Karen Sentoff

Particle Number and Size Distribution of Emissions During Light-Duty Vehicle Cold Start: Data from the Total Onboard Tailpipe Emissions Measurement System

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.

Second-by-Second Characterization of Cold-Start Gas-Phase and Air Toxic Emissions from a Light-Duty Vehicle

Tailpipe pollutants from motor vehicles are linked to environmental concerns and human health issues. Gasoline engine ignition produces a significant portion of trip emissions, but few studies have quantified mobile source air toxic (MSAT) species for light-duty vehicles during cold start. Real-world tailpipe emissions were measured from a 1999 Toyota Sienna minivan with the University of Vermont total onboard tailpipe emissions measurement system. A Fourier transform infrared spectrometer measured 27 gas-phase emissions for cold start, extended idle, and warm-up driving at 1-s temporal resolution. Analysis demonstrated that (a) time to optimal function of emissions control devices was not indicated by one species, but varied for different pollutants; (b) extended idling after cold start produced elevated emissions for MSAT species as compared to warm-up driving; and (c) ambient temperatures ranging from 9.5°C to 38.4°C affected species from each emission category, with the exception of carbon dioxide. Carbon monoxide produced peak emissions three orders of magnitude higher than hot-stabilized conditions for an average of 90 s, regardless of operating conditions, while nitric oxide peak emissions were over an order of magnitude higher during warm-up driving than extended idle. Peak MSAT emissions, up to two orders of magnitude higher than hot-stabilized idle, were maintained or increased during extended idle and decreased to baseline within 100 to 200 s of warm-up driving. Results indicate extended idling after cold starts prolongs elevated concentrations of MSAT emissions, suggesting that recent policy efforts to reduce vehicle idling behavior could limit potential human exposure to the toxic exhaust constituents.

Impacts of Model Resolution on Transportation Network Criticality Rankings

Objective rankings of the criticality of transportation network infrastructure are essential for efficiently allocating limited adaptation resources and must account for network connectivity and travel demand. Road link criticality can be quantified by the total travel delay caused when the capacity of a road segment or link is disrupted or removed. These methods can use standard travel demand models, but the exclusion of lower-volume roads and the aggregate nature of traffic analysis zones may distort resulting criticality rankings. To test the impact of link exclusion and demand aggregation, the authors applied the network robustness index, a well-established link criticality measure, to a hypothetical network with varying levels of network resolution and demand aggregation. The results show a statistically significant change in criticality rankings when demand is aggregated and especially when links are excluded from the network, suggesting that criticality rankings may be distorted when estimated with typical demand models. Application to a road network in Vermont supports the finding on the impact of network resolution on criticality rankings.