Exposure to ultrafine particles from traffic in city streets and the urban atmosphere

Mass-based emission controls (such as Euro - I to IV) are successfully reducing emissions of fine particles from road vehicles, but may have actually increased the emission of ultrafine particles and their persistence in the atmosphere just at a time when growing evidence indicates that these ultrafine particles present the greatest threat to health upon inhalation. These particles have so little mass that they barely register in measurements made by the current urban air quality monitoring network, which measures the mass of particulate matter (PM₁₀). Ultrafine particles also have different dynamic and transport properties to larger particles, leading to more variability in concentrations that may rise and fall independently of PM ₁₀. Current daily and hourly monitoring of PM₁₀ for the purposes of Air Quality Management fails to represent the wide variation and episodicity in exposure of an urban population to the threat from traffic-sourced ultrafine particles. In order to quantify and interpret this variability, data from experiments employing sophisticated, high-resolution instrumentation in UK cities will be presented. This data will illustrate the variability on time scales from minutes to hours of ultrafine particle exposure on busy streets, and will indicate how meteorological factors and urban topography determine the exposure to traffic particle emissions in the surrounding urban environment. Such information has important consequences for the assessment of future emission reduction and air quality improvement strategies, especially localised strategies.