Environmental Problems Caused by Local Air Pollution
A wide range of compounds can cause local air pollution. The emissions of sulfur dioxide, nitrogen oxides, carbon monoxide, fine particulate matter, organic compounds like benzene, toluene and poly-aromatic hydrocarbons (PAH), and heavy metals in particulate matter (lead, cadmium) can cause local concentrations to reach levels which are harmful to human health.
Some compounds like nitrogen oxide and volatile organic compounds cause air pollution problems in stagnant air, as the reactions between these compounds form ozone and other oxidants. Ozone is at present (2006), together with particulate matter, the most serious pollutant in cities in developed countries. It is also becoming very problematic in developing countries.
|Figure 1. Average pollutant concentrations in Dutch cities: red (upper line in 1965): sulfur dioxide; black (middle line in 1965): black smoke; and blue (lowest line in 1965): nitrogen dioxide.|
The earliest local pollution problems were caused by very high sulfur dioxide concentrations in and around industrial and residential areas, such as the London smog disaster of 1952. For more information, see also Impacts of air pollution on local to global scale. Concentrations of 300 to 500 microgram/m3 were typical in European cities 40 years ago. Most problems with sulfur dioxide have been eliminated in developed countries, but very high sulfur dioxide concentrations are still encountered in developing countries, where sulfur-rich coal is used for energy production. Figure 1 reflects these changes.
In developed countries, sulfur dioxide concentrations were brought down by replacing coal with cleaner-burning oil or low-sulfur natural gas in the production of energy or by controlling the emissions with sulfur dioxide scrubbers. The black smoke emissions were reduced by using more efficient burners in energy production and through the installation of particulate control systems such as electrostatic precipitators and filtration systems, among other things. The nitrogen oxide emissions from burners and automobiles were also drastically reduced, but the enormous increase in energy production and transportation has compensated for these lower emission factors.
|Figure 2. The contributions of different origins of air pollution.|
A problem is the differentiation between the contribution of local pollution and pollution of regional origin. In Figure 2, contributions from different sources are represented schematically. In stagnant weather conditions, when regional or inter-regional transport is not taking place, local sources are responsible. Examples of local pollution are the smog in Los Angeles, California and the ozone in Athens, Greece. However, in many cases, a combination of different sources has an impact. For example, five years ago, ozone was attributed to local sources, both in Hong Kong and Guangzhou, China. Recent research has clearly shown that ozone in these locations is often of regional origin. These findings have greatly influenced abatement measures; local measures will reduce the ozone concentrations less than expected if nitrogen oxides and volatile organic compounds are of regional origin.
For at least three decades, research has been carried out to find a connection between human health and air pollution. It became clear, around 10 years ago, that such a connection exists. Improvement of inadequate measurement methods for air quality, and especially advances in epidemiological methodology (the study of statistical behavior of illness), played an important role in determining the link between air quality and public health.
Two epidemiological methods are used for this purpose, namely, time series investigations and large Cohort studies:
- The principle of the time series method is based on comparing the actual mortality with the mortality calculated by way of the known impact of different diseases, accidents, etc. Typically, a small fraction of a few percent is left that cannot be explained this way. This fraction is correlated with different possible factors, and a good correlation with air pollution is found, while no other known factor can give a satisfactory explanation. More detailed investigations show, at least for Europe and the United States, that no correlation is found with, e.g., sulfur dioxide concentrations, but a statistically significant correlation is found with concentrations of fine particles, PM-10 or PM-2.5, and/or ozone.
- The other epidemiological method used to trace the impact of air pollution is the Cohort Method. In the Cohort Method, a large group of people is characterized and divided into groups according to the parameters under investigation. Groups can be formed, e.g., as smoker/nonsmoker, living near road/living further away, etc. Next, mortality and the frequency of diseases are estimated as a function of these parameters, comparing the different groups. Statistical techniques can "tease apart" different parameters and determine their influence on the outcome. Cohort studies are needed to quantify the decrease in lifespan caused by ozone and aerosols (e.g., a large Dutch Cohort Study estimated this decrease in lifespan to be one to three years), but also to discriminate between different factors.
