A pragmatic mass closure model for airborne particulate matter at urban background and roadside sites
Introduction
Airborne particulate matter continues to give rise to concern as a result of its, now well established, adverse effects on human health. Whilst some doubt still exists as to which particle metric (e.g. number, surface area or mass) or size fraction is most closely related to the adverse health outcomes (Harrison and Yin, 2000), current air quality standards in both Europe and North America are expressed in terms of mass concentration and therefore control policies are directed at reducing the mass concentration of particles in the atmosphere.
The formulation of cost-effective abatement strategies for airborne particulate matter is crucially dependent on knowledge of the contributions of individual source categories to airborne concentrations. Such information may be gained from deterministic models, but such models suffer from many weaknesses including poor information on the magnitude of certain particle source categories (e.g. resuspension). The alternative is to use receptor models and there has been some success in the apportionment of both particle mass and specific components of airborne particles to specific source categories using models based on multivariate statistics (Thurston and Spengler, 1985; Chan et al., 1999; Harrison et al., 1996). However, much can be learnt from simpler models based purely on analysis of major chemical components; e.g. in the UK, the Airborne Particles Expert Group (APEG, 1999) used a three component model of atmospheric particles to construct projections of the influence of abatement policies on future airborne concentrations of PM10. The components in the model were primary combustion particles (mainly from road traffic), secondary sulphates and nitrates and coarse particles, including sodium chloride and resuspended soils and road dusts. While such a model is clearly a major over-simplification, it does bring considerable clarification to what is otherwise a very complex problem. Taking the three component model as a starting point, Turnbull and Harrison (2000) devised a slightly more sophisticated multiple regression model using analytical data for sulphate, nitrate, chloride and black smoke (as a marker for combustion aerosol), which accounted for a large proportion of the variance in the mass data.
The aim of the currently described work is to extend and improve those simple models. To date, aerosol chemistry and mass closure models have typically analysed a large proportion of the elements of the periodic table in an attempt to gain as complete a knowledge as possible of the chemical composition (Young et al., 1994; Chan et al., 1997; Kim et al., 2000; Andrews et al., 2000; Chow et al., 2002); however such an approach is too complex to be applicable on a routine basis and despite very extensive chemical analyses has generally failed to account for the full gravimetrically determined mass of particles, an outcome which has been attributed to the presence of unanalysed strongly bound water. In this work we have, therefore, deliberately restricted ourselves to the analysis of a small number of components which can be used as tracers of major aerosol constituents, in order to provide a simple but effective model. In all, some seven chemical components are analysed and the mass reconstructed in a semi-deterministic manner, rather than purely by regression analysis. The method is therefore a hybrid between the comprehensive chemical analysis method on the one hand, and the simpler statistical procedures on the other.
Section snippets
Air sampling
Sampling was conducted at four pairs of sites, three in London and one in Birmingham. Each site pair is known by the area in which it is located, i.e. High Holborn, Elephant and Castle, and Park Lane in London, and Selly Oak in Birmingham. Each site pair comprises a roadside location close to a heavily trafficked highway and a background location within 1 km of the roadside location but at least 50 m from any busy highway. The difference between the roadside and background locations may be taken
Results and discussion
As explained in the introduction, the aim of this work was to provide a much simplified protocol for chemical analysis of airborne particles which nonetheless took account of the major aerosol components. Table 1 shows the species which were analysed, and the range of concentrations encountered. The reasons for selecting these analytes, and the adjustment factors to the concentrations are detailed below.
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Sulphate: This is one of the major secondary components of airborne particles and at inland
Conclusions
Any chemical model of atmospheric aerosol risks being incomplete even when a very wide range of constituents is analysed, and there are problems in converting such analyses to mass since the elements oxygen and hydrogen are not analysed and have to be inferred. Additionally a chemical analysis of water, as opposed to indirect estimation of water content by other means is very difficult and therefore scarcely ever conducted. Therefore a comprehensive analysis, including a wide range of trace
Acknowledgements
The authors are grateful to the UK Department of Transport for financial support of the work through the TRAMAQ programme, and to Roger Barrowcliffe and Alex Newton of ERM who collaborated in organisational aspects of the work and the air sampling.
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