of hydrocarbon refrigerants (i.e., propane, R-441A, and R-443A) by end-use in the 

sector (Scenario 6). The two new emission scenarios considered as a continuation of the February 2014 

analysis are as follows: 

5) Scenario 5: applications in the ref/AC sector: cold storage warehouses, chillers,4 and residential 

and light commercial AC and heat pumps (excluding window AC and large commercial unitary 

AC)5 and

 

1 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. 

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

food systems, and large retail food systems.

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

buildings, hotels, hospitals, and the like.

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 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. 
4

6) Scenario 6: applications assumed in Scenario 5, with the addition of window AC and large 

commercial unitary AC.

As mentioned above, the most conservative scenario from the February 2014 analysis (Scenario 1) and 

the most realistic scenario (Scenario 4) were re-run to reflect updated modeling parameters (see 

discussion below).

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).

6 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 

It was assumed that the hydrocarbon refrigerants would enter the market at a minimal level (i.e., 1%) in 

2016, and reach 100% market penetration by 2025. Emissions were estimated using the latest version of 

the VM (VM_IO file_v4.4_10.13.14),

7 which included updates to several ref/AC end-uses, including 

MVACs, window AC units, ice makers, and small retail food end-uses.8 The year 2040 was chosen as the 

analysis year for all scenarios to conservatively estimate the full impact of hydrocarbons. The equipment 

being modeled have longer lifetimes (i.e., 15 to 27 years) and hydrocarbon refrigerants are not assumed 

to reach full penetration of new systems until 2025, so emissions do not reach a roughly steady state 

until around 2040 (as opposed to the previous February 2014 analysis in which steady state emissions 

were reached in 2030). By 2040, the hydrocarbon refrigerants have reached full market penetration and 

equipment containing hydrocarbon refrigerants has begun to reach the end of its useful life. 

To maintain comparability to the previous February 2014 analysis, 2040 emission estimates from 

Scenario 1 and Scenario 4 were recalculated based on the latest version of the Vintaging Model and 

were analyzed. For this analysis, a 2040 nationwide emission inventory was available for other emission 

sources and was used to estimate baseline ozone concentrations (without hydrocarbon refrigerants)

(EPA, 2016).

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 assumed that 11.9% of annual emissions occurred during each 

 

6 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_10.13.14.

7 The February 2014 analysis was based on Vintaging Model version VM_IO file_V4.4_11.6.12.

8

For the MVAC light-duty vehicle (LDV) and light-duty trucks (LDT) end-uses, operational and servicing leak rates 

were reduced based on a review of recent literature. For the small retail food and ice makers end-uses, revisions 

were made to the overall stock, growth rates, assumed transition scenarios, and lifetimes based on research on

substitutes and growth in the market. For window AC, a review of air conditioner sales data from 2002 through 

2012 increased the quantity of window AC equipment introduced into the market for 2002 and 2004 through 

2008, while decreasing the quantity of equipment sold into the market for 2003 and 2009 through 2012.
5

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. 

For example, emissions due to servicing MVACs are 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 

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 throughout the year. 

Equipment in end-uses with a smaller charge size (i.e., end-uses with a charge size less than 200 kg) 

were assumed to have a 100% release of refrigerant upon disposal.

9 Disposal release rates for 

equipment in end-uses with greater charge sizes (i.e., chillers) and servicing release rates for all end-uses 

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

financial incentives for the end-user. This is discussed further in the February 2014 assessment. The 

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

3.2 Scenarios 1 and 5

Propylene is currently being proposed for use in a variety of ref/AC end-uses and exhibits the highest 

maximum incremental reactivity (MIR)

10 of hydrocarbons. Scenario 1 from the February 2014 analysis 

evaluated the most conservative impact that releases of hydrocarbons could have on ground-level 

ozone. This scenario assumed use and release of propylene across the entire ref/AC sector and was 

updated to illustrate projected 2040 emissions. The analysis presented here also conservatively 

estimates the impacts of emissions in end-uses for which propylene is most likely to be used (Scenario 5) 

and assumes complete release from smaller end-uses at disposal. Table 1 presents the monthly 

emissions in 2040 which were assumed under Scenarios 1 and 5. 

