Remarkable dynamics of nanoparticles in the urban atmosphere

M Dall'Osto,P I Williams,D C S Beddows,R M Harrison,A Thorpe,H Coe,T Dunbar,J F Barlow

doi:10.5194/acp-11-6623-2011

Abstract

Abstract. Nanoparticles emitted from road traffic are the largest source of respiratory exposure for the general public living in urban areas. It has been suggested that adverse health effects of airborne particles may scale with airborne particle number, which if correct, focuses attention on the nanoparticle (less than 100 nm) size range which dominates the number count in urban areas. Urban measurements of particle size distributions have tended to show a broadly similar pattern dominated by a mode centred on 20–30 nm diameter emitted by diesel engine exhaust. In this paper we report the results of measurements of particle number concentration and size distribution made in a major London park as well as on the BT Tower, 160 m aloft. These measurements taken during the REPARTEE project (Regents Park and BT Tower experiment) show a remarkable shift in particle size distributions with major losses of the smallest particle class as particles are advected away from the traffic source. In the Park, the traffic related mode at 20–30 nm diameter is much reduced with a new mode at <10 nm. Size distribution measurements also revealed higher number concentrations of sub-50 nm particles at the BT Tower during days affected by higher turbulence as determined by Doppler Lidar measurements and are indicative of loss of nanoparticles from air aged during less turbulent conditions. These results are suggestive of nanoparticle loss by evaporation, rather than coagulation processes. The results have major implications for understanding the impacts of traffic-generated particulate matter on human health.

Highlights

Urban air pollution is one of the environmental problems of major concern and will, due to growing urbanization, probably become increasingly important in the future
During the REPARTEE field campaigns held for four weeks in both October 2006 and October/November 2007, measurements of particle number concentrations were made using condensation particle counters, and number size distributions using Scanning Mobility Particle Sizers deployed at locations in central London
Recent work (Harrison and Yin, 2008) has shown that elemental carbon is an excellent tracer of diesel exhaust emissions and it is a species with a relatively long atmospheric lifetime which is a useful tracer of ground-level urban emissions

Summary

Introduction

Urban air pollution is one of the environmental problems of major concern and will, due to growing urbanization, probably become increasingly important in the future. Particulate matter is one of the key urban pollutants and much attention has focussed on the smaller size fractions. The size of airborne particles is heavily influenced by the multiplicity of sources and processes which lead to their emission and formation, and by the material from which the particles were formed. Current regulations address the amount of ambient particulate matter (PM) as PM10 (Dp ≤ 10 μm) and PM2.5 (Dp ≤ 2.5 μm) expressed as a mass concentration of particles, and not particle number concentrations. It has been shown that in urban environments, the smallest particles make the highest contribution to the total particle number concentrations, but only a small contribution to particle volume or mass (Harrison et al, 2000). Recent toxicological studies have suggested that ultrafine particles, i.e., the fraction of fine particles below

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