Air Quality in Urban Environments
By R. E. Hester, R. M. Harrison
The Royal Society of Chemistry Copyright © 2009 Royal Society of Chemistry
All rights reserved.
Urban Air Pollution Climates throughout the World
OLE HERTEL AND MICHAEL EVAN GOODSITE
The extent of the urban area, the local emission density, and the temporal pattern in the releases govern the local contribution to air pollution levels in urban environments. However, meteorological conditions also heavily affect the actual pollution levels as they govern the dispersion conditions as well as the transport in and out of the city area. The building obstacles play a crucial role in causing generally high pollutant levels in the urban environment, especially inside street canyons where the canyon vortex flow governs the pollution distribution. Of the pollutants dominating urban air pollution climates, particulate pollution in general together with gaseous and particulate polycyclic aromatic hydrocarbons (PAHs) and heavy metals are those where further field measurements, characterization and laboratory studies are urgently needed in order to fully assess the health impact on the urban population and provide the right basis for future urban air pollution management.
In addition to other adverse health effects, air pollution is estimated to cause about 2 million premature deaths worldwide annually. In this context particulate matter (PM) is generally believed to be the most hazardous of ambient pollutants, and it has been estimated that reducing ambient air concentrations of PM10 from 70 to 20 µg m-3 would lower the number of air quality related deaths by approximately 15%. More than half of the world's population reside in cities, where the highest air pollution exposure and associated negative health impact take place. Furthermore, the projections for the next 50 years indicate that the worldwide urban population will increase by two thirds. Urban air pollution has been increasing in major cities, especially those found in developing countries (such as in: Brazil, Russia, India, Indonesia and China) as a result of rapid urbanisation. The cost to society of the associated health effects is significant and has been estimated to be approximately 2% of the Gross Domestic Product (GDP) in developed countries and 5% of GDP in developing countries (www.unep.org/urban_environment/issues/urban_air.asp). There may also be associated losses in productivity.
1.1 Emission and Formation of Urban Air Pollution
Urban air pollution arises from the competition between emission processes which increase pollutant concentrations, and dispersion, advection and deposition processes that reduce and remove them. This chapter describes the differences in local urban pollutant levels between cities worldwide, and outlines how these differences in pollution levels reflect differences in emission densities and emission patterns, but also in pollutant dispersion and removal processes. The impact on pollution levels of the dispersion and removal processes are governed by the local meteorological conditions, which also vary heavily with the physical location of the city. Air pollution concentrations in an urban environment are naturally the result of local emissions as well as contributions from pollution transport from more remote sources (see Figure 1). The size of the city domain and the density of pollutant emissions govern the local contribution to urban air pollution. Naturally, the temporal pattern in urban air pollution levels is a function of variations in the local releases, but just as important are the variations in the meteorological parameters that govern the dispersion and the pollutant transport in and out of the city.
Besides the influence from temporal variations in emissions and meteorology, the emission release height also plays an important role. Air pollution emitted from a high release height will in many cases be transported out of the urban area before being dispersed down to ground level; depending on the size of the urban domain. Urban industries, power plants and other sources for which the releases come from tall chimneys, contribute therefore only rarely to the local ground level air pollutant concentrations inside the urban area. These pollutant sources contribute primarily to the more regional air pollution.
Pollutant emissions related to vehicular transport, local domestic heating and smaller industries have low release heights [less than 10 m above ground level (a.g.l.)]. These releases are not diluted as efficiently as generally the case for emissions from tall release heights (more than 20 m a.g.l.). The contribution from "low" sources therefore often dominates the pollutant concentrations at ground level inside the urban area. A steady growth in vehicular transport and centralization of domestic heating have made road traffic the most important source of urban air pollution in many countries, including most industrialized nations.
In respect to local contribution from different sectors there are generally significant differences between developed and developing countries. A comparison of two so-called mega-cities (Beijing and Paris) showed that aerosol particles and volatile organic compounds (VOCs) have a complex and multi-combustion source in Beijing, whereas a single traffic pollution source completely dominates the urban atmospheric environment in Paris.
Indoor air quality is a major health concern. In the developing countries, emissions from household use of fossil fuels in the year 2000 was estimated to account for 1.6 million deaths, mainly among women and children in the poorest countries.
