2024-25971. Air Quality: Revision to the Regulatory Definition of Volatile Organic Compounds-Exclusion of (Z)-1-chloro-2,3,3,3-tetrafluoropropene (HCFO-1224yd(Z))

Table 1—Potentially Affected Entities by North American Industrial Classification System (NAICS) Code

Category	NAICS code	Description of regulated entities
Industry	333415	Air-Conditioning and Warm Air Heating Equipment and Commercial and Industrial Refrigeration Equipment Manufacturing.
Industry	811310	Commercial and industrial machinery and equipment (except automotive and electronic) repair and maintenance.
Industry	221116	Geothermal Electric Power Generation.
Industry	221117	Biomass Electric Power Generation.
Industry	221118	Other Electric Power Generation.

This table is not intended to be exhaustive but rather provides a guide for readers regarding entities that might be affected by this deregulatory action. This table lists the types of entities that the EPA is now aware of that could potentially be affected to some extent by this action. Other types of entities not listed in the table could also be affected to some extent. To determine whether your entity is directly or indirectly affected by this action, you should consult your State or local air pollution control and/or air quality management agencies.

II. Background

A. The EPA's VOC Exemption Policy

Tropospheric O₃, commonly known as smog, is formed when VOC and nitrogen oxides (NO_X) react in the atmosphere in the presence of sunlight. Because of the harmful health effects of O₃, the EPA and State governments limit the amount of VOC that can be released into the atmosphere. VOC form O₃ through atmospheric photochemical reactions, and different VOC have different levels of reactivity. That is, different VOC do not react to form O₃ at the same speed or form different amounts of O₃ . Some VOC react more slowly or form less O₃; therefore, changes in their emissions have limited effects on local or regional O₃ pollution episodes. It has been the EPA's policy since 1971 that certain organic compounds with a negligible level of reactivity should be excluded from the regulatory definition of VOC to focus VOC control efforts on compounds that significantly affect O₃ concentrations. The EPA also believes that exempting such compounds creates an incentive for industry to use negligibly reactive compounds in place of more highly reactive compounds that are regulated as VOC. The EPA lists compounds that ( print page 88942) it has determined to be negligibly reactive in its regulations as being excluded from the regulatory definition of VOC (40 CFR 51.100(s)).

The CAA requires the regulation of VOC for various purposes. Section 302(s) of the CAA specifies that the EPA has the authority to define the meaning of “VOC” and, hence, what compounds shall be treated as VOC for regulatory purposes. The policy of excluding negligibly reactive compounds from the regulatory definition of VOC was first laid out in the “Recommended Policy on Control of Volatile Organic Compounds” (42 FR 35314, July 8, 1977) (“1977 Recommended Policy”) and was supplemented subsequently with the “Interim Guidance on Control of Volatile Organic Compounds in Ozone State Implementation Plans” (70 FR 54046, September 13, 2005) (“2005 Interim Guidance”). The EPA 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 regulatory definition of VOC. Compounds that are more reactive than ethane continue to be considered VOC for regulatory purposes and, therefore, are subject to control requirements. The selection of ethane as the threshold compound was based on a series of smog chamber experiments that underlay the 1977 Recommended Policy.

The EPA has used three different metrics to compare the reactivity of a specific compound to that of ethane: (i) the rate constant for reaction with the hydroxyl radical (OH) (known as k_OH); (ii) the maximum incremental reactivity (MIR) on a reactivity per unit mass basis; and (iii) the MIR expressed on a reactivity per mole basis. Differences between these three metrics are discussed below.

The k_OH is the rate constant of the reaction of the compound with the OH radical in the air. This reaction is often, but not always, the first and rate-limiting step in a series of chemical reactions by which a compound breaks down in the air and contributes to O₃ formation. If this step is slow, the compound will likely not form O₃ at a very fast rate. The k_OH values have long been used by the EPA as metrics of photochemical reactivity and O₃ -forming activity, and they were the basis for most of the EPA's early exemptions of negligibly reactive compounds from the regulatory definition of VOC. The k_OH metric is inherently a molar-based comparison, i.e., it measures the rate at which molecules react.

The MIR, both by mole and by mass, is a more updated metric of photochemical reactivity derived from a computer-based photochemical model, and it has been used as a metric of reactivity since 1995. This metric considers the complete O₃ -forming activity of a compound over multiple hours and through multiple reaction pathways, not merely the first reaction step with OH. Further explanation of the MIR metric can be found in Carter (1994).

The EPA has considered the choice between MIRs with a molar or mass basis for the comparison to ethane in past rulemakings and guidance. In the 2005 Interim Guidance, the EPA stated that a comparison to ethane's MIR on the mass basis strikes the right balance between a threshold that is low enough to capture chemicals that significantly affect ozone formation and a threshold that is high enough to allow for the exemption of some other chemicals that may usefully substitute for more reactive compounds. The guidance also stated that EPA will continue to compare chemicals to ethane using k_OH expressed on a molar basis and MIR values expressed on a mass basis during the review of suggested chemicals for VOC-exempt status.^[1] The 2005 Interim Guidance notes that the EPA will consider a compound to be negligibly reactive if it is equally reactive as or less reactive than ethane based on either k_OH expressed on a molar basis or MIR values expressed on a mass basis (70 FR 54046).

