Section 109(d)(1) of the CAA requires that the Agency periodically review and revise, as appropriate, the air quality criteria and the NAAQS for the six ``criteria'' air pollutants, including particulate matter. The EPA Office of Air and Radiation requested AAMMS advice on monitoring issues related to urban visibility.
In October 2006, EPA issued the final rule to revise both the primary and secondary NAAQS for PM (71 FR 61144) and retain PM10 as the indicator for thoracic coarse particles. The secondary standards for PM2.5 and PM10 were set identical to the respective primary standards to address PM-related welfare effects including visibility impairment, effects on vegetation and ecosystems, materials damage and soiling, and effects on climate change. The Agency is considering development of a specific indicator for urban visibility in support of a future secondary NAAQS. CASAC has been asked to provide advice on proposed options for an urban visibility indicator, and any methods and equipment for its direct measurement.