Health Effects of Ozone
The formation of ozone can take several to 24 hours, depending on temperature, amount of received solar radiation, and relative humidity. Episodes of stagnant air are quite often encountered in cities such as Mexico City, Athens, and Barcelona. Under these stagnant conditions, local emissions can lead to high ozone concentrations which can affect human health and damage vegetation. In other cases, as encountered in the Netherlands but also in, for example, the Guangdong Province of China, transport by wind and hence exchange of air generally takes place. Ozone is mainly a regional problem in these areas.
High ozone peak values have serious effects on human health, and air quality standards are based on prevention of impact on human health. Damage to crops is also a severe problem but is more dependent on long-term exposure. An overview of ozone standards and the ozone concentration during a period of stagnant weather in Guangzhou, China are given in Figure 3.
Figure 3. European air quality standards for ozone and ozone concentrations, 8 hours average, in Guangzhou, China.
A preliminary estimate (RIVM) shows that in the Netherlands, 1,900 deaths of 16 million inhabitants are caused by ozone pollution. The impact of ozone on vegetation, trees, and agricultural crops, is well-documented. The damage from ozone to vegetation starts as soon the ozone concentrations exceed 40 ppb. The sum of total of hourly concentrations over 40 ppb is expressed as AOT-40 and damage is expected at values over 3,000 to 4,000.
Figure 5. Ozone exposure expressed as AOT-40 for Europe, 2003.
From Figure 5, it is clear that ozone is a very serious problem in southern Europe, as it is in many areas in the U.S. and in developing countries.
Health Effects of Particulate Matter
Epidemiological research has established that particle pollution has a large impact on human health. An overview of mass, number, and lifetime of a typical city aerosol is given in Figure 6. For general background information on particles, see Aerosols; for details about emissions of particles, see Air pollution emissions.
The large number of very small particles is caused by high-temperature processes. Recently, transportation emissions have increased greatly. Otto (cars with ignition) and Diesel engines cause great emissions of very small particles with very little mass and short lifetimes, because they coagulate very quickly. Aerosol particles between 0.2 and 2 micron present a large fraction of the mass; they are present in smaller numbers than the small particles, but they have relatively long lifetimes. Large particles are very low in number, have large mass, and short lifetimes; they “fall” out of the atmosphere, a process called deposition.
The epidemiological research has not yet revealed, with certainty, which fraction of aerosol, PM-10 (mass of aerosol particles with a diameter < 10 ?m), PM-2.5 (mass of aerosol with a diameter < 2.5 ?m) or ultra-fine particles or specific compounds in aerosol particles are responsible for the observed health effects. Many indicators point in the direction of ultra-fine particles.
Figure 6. Number of aerosols per cm3 in green, right axis; mass in red, left axis; and lifetime in hours, striped bars, right axis. All axes are logarithmic.
The effect of aerosols on human health has been traced in different locations. Increased mortality as a result of this pollution is detected everywhere. Initial studies found that the largest effects were observed in “clean areas,” such as the U.S. The mortality trends also showed a gradient going from Western to Eastern Europe.
In more recent studies, the same sensitivities for aerosol have been found, independent of location. The extra mortality is roughly the same in the U.S., Western and Eastern Europe, and China with 2–4% more deaths with an increase in concentration of 100 ?g*m-3.
Black smoke is a proxy (can be taken as indicative) for transportation emissions. The effect seems to be linear and no effect threshold has been found.
Time series epidemiological research indicates that the increased death rate is 2% per 100 ?g*m-3, for the Netherlands. This means, at the Dutch ambient yearly PM-10 level of 32 ?g*m-3, that an additional 1,700 to 3,000 acute deaths per year can be attributed to aerosols, of a population of 16 million. It is much more difficult to assess the chronic effects of exposure to aerosols and the estimates of these chronic effects are quite uncertain, but could be, according to a large Dutch Cohort Study, in the order of 10,000 to 15,000 extra deaths per year for the Netherlands. In comparison, the number of deaths attributed to traffic accidents in the Netherlands is about 1,300 per year.
In this large Dutch Cohort Study, groups have been sorted out as a function of living near or further away from roads with heavy traffic. Emissions from traffic clearly generate large gradients in aerosol number concentrations. However, the extra mass contribution to PM-2.5 or PM-10, but also of soot (indicated as EC, elemental carbon), as a function of the distance from roads, is limited.