 

9

In April 2015, EPA published a final rule to exempt four hydrocarbon refrigerant substitutes from specific enduses, subject to use conditions, from Clean Air Act Section 608's prohibition on venting, release, or disposal, 

including: (1) isobutane and R-441A in retail food refrigerators and freezers (stand-alone units only); (2) propane in 

household refrigerators, freezers, and combination refrigerators and freezers; (3) ethane in very low temperature 

refrigeration equipment and equipment for non-mechanical heat transfer; (4) R-441A, propane, and isobutane in 

vending machines; and (5) propane and R-441A in self-contained room AC for residential and light commercial AC

and heat pumps. 

10 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. Please refer to the February 2014 analysis for 

additional information.
6

Table 1: 2040 National Refrigerant Emissions under Scenarios 1 and 5

Month 

Scenario 1 Emissions (MT) Scenario 5 Emissions (MT)

Propylene Propylene

January 10,280 4,716

February 10,280 4,716

March 10,280 4,716

April 15,127 8,576

May 15,127 8,576

June 15,127 8,576

July 15,127 8,576

August 15,127 8,576

September 10,280 4,716

October 10,280 4,716

November 10,280 4,716

December 10,280 4,716

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

Figure 1 presents the percent contribution of propylene emissions by end-use under Scenarios 1 and 5. 

As illustrated in these charts, residential and light commercial AC end-uses presented the most 

significant source of emissions across all four modeled scenarios. 

Figure 1. Percent Contribution of End-Use to Total Emissions under Scenarios 1 and 5

Note: Emissions under the residential and light commercial AC and 

heat pumps end-use (Res and Comm AC) consists of 48% from 

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

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

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 8% from light-duty vehicles, 11% 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 86% from 

residential unitary AC and 5% from small commercial unitary AC, 1% 

from PTACs, and 1% from water and ground source HPs (relative to 

total emissions in Scenario 5). Window AC and large commercial 

unitary AC were not included in this scenario.

 

Retail Ref

5%

Household 

Ref

1%

IPR

7%

Transport 

Ref

Dehumidif 10%

ers

<1%

MVAC

19%

Chillers

2%

Res and 

Comm AC

55%

Cold 

Storage

1%

Scenario 1 Cold 

Storage

3%

Chillers

5% Res and 

Comm AC

92%

Scenario 5
7

3.3 Scenario 4

Scenario 4 from the February 2014 analysis evaluated the most realistic impact that releases of 

hydrocarbons (i.e., propane, isobutane, or propylene) could have on ground-level ozone. This scenario 

assumed use of hydrocarbons only 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 

and was updated to show 2040 emissions. Estimated emissions for this scenario are shown in Table 2.

 Table 2: 2040 National Refrigerant Emissions of under Scenario 4

Month

Scenario 4 Emissions (MT) (“Most Realistic Scenario”)

Propane Isobutane Propylene

January 1,534 99 198

February 1,534 99 198

March 1,534 99 198

April 1,575 102 207

May 1,575 102 207

June 1,575 102 207

July 1,575 102 207

August 1,575 102 207

September 1,534 99 198

October 1,534 99 198

November 1,534 99 198

December 1,534 99 198

Note: Shaded months indicate those used in the analysis

Figure 3 presents the percent contributions of each refrigerant and each end-use to the total emissions 

under 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 58% from

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

Scenario 4).

Retail Food

10%

Transport Ref

25%

Household Ref

3%

Res and 

Comm AC

62%

By End-Use

Propane

84%

Isobutane

5%

Propylene

11%

By Refrigerant
8

3.4 Scenario 6

An additional, more realistic scenario (Scenario 6) was developed to model the type of hydrocarbon or 

blend (i.e., R-441A, R-443A, or propane) likely to be used in ref/AC end-uses. The reader is referred to 

Appendix B for additional information regarding the assumptions used under this scenario. In addition, 

Appendix C presents the relative contribution to total emissions of each refrigerant by end-use. 

