Estimating the reduction of urban PM10 concentrations by trees within an environmental information system for planners

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Abstract

Trees have been widely quoted as effective scavengers of both gaseous and particulate pollutants from the atmosphere. Recent work on the deposition of urban aerosols onto woodland allows the effect of tree planting strategies on airborne aerosol concentrations to be quantified and considered within the planning process. By identifying the potential planting locations in the local authority area, and applying them within a dispersion and deposition model, the potential magnitude of reduction in the ambient concentration of PM10, achievable through urban tree planting, has been quantified for two UK cities. As part of the Environmental Information Systems for Planners (EISP), flow diagrams, based on planning decisions, have incorporated output from the model to make decisions on land use planning ranging from development plans and strategic planning, to development control. In this way, for any new developments that contribute to the local PM10 level, the mitigation by planting trees can be assessed, and in some cases, reductions can be sufficient to meet air quality objectives for PM10.

Introduction

The Air Quality Framework Directive (Directive 96/62/EC) was adopted across the European Union in 1996. The framework was established to set targets on air quality and emissions of 12 pollutants (SO2, NO2, particulate matter, lead, carbon monoxide, benzene, ozone, PAHs, cadmium, arsenic, nickel and mercury). The UK framework, the Air Quality Strategy for England, Scotland and Northern Ireland (Defra, 2000) has set objectives on eight of these pollutants for protecting human health (benzene, 1,3-butadiene, carbon monoxide, lead, NO2, ozone, particulate matter, and SO2). Seven of these eight have been adopted for the purposes of Local Air Quality Management (LAQM), Table 1. Ozone has been omitted from LAQM as it is considered a transboundary pollutant and not applicable to local level objectives. LAQM is the responsibility of local authorities, and it is their statutory duty to assess and review air quality and make sure that the national air quality objectives will be achieved in their area. Furthermore, the land use planning system is integral in improving air quality. Local authorities must consider the links between air quality and land use policy whether it be strategic planning in the form of development plans or planning applications in the form of development control.

In a collaborative effort, funded by the UK Natural Environment Research Council (NERC) and the Office of the Deputy Prime Minister (ODPM), through its URGENT research programme (Urban Regeneration and the Environment), a demonstration version of an Environmental Information System for Planners (EISP) has been developed to provide guidance for local authorities, taking into account the planning processes, legislation and planning guidance (Culshaw et al., 2006).

Under the principles of sustainable development there is a requirement to protect the environment as well as respecting environmental limits (DETR, 1999). The planning system plays a key role in sustainable development as it determines the location of new development and guides land use strategies. To help achieve this there is a recently identified need for decision systems that link the planning framework with the relevant scientific knowledge and environmental data. The EISP is a valuable online resource for assisting in the decisions making of planners, and at the same time provides practical and current environmental information, models, and data (e.g. air quality guidelines, groundwater pollution maps and flood risk maps and models).

This paper concentrates on the air quality module of the EISP system and in particular focuses on particulates. From 1997–2002, 25–55% of the EU urban population was potentially exposed to ambient air concentrations of PM10 in excess of the EU 24 h mean objective set to protect human health (EEA, 2005). In the UK, current health guidelines relate to the mass concentration contained in particles with an aerodynamic diameter of 10 μm or less (so-called PM10), as a proxy for the aerosol fraction that can be respired. The full organization and development of the EISP, the other modules, GIS and browser technology, have been described in the final NERC report on EISP to the Office of the Deputy Prime Minister (Culshaw et al., 2006). However, examples of the web browser interface used in the air quality module are shown in this paper.

Particulates are one of the 7 pollutants covered under LAQM and have varying effects on human health. People with pre-existing respiratory and/or cardiac disorders are at most risk of acute effects from exposure to particles (COMEAP, 1995). High levels of PM10 cause increased breathing difficulties in people with asthma, chronic bronchitis, emphysema and other lung conditions. They may also cause premature death in older people with heart and lung disease. No threshold concentration has been observed below which the association of PM10 concentrations and human health effects breaks down. For this reason, PM10 is now thought to be the most important of the commonly occurring air pollutants. (British Lung Foundation Factsheet).

Particles are generated from primary or secondary sources:

  • Primary sources are carbon particles from the incomplete combustion of fuel, mining, quarrying, and from brake and tyre wear in motor vehicles. Particles from combustion sources are generally less than 2.5 μm diameter and often less than 1μm in size, however, particles generated from mechanical processes (mining quarrying) are generally greater than 2.5 μm in size. A certain amount of particulate matter is emitted naturally, for example wind blown dust and sea salt, and biological particles such as pollen and fungal spores. These naturally occurring particles also often exceed 2.5 μm in size.