The results of this Cohort Study indicate that extra mortality (number of deaths per given number of inhabitants) due to cardiopulmonary diseases is twice as high near roads in comparison to locations further away from roads. The implication is that Dutch "background" aerosol concentrations have a clear impact on human health, but that this risk is twice as high near major roads.
The results of this Dutch Cohort Study indicate that PM-2.5 or ultra-fine particles (aerosol with a diameter < 0.1 ?m) could have a large impact on human health. Very small particles are retained to a large extent in human lungs while those with the main mass (0.1 to 2 ?m) are not very effectively absorbed. It is clear that extra mortality due to aerosols in, for example, Chinese cities could be quite high. Average PM-2.5 concentrations in large Chinese cities are probably between 50 and 120 ?g*m-3.
If the same increased mortality as found in Europe is assumed, and linear extrapolation is made (which are both quite uncertain assumptions but no sufficient data are available to make better estimates), then an extra mortality of 5,000 to 10,000 due to acute effects and 20,000 to 50,000 due to chronic effects per year could be expected for a city like Beijing, China. Apart from this extra mortality, a large number of extra cases of non-mortal illnesses, such as asthma and bronchitis as well as heart diseases, can be predicted.
Figure 7. Shortening of lifespan due to aerosols in Europe.
It is obvious that the impact of aerosols on human health will result in a large economic loss. As a result, air quality standards were introduced five years ago in Europe (yearly average PM-10 of 40 ?g*m-3) and in the U.S. (annual PM-2.5 standard of 15 ?g*m-3) and measures are being taken to meet these standards. The fact that no mechanism has been found to explain the effects of aerosols, and that correct measurements of mass and chemical composition of aerosols pose many problems, are severe hindrances to formulating and implementing effective air quality standards for aerosols. However, these new findings of the impact of aerosols, and especially the important role of transportation emissions, are already leading to changes in the development of road infrastructure, among other things, in Europe.
Recently, overviews have been compiled to analyze the impact of aerosols (PM-2.5) on human health, in terms of decreased lifespan in months, in Europe (Clean Air for Europe Report). In Figure 7, the rather large impact of aerosols on diminished life expectancy (dark areas) in, for example, the Netherlands, is clear.
The wide-ranging impacts of particulate matter are still greatly uncertain, and as long as the mechanisms of the impact on human health are not known, it will be difficult to reach absolute certainty. However, preliminary results are quite serious and it is important to determine what these effects could mean in cities of developing nations, where the concentrations of fine particulates are up to 10 times higher than in developed nations.
Visibility and Aerosols
|Figure 8. Interaction between aerosols and incoming solar radiation.|
|Figure 9. Connection between aerosol mass and visibility, in km.|
Aerosols have a large impact on visibility, as we see objects by their reflected light. Aerosols of a certain size, between 0.1 and 1.5 ?m in diameter, scatter visible light. Not only is the amount of light emitted by the object we are looking diminished due to the presence of aerosols, but the sunlight that is scattered by the particles forms a background haze that also limits visibility. Two types of scattering, "Mie" and "Rayleigh" scattering are observed. Rayleigh scattering is scattering that occurs primarily as a result of gas molecules, limiting the horizontal visibility. It is a function of the wavelength of the light and limits the visibility to approximately 130-260 km (80-160 miles) depending on the color of the light. If the wavelength of the incoming light and the size of a particle are about the same, the light can be scattered in all directions, including backward (back scatter). This scattering by particles is referred to as Mie scatter (see Figure 8).
Particles between 0.1 and 1.5 micron, with about the same diameter as the wavelength of incoming solar visible light, scatter the visible light and reduce visibility. In addition, particles can also absorb visible light (such as carbon-based particles). Some light may pass through the particles or aerosols if no absorption takes place. Very small particles have minimum influence on the light. For a given size distribution of particles, a relation can be developed between the visibility and the aerosol mass as shown in Figure 9.
In Figure 9, the uncertainty is given by the gray band. The main source of this uncertainty is variation in size distribution, which can vary greatly in different locations. Thus, one can derive the aerosol concentration from visibility only with rather large uncertainty.
Reduction of visibility is a pollution problem, especially in those locations which are known for their scenic beauty. In mega-cities, such as Mexico City or Beijing, China, visibility can be reduced to a few hundred meters due to the impact of aerosols.