Estimated emissions for this scenario are shown in Table 3.

 Table 3: 2040 National Refrigerant Emissions of under Scenario 6

Month

Scenario 6 Emissions (MT)

Propane Isobutane Propylene Ethane n-Butane

January 2,157 237 2,886 5 64

February 2,157 237 2,886 5 64

March 2,157 237 2,886 5 64

April 3,667 423 5,107 6 69

May 3,667 423 5,107 6 69

June 3,667 423 5,107 6 69

July 3,667 423 5,107 6 69

August 3,667 423 5,107 6 69

September 2,157 237 2,886 5 64

October 2,157 237 2,886 5 64

November 2,157 237 2,886 5 64

December 2,157 237 2,886 5 64

Note: Shaded months indicate those used in the analysis

Figure 3 presents the percent contributions of each refrigerant and each end-use to the total emissions 

under Scenario 6.

Figure 3. Percent Contribution to Total Emissions under Scenario 6

Note: Emissions under the residential and light commercial AC and 

heat pumps end-use (Res and Comm AC) consists of 79% from

residential unitary AC, 8% from window AC, 5% from small 

commercial unitary AC, 0.5% from large commercial unitary AC, 1% 

from PTACs, and 1% from water and ground source heat pumps

(relative to total emissions in the Scenario 6).

Cold 

Storage

3%

Chillers

4%

Res and 

Comm AC

93%

By End-Use

Propane

40%

Isobutane

4%

Propylene

55%

Ethane

0%

n-Butane

1%

By Refrigerant
9

4 Impact on Ground Level Ozone Concentrations

Using EPA’s CMAQ photochemical model version 5.0.2, 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. This methodology is 

consistent with the approach used in the February 2014 analysis; however, updated emission estimates 

and model versions were used when available. 

4.1 Methodology

The projected emissions of propylene under Scenarios 1 and 5, and the mixture of hydrocarbons under 

Scenarios 4 and 6 were input into the CMAQ to predict impacts on 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 1, 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 was 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 CMAQ simulation was performed using 2011 hourly meteorological data at 36-km horizontal 

resolution with 14 vertical layers to identify 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 2040 nationwide emission inventory (EPA, 2016), as previously 

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

hydrocarbon refrigerant). This inventory reflects the best estimate of nationwide 2040 hydrocarbons 

and NOx emissions whose ratio regulates the speed and production of ground-level ozone formation. 

Split factors for use in the carbon bond 05 chemical mechanism for butane, ethane, isobutane, 

propylene, and propane were determined from the Emit Worksheet in the emitdb.xls workbook form.

11

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)

Ethane 

(ETHA)

Molecular 

Weight 

(g/mole)

Unreactiv

e (UNR)

Molecular 

Weight 

(g/mole)

Butane 1.0 14.53 0.0 -- 0.0 -- 0.0 --

Ethane 0.0 -- 0.0 -- 1.0 30.07 0.0 --

Isobutane 1.0 14.54 0.0 -- 0.0 -- 0.0 --

Propylene 0.34 14.03 0.66 28.06 0.0 -- 0.0 --

Propane 0.5 14.70 0.0 -- 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. 

 

11 The Emit Worksheet in the emitdb.xls form is available at: http://www.engr.ucr.edu/~carter/emitdb/.
10

4.2 CMAQ Results

Table 5 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 

2040 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)

Propylene

Scenario 1 7.80 2.13 1.14

Scenario 5 4.49 1.21 0.65

Combination of Propane, Isobutane, Propylene, Ethane, and n-Butane

Scenario 4 2.48 0.76 0.44

Scenario 6 2.57 0.77 0.44

ppb = parts per billion; O3 = ozone

For Los Angeles, propylene, as anticipated due to its high MIR, resulted in the most significant increases 