The molar comparison of MIR is more consistent with the original smog chamber experiments, which compared equal molar concentrations of individual VOC, supporting the selection of ethane as the threshold, while the mass-based comparison of MIR is consistent with how MIR values and other reactivity metrics are applied in reactivity-based emission limits. It is, however, important to note that the mass-based comparison is less restrictive than the molar-based comparison in that more compounds would qualify as negligibly reactive.

Given the two goals of the exemption policy articulated in the 2005 Interim Guidance, the EPA believes that ethane continues to be an appropriate threshold for defining negligible reactivity. And, to encourage the use of environmentally beneficial substitutions, the EPA continues to believe that a comparison to ethane on a mass basis strikes the right balance between a threshold that is low enough to capture compounds that significantly affect O₃ concentrations and a threshold that is high enough to exempt some compounds that may usefully substitute for more highly reactive compounds.

The 2005 Interim Guidance also noted that concerns have sometimes been raised about the potential impact of a VOC exemption on environmental endpoints other than O₃ concentrations, including fine particle formation, air toxics exposures, stratospheric O₃ depletion, and climate change. The EPA has recognized, however, that there are existing regulatory or non-regulatory programs that are specifically designed to address these issues, and the EPA continues to believe in general that the impacts of VOC exemptions on environmental endpoints other than O₃ formation can be adequately addressed by these programs. The VOC exemption policy is intended to facilitate attainment of the O₃ National Ambient Air Quality Standards (NAAQS), and VOC exemption decisions will continue to be based primarily on consideration of a compound's contribution to O₃ formation. However, if the EPA determines that a particular VOC exemption is likely to result in a significant increase in the use of a compound and that the increased use would pose a significant risk to human health or the environment that would not be addressed adequately by existing programs or policies, then the EPA may exercise its judgment accordingly in deciding whether to grant an exemption.

The EPA has provided the foregoing discussion of its VOC exemption policies as background for its assessment of the petition to list HCFO-1224yd(z) as an exempt compound and its proposed action to grant the petition. However, the EPA is not reopening the 2005 Interim Guidance or other aspects of its VOC exemption policy in this proposed rule and is not seeking comment on these issues.

B. Petition To List HCFO-1224yd(Z) as an Exempt Compound

The AGC Chemicals Americas, Inc. (“AGC”) submitted a petition to the EPA on July 29, 2020, requesting that (Z)-1-chloro-2,3,3,3-tetrafluoropropene (also known as HCFO-1224yd(Z); CAS number 111512-60-8) be exempted from the regulatory definition of VOC. The petition stated that HCFO-1224yd(Z) has low reactivity ( i.e., 0.052 ± 0.011g of O₃ /g of HCFO-1224yd(Z)) ( print page 88943) compared to the MIR of ethane (0.28 g O₃ /g). The petitioner indicated that HCFO-1224yd(Z) may be used in refrigeration which uses a turbo-type refrigerator, a binary generator, a heat recovery heat pump, etc. As a refrigerant, this compound will not be generally emitted into the atmosphere on a continuous basis in significant amounts. Refrigerators will be initially charged and then serviced with HCFO-1224yd(Z) with minimal losses of refrigerant to the atmosphere over time, and they will be subject to EPA's regulations related to servicing and “venting.” HCFO-1224yd(Z) has been approved by EPA through its Significant New Alternatives Policy (SNAP) program as an acceptable substitute for use in new and retrofitted centrifugal chillers, positive displacement chillers and industrial process refrigeration.^[2]

AGC has developed HCFO-1224yd(Z) to support reductions in emissions of greenhouse gases (GHGs). The global warming potential (GWP) for HCFO-1224yd(Z) is 0.88 for a time horizon of 100 years. HCFO-1224yd(Z) is relatively short-lived in the atmosphere, with a lifetime of approximately 20 days. HCFO-1224yd(Z)'s ODP is almost zero (0.00023) and, leading to an environmental impact that is estimated to be low especially when compared to the existing alternatives (Tokuhashi et al., 2018). Hence, HCFO-1224yd(Z) can serve as a replacement for compounds in several centrifugal and positive displacement chillers such as ammonia absorption, carbon dioxide, and HFO-1336mzz(Z) among others with GWP ranging between zero and 630. For industrial process refrigeration, HCFO-1224yd(Z) has a GWP lower than or comparable to that of acceptable existing substitutes for new or retrofit equipment with GWP ranging between zero and 14,800.