In the present paper we focus on ambient air quality and related impact on human health. The actual ambient air pollutant load greatly varies from one city to another, but, generally, major urban areas throughout the world have poor air quality, and, among these, the cities in the developing countries face the greatest challenges. WHO has compiled a survey on typical ranges in ambient air concentrations of four indicator pollutants, which are summarized in Table 1.
1.2 Urban Pollution Levels and Indicators
The highest urban air concentrations of the classic pollutants like PM10 and SO2 are found in Africa, Asia and Latin America, whereas the highest levels of secondary pollutants like O3 and NO2 are observed in Latin America and in some of the larger cities and urban air sheds in the developed countries. The environmental and human health impacts are particularly severe in cities of about 10 million or more inhabitants – also known as mega-cities. Urban air pollution has become one of the main environmental concerns in Asia, and especially in China where the pollution load in mega-cities like Beijing, Shanghai, Guangzhou, Shenzhen and Hong Kong is substantial. In these cities, between 10 and 30% of days exceed the so-called Grade-II national air quality standards by a factor of three to five times that of the WHO AQG (air quality grade). These cities have experienced a 10% growth in traffic each year over the last 5 to 6 years and, even with enhanced emission controls, NO2 and CO concentrations have remained almost constant over the same period of time.
Use of air quality indices (AQIs) are common tools in environmental management. A description of widely used indices and how they are expressed mathematically is given in Gurjar et al. AQIs can be designed to handle single or multi pollutants and may be used for comparing the loads in different cities or for describing the current load in relation to average loads or air quality standards and target values. In a multi component AQI (they applied the term MPI) ranging over mega-cities throughout the world, the highest MPI values were found for Dhaka, Beijing, Cairo and Karachi with values about double those of Delhi, Shanghai and Moscow (Figure 2).
2 Sources in the Vicinity of the City
Airports are usually located in the vicinity of larger cities and often mentioned as potential sources of high pollution loads in the urban areas. In recent years, several studies have thus been carried out to determine the potential impact of airport emissions. These studies generally point at an influence from the road traffic going to and from the airport, whereas the impact of aircraft emissions has been found to be very limited. In a study carried out in Frankfurt Airport, signals from specific aircraft emissions generally could not be identified, whereas emissions from vehicle traffic on surrounding motorways had measurable impact on the air quality. A study from Munich Airport had similar findings. A study inside Heathrow Airport has shown that between 5 and 30% of the local NOx contribution is related to aircraft, whereas the remaining 95 to 70% is from road traffic.
A recent study has indicated that ship traffic is responsible for about 60,000 lung cancer and cardiopulmonary deaths annually, but this outcome is linked to the contribution from ship emissions to the background PM load and not particularly related to the urban air quality. Harbours may be a local source contributing to urban pollution, but studies indicate that local road traffic often dominates the contribution from harbours. A study in the harbour of Aberdeen thus showed a gradient of increasing NO2 and soot concentrations from the harbour towards the city centre, indicating the contribution from the harbour had very limited impact on the local air quality in comparison with the emissions taking place in the urban environment.
Wood combustion in households is a growing concern in areas with many wood stoves that have relatively high local emissions of PM in comparison with other anthropogenic pollution sources. Investigations of wood combustion and air quality in developed countries like New Zealand, Sweden, USA and Denmark have documented that residential wood combustion may significantly elevate the local PM concentrations in outdoor air. As an example, emission inventories for Denmark point at wood combustion as the largest anthropogenic source of primary particle emissions.
3 Impact of the Geography, Topography and Meteorology
The location of the city has significant impact on the dispersion conditions, mainly since it affects the local meteorological conditions. The classical example is Los Angeles situated in a valley with frequent stagnant conditions during temperature inversions. The stagnant conditions lead generally to low wind speeds, and little air exchange between the valley and the surrounding areas. Hot and sunny climate and high emissions from traffic, industry and domestic heating makes the valley act like a large pollutant reaction chamber. This leads to high concentrations of photochemical products like ozone, nitrogen dioxide and peroxy acetyl nitrate (PAN).
