Aggressive drive cycles can cause substantial discrepancies between particulate matter (PM) measurement methods and increased emissions variability. Previous work demonstrated good agreement, within similar to 10%, between aerosol instruments and gravimetric determinations of PM mass emissions from gasoline direct injection engine (GDI) vehicles run over the Federal Test Procedure (FTP). In contrast, the present study reveals discrepancies of a factor of three or more for these vehicles run over the US06 portion of the supplemental FTP test. Two aspects of this are examined: 1) Changes in particle composition and morphology and 2) variability associated with vehicle history and test preparation. PM emissions during the US06 cycle are often accompanied by a strong nucleation mode. The organic to elemental carbon ratio increases relative to the FTP cycle. Also, the black carbon to elemental carbon ratio decreases, suggesting the formation of brown soot. The origins of these changes are not entirely clear, but are likely associated with the high engine exhaust temperatures during the US06 cycle. This also contributes to the higher test to test variability observed for the US06 versus FTP cycle. High temperatures can thermally desorb and pyrolyze various materials, including heavy hydrocarbons, ash, and inorganic salts deposited in the exhaust pipe, catalytic converter, and muffler that provide an additional PM source not present during FTP driving. The first day's PM mass emissions are often substantially higher than on following days for the US06 cycle, but not the FTP. There is also a distinct drop in PM emissions between first and subsequent US06 cycles run in series.