  • Secondary particles are formed in the atmosphere by chemical reaction or the condensation of gases, and sulphate and nitrate aerosols. Particles from these sources are less than 2.5 μm in size.

The Expert Panel on Air Quality Standards (EPAQS), the UK government's independent advisory group on air quality standards, has yet to establish an air quality limit for PM2.5. Historically in the UK, only PM10 have been measured and monitored as an air quality standard. There is strong evidence to suggest that particles less than 2.5 μm are of a greater human health concern than coarser particles. There are a limited number of European studies into the impacts of PM2.5 with many of the studies being carried out in the USA. Due to this fact EPAQS have decided that there is not yet enough European studies to base a European standard for PM2.5, and currently PM10 provides a good basis for monitoring both components. As a consequence the model currently only focuses on PM10 and does not differentiate between the two size classes. Further work on the model could be carried out either through scaling components by ratio or principally, when more direct measurements on PM2.5 have been carried out.

Particles are either deposited through precipitation (via rain or snow), a process known as wet deposition, or they are deposited directly to surfaces, including vegetation, through a process known as dry deposition. The rate of dry deposition varies greatly with particle size with larger particles having high deposition velocities as the efficiency of impaction increases (Slinn, 1982). Trees generally have large leaf areas and their aerodynamical roughness promotes vertical transport. As a result trees are effective scavengers of both gaseous and particulate pollutants from the atmosphere, with dry deposition rates to forest exceeding those to grassland by typically a factor of 3–20 (Gallagher et al., 2002; Fowler et al., 2004). This implies that the conversion of grassland and other smooth surfaces to trees can be used to promote the removal of particulates from the atmosphere.

Work carried out by Nowak and Crane (2000) using the Urban Forest Effects (UFORE) Model predicted air quality improvement in New York, through pollution removal by trees, to be 0.47% for PM10 based on a daytime in-leaf season average. However, where tree cover was 100%, reductions of 13% were estimated over a period of one hour.

Within the air quality module of the EISP, the Local Air Quality Management (LAQM) objectives or limits for PM10, have been used as the primary constraint (or test). Once this primary constraint has been triggered the user works through a series of questions, until the end of the decision flow is reached. Within this decision flow a model has been incorporated that provides a tool for showing the ameliorating effect on high PM10 concentrations (e.g. from new industrial processes) by planting trees across the whole of the local authority area.

Section snippets

Overview

The model used for decision support on the air quality modules of the EISP is based on lookup tables of the % reduction achievable for each 1 km grid cell if a certain fraction of the available area within the local authority were planted up with trees. This available area is expressed as the ‘future planting potential’ or FPP. This information was combined with decision flow diagrams to enable planners to examine the possibility of reducing PM10 concentrations to within the air quality limits

Wolverhampton City Council

Fig. 6 shows the modelled PM10 concentrations for the Wolverhampton city area. The concentration map shows that areas in the south-east corner of the council domain have the highest PM10 concentrations. These are due to the large amount of industrial processes, particularly iron and steel works, and main arterial roads (edge of M6 corridor) in this area. In contrast, the west of the council domain has lower concentrations due to the dominance of residential areas and little industry.

Fig. 7

Benefits

The Air Quality Framework Directive for Europe aims to improve and protect ambient air quality, and protect people's health and the environment from the adverse effects of air pollution.

By incorporating the modelled results into a series of flow diagrams planners can investigate and quantify the visible benefits of planting trees to reduce PM10 concentrations. Since local authorities in the UK are required to review air quality through Local Air Quality Management (LAQM), and the corresponding

Conclusions

Modelling the capture of trees across two UK local authority domains has shown that trees are capable of reducing PM10 concentrations across the whole domain. Reductions of 7–20% can be achieved, but at a cost of planting huge numbers of trees. Appreciative reductions also depend on the availability of suitable planting areas. For example, built up or industrial areas, which are often absent of suitable planting area, are where reductions in PM10 are principally needed. However, smaller

Acknowledgements

The work described in this paper was jointly funded by the UK government's Department of Transport, Local Government and the Regions (DTLR) and the Natural Environmental Research Council (NERC) through its Urban Regeneration and the Environment (URGENT) Programme. The paper is published with the permission of the Executive Director of the British Geological Survey (NERC) and the Director of the Centre for Ecology and Hydrology (NERC). We would like to thank Phil Hickey, Paul Mellon and Lorraine

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