ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-6-4801-2006 On the time-averaging of ultrafine particle number size spectra in vehicular plumes Yao X. H. 1 Lau N. T. 1 Fang M. 1 Chan C. K. 2 Institute for the Environment, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, People's Republic of China Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, People's Republic of China 24 10 2006 6 12 4801 4807 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/6/4801/2006/acp-6-4801-2006.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/6/4801/2006/acp-6-4801-2006.pdf

Ultrafine vehicular particle (<100 nm) number size distributions presented in the literature are mostly averages of long scan-time (~30 s or more) spectra mainly due to the non-availability of commercial instruments that can measure particle distributions in the <10 nm to 100 nm range faster than 30 s even though individual researchers have built faster (1–2.5 s) scanning instruments. With the introduction of the Engine Exhaust Particle Sizer (EEPS) in 2004, high time-resolution (1 full 32-channel spectrum per second) particle size distribution data become possible and allow atmospheric researchers to study the characteristics of ultrafine vehicular particles in rapidly and perhaps randomly varying high concentration environments such as roadside, on-road and tunnel. In this study, particle size distributions in these environments were found to vary as rapidly as one second frequently. This poses the question on the generality of using averages of long scan-time spectra for dynamic and/or mechanistic studies in rapidly and perhaps randomly varying high concentration environments. One-second EEPS data taken at roadside, on roads and in tunnels by a mobile platform are time-averaged to yield 5, 10, 30 and 120 s distributions to answer this question.

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