A methodology for quantifying PM2.5 emissions from refinery catalytic cracking operations during routine operations as well as during the emission events was developed. PM2.5 samples collected on filters were obtained from four continuous ambient monitoring sites in the greater Houston,TX, area, i.e., Clinton Drive, Channelview, East Houston, and Kingwood. Rare earth element (REE) relative abundance sequence, enrichment factor analysis and ratios of La to light REE (Ce, Pr, Nd, and Sm) indicated that REE enrichment in the Houston atmosphere could be predominantly attributed to catalytic cracking operations. Enrichment factors of these elements in ambient PM2.5 during the emission event with respect to FCC catalyst samples were all close to unity when normalized by Nd. In all cases, excellent correlation coefficients were obtained with near zero intercept values. FCC emission contributed only 0.10-0.15 μg/cu m to PM2.5 levels during routing operation. This is an abstract of a paper presented at the 101st AWMA Annual Conference and Exhibition (Portland, OR 6/24-27/2008).