Photochemical reactivity could replace existing VOC regulations

Written on: July 1, 2023 by Nicholas Georges

If you’ve had any conversations recently with the rulemaking staff at the California Air Resources Board (CARB), chances are photochemical reactivity has come up in the course of the discussion. For those who aren’t familiar, photochemical reactivity is a measure of the tendency of a chemical to break down in the atmosphere in the presence of sunlight and other pollutants to form ozone. Reactions in the atmosphere are very complex and the amount of ozone produced can vary based on atmospheric conditions and the inherent reactivity of the molecules.

As you probably know, CARB is planning to undergo another rulemaking on consumer products. It should not come as a surprise that sizeable emissions reductions are needed to meet the California State Implementation Plan (SIP) commitments to continue improving air quality. For more than 30 years, CARB has regulated the volatile organic compound (VOC) content of consumer products as part of an overall strategy to reduce ground-level ozone; however, the next rulemaking will not just focus on VOCs. CARB is zeroing in on ozone-forming potential (OFP) when determining emission reductions. Maximum Incremental Reactivity (MIR) is one way to measure photochemical reactivity by estimating how much ground-level ozone can potentially be formed, provided that the other components necessary for the reaction to generate ozone are present. While various conditions will change the amount of ozone that can be generated, it is generally understood that a substance with a higher MIR will form more ozone under most conditions than a substance with a lower MIR.

Why are we suddenly shifting focus from VOCs to measurements that represent OFP? After decades of regulating consumer products, there are not many areas left for CARB to regulate and obtain the VOC reductions they are pursuing—and MIR-based limits would offer a more targeted approach for reducing ground-level ozone.

However, this raises a question: Why regulate consumer products by VOC content if MIR is a more targeted approach? The answer to that question is best summarized by a 1995 U.S. Environmental Protection Agency (EPA) report to Congressi (emphasis added):

To be most effective, ozone control strategies ideally should be based not only on mass VOC and NOx emissions but should consider the relative photochemical reactivity of individual species, the VOC-to-NOx ratios prevalent in specific airsheds, and other factors which could work together to minimize the formation of ozone with minimum adverse impacts. Reactivity data on VOC, especially those compounds used to formulate consumer and commercial products, is extremely limited. Better data, which can be obtained only at a great expense, is needed if EPA is to consider relative photochemical reactivity in any VOC control strategy. In the meantime, a practical approach is to act on the basis of mass VOC emissions.

While MIR is the more scientifically robust approach, the science was not in place to be used for regulatory purposes, so CARB targeted the low-hanging fruit that a mass-based VOC approach offered at the onset of these regulations. Both CARB and Industry have long recognized the need to measure the differences in reactivities of VOCs and the potential amount of ground-level ozone, so they funded Dr. William (Bill) Carter’s environmental chamber studies of the MIR of VOCs at the Statewide Air Pollution Research Center (SAPRC) at the University of California, Riverside. The experiments consisted of repeated six-hour indoor chamber irradiations of a simplified mixture of ozone precursors with NOx in excess, alternating tests of varying amounts of an added VOC.

Dr. Carter’s work has paved the way for consumer products to be regulated by MIR—and there’s precedence to do so. CARBii (and EPAiii) already regulate aerosol coatings based on reactivity, not mass-based VOC content. In addition, CARB added an alternative compliance optioniv for multi-purpose lubricants and expanded the Innovative Product Exemptionv for hairspray, dry shampoo and personal fragrance products to utilize photochemical reactivity.

Exploring MIR-based limits for various product categories will be more complex and likely require companies to make adjustments on their end to comply; however, switching to MIR and utilizing solvents with lower MIR values means that formulators can utilize VOCs with low MIRs, which will increase opportunities for innovation.

The Household & Commercial Products Association (HCPA) will host a MIR webinarvi on Aug. 8. For more information or to register, please contact me at SPRAY

 Study of Volatile Organic Compound Emissions from Consumer & Commercial Products Report to Congress. EPA-453/R-94-066-A. March 1995.
ii  Cal. Code Regs. Title 17, §§ 94520-28
iii 40 CFR Part 59 Subpart E
iv Cal. Code Regs. Title 17, § 94509 (r)
v  Cal. Code Regs. Title 17, § 94511
vi  link