Diesel exhaust particles consist mainly of nanoparticles, the surfaces of which are covered with various organic chemicals such as polycyclic aromatic hydrocarbons (PAHs), some of which are known to be mutagenic or carcinogenic. Because some of these chemicals are volatile or semi-volatile, the fact that a differential mobility analyzer (DMA) operates at normal ambient pressure represents an advantage over many impactor-type samplers, which operate at half atmospheric pressure or less. In this study, we used twin custom-made DMAs as a nanoparticle sampler, and increased the sampling flow rate for each DMA separately. The sampler was used to sample ambient aerosol particulate matter (PM) at the side of a road carrying heavy traffic over a 4-day period. The average sizes of the aerosol particles collected were 80 and 240 nm. The PAHs on these particles were collected on quartz fiber filters and measured by direct-injection thermal-desorption gas chromatography/mass spectrometry. Twelve PAHs (3- to 6-ring PAHs), including benzo(a)pyrene, were analyzed quantitatively. The nanoparticles collected by the DMA sampler were richer in 5- to 6-ring PAHs than PM2.5 particles sampled in parallel. Scanning electron microscopy of the nanoparticles deposited on the DMA electrodes showed that the 240-nm particles (as classified by the DMA) were agglomerates of soot particles with a unit size of around 50nm or less, whereas the 80-nm particles consisted of single nanoparticles or agglomerates of a few particles.