The particle size distribution, volatility, and effective density of particulate matter emitted from a homogeneous charge compression ignition (HCCI) engine fueled by port injected compressed natural gas were measured and compared to emissions emitted from the same engine during motoring and spark ignition for two compression ratios. The particle concentration and geometric mean diameter were greater at high compression ratio, and also, the total particulate mass was lower for spark ignition and homogeneous charge compression ignition than during motoring. Volatility tests showed that all operating conditions have less than 5% of the particulate matter remaining when denuding the sample at 100 degrees C. Effective density measurements also show that the particles for each operating mode have a relatively constant density with respect to particle size (approximately 850 kg/m(3)). (C) 2014 Elsevier Ltd. All rights reserved.