On November 12, 2024, the U.S. Environmental Protection Agency (EPA) proposed to exempt the compound (Z)-1-chloro-2,3,3,3-tetrafluoropropene (HCFO-1224yd(Z), CAS number 111512-60-8) from being classified as a volatile organic compound (VOC) due to its minimal impact on tropospheric ozone formation. Public comments are welcome via https://www.regulations.gov/ until January 13, 2025.
The decision is supported by studies of the compound's rate constant with hydroxyl radicals (kOH) and its maximum incremental reactivity (MIR), both of which indicate significantly lower reactivity than ethane, justifying its VOC exemption.
Tropospheric Ozone: Formation and Impacts
Tropospheric ozone, also known as ground-level ozone or smog, forms when volatile organic compounds (VOCs) and nitrogen oxides (NOx) react under sunlight. Given ozone's health risks, the U.S. EPA and state governments rigorously regulate VOC emissions. However, the varying reactivity of VOCs means that reductions in emissions do not uniformly impact local or regional air quality.
Strategic Regulatory Framework
Since the early 1970s, EPA policy has focused on excluding low-reactivity organic compounds from the VOC regulatory scope. This allows for targeted control efforts on compounds significantly affecting ozone levels, and encourages industry to adopt safer, less reactive alternatives.
Empowered by Section 302(s) of the Clean Air Act, the EPA defines "VOC" and identifies which substances are regulated. This approach was first outlined in the 1977 "Policy on Control of Volatile Organic Compounds" and further detailed in the 2005 "Interim Guidance on Control of Volatile Organic Compounds in Ozone State Implementation Plans."
The recent proposal to exempt HCFO-1224yd(Z) aims to lessen environmental impacts and promote the use of safer alternatives. EPA continues to monitor this compound through programs like SNAP and the New Chemicals program to mitigate unforeseen environmental risks.
