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A study of the sources of variability in particulate measurements using the Heavy-Duty Transient Test (40 CFR Subpart N) has been conducted. It consisted of several phases: a critical examination of the test procedures, visits to representative facilities to compare and contrast facility designs and test procedures, and development of a simplified model of the systems and procedures used for the Heavy-Duty Transient Test. Some of the sources of variability include; thermophoretic deposition of particulate matter onto walls of the sampling system followed by subsequent reentrainment in an unpredictable manner, the influence of dilution and cooling upon the soluble organic fraction, inconsistency among laboratories in the engine and dynamometer control strategies, and errors in measurements of flows into and out of the secondary dilution tunnel.

The effects of fuel sulfur content and dilution conditions on diesel engine PM number emissions have been researched extensively through steady state testing. Most results show that the concentration of nuclei-mode particles emitted increases with fuel sulfur content. A few studies further observed that fuel sulfur content has little effect on the emissions of heavily-used engines. It has also been found that primary dilution conditions can have a large impact on the size and number distribution of the nuclei-mode particles. These effects, however, have not yet been fully understood through transient testing, the method used by governments worldwide to certify engines and regulate emissions, and a means of experimentation which generates realistic conditions of on-road vehicles by varying the load and speed of the engine.

The effectiveness of a centralized Inspection/Maintenance (I/M) program, implemented in a major U.S. metropolitan area in 1991, is directly measured through ambient CO monitor data. A multi-factoral analysis is developed which quantifies effects due to the interaction of hourly traffic levels with wind vector and ambient temperature conditions, allowing a better measure of I/M effectiveness. The time-trend of the measured CO levels is seen to closely match those predicted by the analysis throughout the 7 year study period. An average ambient CO reduction of 1.3 ± 1.4 percent attributable to I/M is measured, with individual results of +1.5 percent, +5.8 percent, and -3.4 percent obtained for the three monitor locations studied.