Abstract:The US Clean Air Act Amendments require PM10 transportation conformity and attainment demonstrations. This study examines the policy implications and validity of a proposed PM10 transportation conformity method called equivalent emissions (EE) that uses a linear, non-chemical model to incorporate emissions trading into PM10 transportation conformity determinations. We evaluate the new method by comparing predictions from EE to predictions from a mechanistic air quality model that uses non-linear chemical mechanisms to calculate the formation of secondary PM10. Results indicate that the EE method over estimates reductions of secondary PM10 formation allowing the primary fraction to rise while secondary PM10 is not actually declining in the atmosphere. Thus, conformity could be established between air quality and transportation plans using EE, resulting in projects being funded that might prolong public exposure to unhealthy levels of PM10 depending on the specifics of the non-attainment area.