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

maximum increase in the 8-hr ozone concentration is 7.80 ppb and under Scenario 5, the maximum 

increase in the 8-hr ozone concentration is 4.49 ppb. Scenario 4 (i.e., the most realistic scenario) 

resulted in a maximum increase of 2.48 ppb in the 8-hr ozone concentration. Scenario 6 resulted in a 

peak 8-hr ozone increase of 2.57 ppb. This increase in ozone is estimated to be about 95% associated 

with the use of propylene as a refrigerant and 4% from propane under this scenario. For all four 

scenarios, Los Angeles showed the largest ground-ozone concentration increases of the three cities

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

increase for propylene in Scenario 1 and Scenario 5 and the hydrocarbon combination in Scenario 4 and 

Scenario 6 for Los Angeles. In these figures, red, orange, and yellow represent modeled changes in 

ozone concentrations.
11

Figure 4. Maximum 8-hour Ozone Increase for Los Angeles in 2040 

(Clock-wise from Left: Scenario 1 Propylene, Scenario 5 Propylene, Scenario 4 Combination, and 

Scenario 6 Combination)

For Houston under Scenario 1, propylene resulted in a maximum increase in the 8-hr ozone 

concentration of 2.12 ppb. For Scenario 5, propylene results for the Houston region show that the

maximum increase in the 8-hr ozone concentration is 1.21 ppb. Scenario 4 (i.e., the most realistic 

scenario) resulted in a maximum increase of 0.76 ppb in the 8-hr ozone concentration. Scenario 6 (i.e., 

the more realistic scenario) resulted in a peak 8-hr ozone increase of 0.77 ppb downwind of Houston.

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

for propylene in Scenario 1 and Scenario 5 and the hydrocarbon combination in Scenario 4 and Scenario 

6 for Houston. The further downwind location of the maximum increase in Scenario 4 and 6 reflects the 

slower reactivity of the refrigerant mixture and also occurs on a different day from each other as well as 

relative to Scenario 1 and 5. In these figures, red, orange, and yellow shading represent modeled 

changes in ozone concentrations. 
12

Figure 5. Maximum 8-hour Ozone Increase for Houston in 2040 

(Clock-wise from Left: Scenario 1 Propylene, Scenario 5 Propylene, Scenario 4 Combination, and 

Scenario 6 Combination)

For Atlanta, propylene under Scenario 1 resulted in a maximum increase in the 8-hr ozone 

concentration of 1.14 ppb. For Scenario 5, results for the Atlanta region show that the maximum 

increase in the 8-hr ozone concentration is 0.65 ppb. Scenario 4 (i.e., the most realistic scenario) 

resulted in a maximum increase of 0.44 ppb in the 8-hr ozone concentration. Scenario 6 (i.e., a more 

realistic scenario) resulted in a peak 8-hr ozone increase of 0.44 ppb for Atlanta. Figure 6 shows the 

maximum increases and spatial extent of the 8-hour ozone concentration increase for propylene in 

Scenario 1 and Scenario 5 and the hydrocarbon combination in Scenario 4 and Scenario 6 for Atlanta. 

The further downwind location of the maximum increase in Scenario 4 and 6 reflects the slower 

reactivity of the refrigerant mixture under this scenario and also occurs on a different day. In these 

figures, yellow, orange, and red shading represent modeled changes in ozone concentrations.
13

Figure 6. Maximum 8-hour Ozone Increase for Atlanta in 2040 

(Clock-wise from Left: Scenario 1 Propylene, Scenario 5 Propylene, Scenario 4 Combination, and 

Scenario 6 Combination)

4.3 Comparison to National Ambient Air Quality Ozone Standard

The current national ambient air quality 8-hour average ozone standard (NAAQS) is 70 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 

70 ppb 8-hr National Ambient 

Air Quality Ozone Standard

Propylene (Scenario 1) 7.8 11.1%

Propylene (Scenario 5) 4.5 6.4%

Combination (Scenario 4) 2.5 3.5%

Combination (Scenario 6) 2.6 3.7%

ppb = parts per billion; O3 = ozone

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 ref/AC 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 5 and 6 in EPA’s Vintaging Model do 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.