Toxicity of HCFO-1224yd(Z) is comparable to or lower than that of other available substitutes in the same end uses. The toxicity risks are evaluated through the SNAP program but can also be minimized through the application of recommended guidance in the Occupational Alliance for Risk Science's Workplace Environmental Exposure Level (OARS WEEL), the American Society of Heating, Refrigerating and Air-Conditioning Engineers safety standards 15 (ASHARE 15) and other industry standards, as well as the safety data sheet (SDS) and other safety precautions related to refrigeration and air conditioning industry.

To support its petition, AGC provided a document on ground-level atmospheric ozone formation potential from the reactivity of HCFO-1224yd(Z) with the hydroxyl OH based on calculations using SARPC-11 atmospheric chemical mechanism.^[3] AGC's supplemental technical report supplied a MIR of HCFO-1224yd(Z) of 0.052 ± 0.011 g O₃ /g HCFO-1224yd(Z) on the mass-based MIR scale. This reactivity is significantly lower than that of ethane (0.29 ± 0.07 g O₃ /g ethane). The report also addressed uncertainties around the MIR value calculated and stipulated that the relative impact on ozone formation will be small when compared to variability in atmospheric conditions. The report raised a warning around the chemical mechanism used to predict ozone formation potential to caution about the need to test whether the predicted value can be observed in an environmental chamber experiment. The petition did not include a value for the rate constant k_OH for the gas-phase reaction with OH radicals.

To address the potential for stratospheric O₃ impacts, the petitioner specified that, because the atmospheric lifetime of HCFO-1224yd(Z) due to loss by OH reaction was estimated to be relatively short, even though HCFO-1224yd(Z) contains chlorine, it is not expected to contribute to the depletion of the stratospheric O₃ layer more than other alternatives listed acceptable by EPA's SNAP program (USEPA, 2019).

III. The EPA's Assessment of the Petition

The EPA is proposing to respond to the petition to revise the EPA's regulatory definition of VOC for exemption of HCFO-1224yd(Z). This action is based on consideration of the compound's low contribution to tropospheric O₃ and the low likelihood of risk to human health or the environment, including stratospheric O₃ depletion, toxicity, and climate change. Additional information on these topics is provided in the following sections.

A. Contribution to Tropospheric Ozone Formation

The rate constant k_OH for the gas-phase reaction with OH radicals is measured to be (5.84 ± 0.030) 10⁻¹³ cm³ /molecule-sec at ~298 degrees Kelvin (K) (Tokuhashi et al., 2018). This k_OH is more than twice the k_OH of ethane (2.4 × 10⁻¹³ cm³ /molecule-sec at ~298 K; Atkinson et al., 2006) even when uncertainty is considered and, therefore, suggests that it is more reactive than ethane. In most cases, chemicals with high k_OH values also have high MIR values, but the products that are formed here in subsequent reactions are expected to be polyfluorinated compounds, which do not contribute to O₃ formation (Osterstrom et al., 2017). In the case of HCFO-1224yd(Z), while the k_OH is relatively high, the calculated maximum incremental reactivity MIR is very low when compared to that of ethane based on Carter (2020), provided by the petitioner, and reviewed by EPA.

Carter (2020) estimates that HCFO-1224yd(Z) has a MIR value of 0.052 ± 0.011 g O₃ /g VOC versus 0.29 ± 0.07 g O₃ /g VOC for ethane. Therefore, the EPA considers HCFO-1224yd(Z) to be negligibly reactive and eligible for VOC-exempt status following the Agency's long-standing policy that compounds should so qualify where either reactivity metric (k_OH expressed on a molar basis or MIR expressed on a mass basis) indicates that the compound is less reactive than ethane. While the overall atmospheric reactivity of HCFO-1224yd(Z) was not studied in an experimental smog chamber, the chemical mechanism derived from other chamber studies (Carter, 2011) was used to model the complete formation of O₃ for an entire single day under realistic atmospheric conditions by Carter (2020). The EPA has assessed the Carter study provided by the petitioner and believes the calculated MIR value is reliable.^[4]

Table 2 presents three reactivity metrics for HCFO-1224yd(Z) as they compare to ethane. ( print page 88944)

Table 2—Reactivities of Ethane and HCFO-1224 yd (Z)

Compound	k OH (cm3 /molecule-sec)	Maximum incremental reactivity (MIR) (g O 3 /mole VOC)	Maximum incremental reactivity (MIR) (g O 3 /g VOC)
Ethane	2.4 × 10 −13	8.4	0.28 ± 0.07
HCFO-1224yd(Z)	(5.84 ± 0.030) × 10 −13	7.7	0.052 ± 0.011
Notes:
k OH value at 298 K for ethane is from Atkinson et al. (2006; page 3626).
k OH value at 300 K for HCFO-1224yd(Z) is from Tokuhashi et al., 2018 (table 1).
Mass-based MIR value (g O 3 /g VOC) of ethane is from Carter (2011).
Mass-based MIR value (g O 3 /g VOC) of HCFO-1224yd(Z) is from Carter 2020.
Molar-based MIR (g O 3 /mole VOC) values were calculated from the mass-based MIR (g O 3 /g VOC) values using the number of moles per gram of the relevant organic compound.