Phase 1 of the Advanced Collaborative Emissions Study (ACES) included detailed chemical characterization of exhaust species emitted from four 2007 model-year heavy heavyduty diesel engines (HHDDE) manufactured by Caterpillar, Cummins, Detroit Diesel, and Volvo. The work started in March, 2007, and was completed in August, 2008. ACES Phase 1 objectives were: (1) To quantify the reduction in both regulated and unregulated emissions from advanced diesel engines; (2) To provide regulated and unregulated emissions data for this new engine technology; (3) Make data available to support the selection of one engine for the ACES Phase 3 health study; and (4) To provide initial guidance for the ACES Phase 3 health study using the regulated and unregulated emissions information from ACES Phase 1. Based on the results of the exhaust emissions characterization, one of the four ACES engines, Engine B, was selected for the ACES Phase 3 health study to be conducted at Lovelace Respiratory Research Institute (LRRI). The engine selection process is described in a Health Effects Institute (HEI) Report, included in Appendix A. An additional backup engine to the one selected for the health study, Engine B, was provided by the engine manufacturer and was tested for regulated emissions at a barometric pressure of 99.3 kPa, representing SwRI's elevation in San Antonio, Texas, and at a simulated barometric pressure of 82.6 kPa, representing LRRIs higher elevation in Albuquerque, New Mexico. The high elevation simulation was performed on the backup engine only.