5.2 Air Quality Modeling 

 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.
15

 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.

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 could be a concern for local air quality 

under the more conservative propylene scenarios. Propylene refrigerant could potentially increase 

ground level ozone by a maximum of 11% on a given day in the most extreme case (Scenario 1) and up 

to a 6% increase for the more realistic propylene-only scenario (Scenario 5). The more realistic Scenario 

6 showed almost a 4% increase in ground level ozone. In most cases these maximum levels of increase 

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

these areas is not limited by VOC emissions, but by other compounds such as nitrogen oxides [NOx]). 

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

maximum increase of less than 4% in ground level ozone. 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. 
16

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.

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

United States Environmental Protection Agency (EPA). 2016. Draft Final Environmental Impact 

Statement, Phase 2 Greenhouse Gas Emission Standards for Medium- and Heavy-Duty Engines and 

Vehicles. Washington, D.C. (In progress).
17

Appendix A

Table A-1. Scenario 5 Propylene Modeling Assumptions

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

Rate^

Chillers

Centrifugal Chillers Large 2% 10%

Screw Chillers Large 1% 10%

Scroll Chillers Large 0.5% 10%

Reciprocating Chillers Large 1.5% 10%

Cold Storage 

Warehouses Cold Storage Large 8% to 16% 10%

Residential and Light 

Commercial Air 

Conditioning and 

Heat Pumps

Residential Unitary AC Small 11% 100%

Small Commercial Unitary 

AC Small 9% 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.
Appendix B

Table B-1: Scenario 6 Modeling Assumptions

SNAP End-Use Vintaging Model End-Use HC*

Market 

Penetration**

Annual Leak 

Rate

Disposal 

Release Rate

Chillers

Centrifugal Chillers Propylene 100% 2% 10%

Screw Chillers Propylene 100% 1% 10%

Scroll Chillers Propylene 100% 0.5% 10%

Reciprocating Chillers Propylene 100% 1.5% 10%

Cold Storage 

Warehouses Cold Storage Propylene 100% 8% to 16% 10%

Residential and 

Light 

Commercial Air 

Conditioning 

and Heat 

Pumps

Window Units

R-441A 33% 0.65% 100%

R-443A 33% 0.65% 100%

Propane 33% 0.65% 100%

Residential Unitary AC R-443A 100% 11% 100%

Small Commercial Unitary AC R-443A 100% 9% 100%

Large Commercial Unitary AC R-443A 100% 8% 100%

Water & Ground Source HPs R-443A 100% 4% 100%

PTAC/PTHP R-443A 100% 4% 100%

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

isobutane. 

**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.
2

Appendix C

Table C-1: Relative Contributions to Total and VOC Emissions by Refrigerant for Scenario 6

SNAP End-Use Vintaging Model End-Use HC

Relative Contribution 

to Scenario 2040 

Emissions

Chillers

Centrifugal Chillers Propylene 1.4%

Screw Chillers Propylene 1.5%

Scroll Chillers Propylene 0.4%

Reciprocating Chillers Propylene 0.0%

Cold Storage 

Warehouses Cold Storage Propylene 1.9%

Residential and 

Light Commercial 

Air Conditioning 

and Heat Pumps

Window Units

Propane 5.1%

Isobutane 0.3%

Propylene 1.4%

Ethane 0.1%

n-Butane 0.9%

Residential Unitary AC

Propane 32.3%

Isobutane 4.0%

Propylene 44.5%

Small Commercial Unitary AC

Propane 1.9%

Isobutane 0.2%

Propylene 2.6%

Large Commercial Unitary AC

Propane 0.2%

Isobutane 0.0%

Propylene 0.3%

Water & Ground Source HPs

Propane 0.2%

Isobutane 0.0%

Propylene 0.2%

PTAC/PTHP

Propane 0.2%

Isobutane 0.0%

Propylene 0.3%