This report is part of a project to characterize cotton gin emissions from the standpoint of stack sampling. In 2006, the Environmental Protection Agency (EPA) finalized and published a more stringent standard for particulate matter with nominal diameter less than or equal to 2.5 µm (PM2.5). This created an urgent need to collect additional cotton gin emissions data to address current regulatory issues, because current EPA AP-42 cotton gin PM2.5 emission factors did not exist. The objective of this study was the development of PM2.5 emission factors for cotton gin master trash systems based on the EPA-approved stack sampling methodology, Method 201A. The project plan included sampling seven cotton gins across the Cotton Belt. Five of the seven gins had master trash systems. In terms of capacity, the five gins were typical of the industry, averaging 37.1 bales/h during testing. Some test runs were excluded from the test averages because they failed to meet EPA Method 201A test criteria. Also, other test runs, included in the analyses, had cotton lint fibers that collected in the ≤ 10 µm and/or ≤ 2.5 µm samples. This larger lint material can impact the reported emissions data, but EPA Method 201A does not suggest methods to account for these anomalies. Average measured master trash system PM2.5 emission factor based on the five tests (13 total test runs) was 0.0042 kg/227-kg bale (0.0093 lb/500-lb bale). The master trash system average emission factors for PM10 and total particulate were 0.036 kg/bale (0.080 lb/bale) and 0.143 kg/bale (0.314 lb/bale), respectively. The master trash system PM2.5 emission rate from test averages ranged from 0.081 to 0.33 kg/h (0.18-0.73 lb/h). System average PM10 emission factors were higher and system average total particulate emission factors were lower than those currently published in EPA AP-42. The ratios of master trash system PM2.5 to total particulate, PM2.5 to PM10, and PM10 to total particulate were 2.9, 11.5, and 25.5%, respectively.