During the construction and evaluation of a sampler for measurement and characterization fine particulate matter (PM) emissions from stationary combustion equipment, several technical challenges were noted. The sampler design incorporated dilution, cooling and moisture addition to the stack gas inside an inert dilution tunnel to closely simulate near-ambient conditions to promote atmospheric transformation of source particles. The automated and on-line process control capabilities of the system allows for simulation of a range of ambient-like humidity and temperature conditions for PM sampling, while providing reproducible particulate mass emission results. Subsequent analyses of the size segregated PM2.5, PM10 and total particulate samples yield concentrations of particle mass, carbons, acidic species and trace elements. The first-generation sampling system was applied on a 150-kW oil-fired boiler and a 0.7-MWth coal-fired pilot scale boiler to provide source PM characteristic profiles. Challenges noted during the initial studies included (a) difficulty in achieving optimum dilution and residence time while sustaining isokinetic sampling for high flue gas velocities; (b) inaccuracies in measurement and control of sampling system flow rates to maintain a balanced flow system; and (c) particle depositions in several system components. A second-generation system was later constructed that provided a higher dilution up to 80-fold and the extended residence times up to 80 s. Reliable measurement and control of the gas flow rates were achieved using a CO, tracer technique. Samplings from the combustion units with stack velocities ranging from 3 to 10 m/s were successfully performed. For PM measurement on boilers with a stack velocity higher than 10 m/s, a flue pre-separator or splitter is required. The sampler's overall design is being further modified for additional improvements including a suitable splitter design. This paper focuses primarily on the technical issues relating to source PM sampling equipment, while initial PM2.5 mass emission results from the combustion of a No. 4 fuel oil are reported simply to illustrate its capabilities. (C) 2004 Elsevier B.V All rights reserved.