A comparison of nitrogen oxide (NOx) levels in the street Via Senato in Milan, Italy and the street Jagtvej in Copenhagen, Denmark showed similar concentrations at the two sites despite much higher traffic in the street of Copenhagen. It was shown mainly to be a result of generally lower wind speeds in Milan compared with Copenhagen. High wind speeds and neutral conditions prevail in Copenhagen, whereas low wind speeds and stable or near stable conditions are frequent in Milan. Copenhagen has a cold coastal climate whereas Milan has a warm sub-tropical climate and the local wind conditions are furthermore affected by the location inside the Po Valley.
Some cities have characteristic wind systems as a result of local topography. An example of such effects is the rising air over a warm mountain side during daytime often leading to local formation of clouds and release of precipitation. During night the system turns around and the cooling of the air in the mountain valley leads to stable conditions that may cause local air pollution problems. The impact of Katabatic winds is another example, which affects cities along the Norwegian coast. The Katabatic winds are formed when cold air masses move down-slope (Katabatic is Greek for moving down hill) and meeting the colder snow and glacier covered areas, which then cool the air mass further, before the air ?oats out through a narrow cleft at the bottom of the hill. Usually the impact on air pollutant concentrations is moderate, but they may lead for example to high levels of local dust. Yet another example is the warm and dry Foehn wind formed on the back-side of a mountain chain, e.g. on the north side of the Alps. When the wind is forced over the mountain, the air is cooled and releases moisture. The air subsequently becomes warmer when it is moving downhill again. This system may then form an inversion and, e.g. reduce dispersion of local air pollutants.
The ambient temperature in the urban atmosphere of larger cities is generally a couple of degrees Celsius higher than that found in the surrounding rural areas. This feature is termed the urban heat island effect, and the explanation is that the city has a smaller albedo and therefore absorbs more energy compared with the surrounding rural areas. There is in addition a high consumption of energy inside the city, as a result of domestic heating and intense road traffic, which again contributes to release of heat. Finally, the buildings and other urban constructions form a shield for the wind, and this shielding leads to less cooling of the surfaces inside the city. Since the buildings act as heat reservoirs, the city has furthermore a less pronounced diurnal temperature variation compared with the rural area.
In calm weather, an urban circulation cell may be formed by warm air rising from the city. Some distance away, this heated air sinks and returns to the city at a low altitude. A similar phenomenon is known in coastal regions, where a sea breeze may be formed as a result of the temperature difference between the sea and land surfaces. A study in London showed that the heat island circulation over the city means that the wind speed is never below about 1 m s-1 (ref. 24). This study shows that the heat island effect is very important during low wind speed conditions in London where it may dominate the dispersion and thereby be limiting for the highest local air pollution concentrations, and this effect may thereby be the limiting factor for the highest pollution concentrations in the urban environment.
4 Pollutant Dispersion in Urban Streets (see also chapter by Salmond and McKendry)
Trafficked streets are air pollution hot spots in the urban environment (Figure 1). The concentration inside the urban street may be considered as the result of two contributions, one from emissions from the local traffic in the street itself and one from background pollution entering the street canyon from above roof level:
c = cb + cs
where c is the concentration in the street, cb the urban background contribution and cs the contribution from traffic inside the street itself. The background contribution furthermore arises from two contributions; the first of these is the contribution from nearby sources in the urban area (typically this will mainly be traffic in surrounding streets), and the other contribution consists of regional (sources within a distance of a few hundred km) and long range transported (sources placed up to thousands of km away) pollution.
Naturally, the pollutant levels in the urban streets are strongly affected by traffic emissions taking place inside the street itself. However, the concentration level and the distribution of air pollution inside the street are to a large extent governed by the surrounding physical conditions. These physical conditions heavily affect the wind speed and especially the wind direction inside the street. The special airflow generated inside the streets and around building obstacles may result in very different concentration levels at different locations in the street. The classical example is the street canyon vortex flow (Figure 3), which physically governs the pollutant distribution inside the street canyon. The street canyon is characterised by the presence of tall buildings on both sides of the street. Within the vortex flow relatively clean air from rooftop height is drawn down at the windward face of the street, across the road at street level, in the reverse of the wind direction at roof top, bringing pollutants in the road to the leeward face of the canyon. This results in pollution concentrations up to 10 times higher on the leeward side compared with the windward side of the street. (Continues...)
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