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Gasoline particulate filters (GPFs) are an important aftertreatment system that enables gasoline direct injection (GDI) engines to meet current emission standardsn note of GPFs may need to improonont accumulates on the GPF during engine operation. GPFs are often ‘pa during vehicle operation when the exhaust is sufficiently hot and it contains sufficient oxygen. This paper explores the effect that engine-out soot emissions and the frequency of GPF regeneration have on GPF filtration efficiency. Two GPF technologies were tested on two engine dynamometers as well as two production vehicles on a chassis dynamometer. The engines span a wide range of engine-out particle emissions (a range of almost one order of magnitude). The filtration efficiency of the GPFs were measured with a regulation-compliant particle number system (non-volatile particles > 23 nm), as well as with a particle counter with a lower cutoff of 2.5 nm, and with a differential mobility spectrometer.