FACT SHEET Revision to 40 CFR part 50, appendix D, Reference Measurement Principle and Calibration Procedure for the Measurement of Ozone in the Atmosphere (Chemiluminescence Method) ACTION • On February 15, 2023, the U.S. Environmental Protection Agency proposed to update the absorption cross-section value for ultraviolet (UV)-based ozone photometer measurements and the dates of publication for two references, as well as adding a new reference for the updated cross-section value. • The proposed revisions consist primarily of updating the current ozone absorption cross- section to the recommended, consensus-based cross-section value of 1.1329xl0"17 cm2 molecule"1 or 304.39 atm"1 cm"1, with an uncertainty of 0.94 atm"1 cm"1 at 0°C and 1 atmosphere (atm), updating dates publication dates for two references, and the addition of the reference for the recommended, consensus-based cross-section value. • The new value is 1.2% lower than the current value of 308 atm"1 cm"1 in 40 CFR part 50, appendix D, and reduces the uncertainty in the value to 0.31%. The change in ozone absorption cross-section could result in increases in measured ozone concentrations but given the existing sources of potential variability in monitoring data, it is unlikely that there would be any consistent measurable and predictable effect on reported data. • The proposed ozone cross-section value will result in an improvement in the accuracy of surface ozone monitoring measurements and reduce the uncertainty in measured ozone concentrations. • The proposed update to the cross-section was brought to EPA's attention by The Gas Analysis Working Group of the Consultive Committee for Metrology in Chemistry and Biology (CCQM-GAWG) of the Bureau of Weights and Measures in France (BIPM) and the National Institute of Standards and Technology (NIST). • EPA is proposing to adopt and implement the new cross-section value as it represents a more accurate value with less uncertainty and is an advancement science and improvement in the UV photometer measurement method. ------- HOW TO COMMENT EPA will accept comments on the proposal for 30 days after publication in the Federal Register. Please identify comments with Docket ID No. EPA-HQ-OAR-2022-0007. You may submit comments by one of the following methods: • Federal eRulemaking Portal: https://www.regulations.gov/ (our preferred method). Follow the online instructions for submitting comments. • E-mail: oppdocket@epa.gov. Include Docket ID No. EPA-HQ-OAR-2022-0007 in the subject line of the message. • Mail: U.S. Environmental Protection Agency, EPA Docket Center, Air and Radiation Docket, Mail Code 28221T, 1200 Pennsylvania Avenue NW, Washington, DC 20460. • Hand Delivery or Courier (by scheduled appointment only): EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution Avenue, NW, Washington, DC 20004. The Docket Center's hours of operations are 8:30 a.m.-4:30 p.m., Monday-Friday (except Federal Holidays). Instructions: All submissions received must include the Docket ID No. for this rulemaking. Comments received may be posted without change to https://www.regulations.gov/, including any personal information provided. For the full EPA public comment policy, information about CBI or multimedia submissions, and general guidance on making effective comments, please visit httys://www. eya. gov/dockets/commenting-eya-dockets. FOR MORE INFORMATON For further information about the rulemaking, contact Joann Rice at EPA's Office of Air Quality Planning and Standards at (919) 541-3372. ------- |