Particle Number and Size Distribution of Emissions during Light-Duty Vehicle Cold Start

Abstract

Vehicle emissions during cold start are known to be significantly higher than after optimal vehicle operating temperatures are reached. There are limited data, however, on particle number and size distributions during cold start. Cold-start tailpipe emissions from a 1999 Toyota Sienna minivan were quantified at ambient temperatures between 20°C and 37°C using a novel system, the total onboard tailpipe emissions measurement system (TOTEMS), assembled to quantify the full suite of exhaust emissions from light-duty vehicles. TOTEMS particle number distributions were measured from 5.6 to 562 nm using an engine exhaust particle sizer (EEPS) and total 3- to 3,000-nm particle counts were measured using an ultrafine condensation particle counter (UCPC) with 1-s temporal resolution during cold start and warm-up driving. Second-by-second particle number distributions from five cold-start emissions tests showed similar particle emissions patterns, allowing for three different cold-start phases to be identified based on particle number emissions behavior. Cold-start duration ranged from 165 to 230 s and increased with decreasing ambient temperature. Different particle sizes during each phase were emitted for different lengths of time, with the most abundant particles in the nanoparticle (diameter <50 nm) range. The mean particle number distributions showed more than 99% of total particle number below 100 nm. Concentrations of ultrafine particles (<100 nm) during cold start were at least 10 to 100 times (EEPS), and as much as 1,000 times (UCPC), above hot-stabilized idle emissions. Observations also suggest the presence of tiny particles below 6 nm during cold start.