Urban air quality
– A significant threat to human health?
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Urban air pollution poses a significant threat to human health, the environment and the quality of life of millions of people in some of the world’s largest cities. (e.g., New Delhi, Hong Kong, Beijing, Jakarta, Los Angeles and Mexico City). |
On the other hand, such countries still face the problem of polluted air even though air quality has been improving gradually over the last two decades. For example, many large cities in Europe still exceed the specific air quality standards for ambient pollutants. The Helsinki metropolitan area in Finland, for example, is one of the cleanest cities in Europe but still the daily limits are exceeded every year.
How can air pollutants affect human health?
A number of epidemiological studies have shown that ambient air pollutants, especially fine and ultra fine particulates, have an adverse effect on human health. This is due to the fact that these tiny particulates can penetrate deep into the lungs and even be dissolved in the blood. The health effects can be especially hazardous for sensitive groups, like children, senior citizens, and people with asthma, obstructive lung disease (chronic obstructive pulmonary disease - COPD), and cardiovascular diseases. According to the World Bank, every year an estimated 800,000 people die prematurely from lung cancer, cardiovascular and respiratory diseases caused by outdoor air pollution. In Hong Kong, for example, it is estimated that by improving the air quality from the existing “average” level to “good” level, 64,000 hospital days would be saved.
What are the main pollutants in urban air?
Scientists have defined the main air pollutants that pose a health risk for humans in urban areas. These pollutants are: inhalable particulates (PM10) and fine particulates (PM2.5), nitrogen oxides (NOx), sulphur dioxide (SO2), ozone (O3), carbon monoxide (CO), and lead (Pd). For each of these pollutants, the World Health Organization has defined air quality guidelines for protection against harmful health effects. The guidelines define the specific limits or concentrations that should not be exceeded. Countries often have their own standards that are similar but the specific levels can vary slightly.
To measure air quality, networks of monitors record the concentrations of the major pollutants at more than 3,000 locations in the world each day, mostly in developed countries. These raw measurements are then converted into Air Quality Index (AQI) values. The AQI has been developed to make it easier for people to understand the significance of monitored and forecast air pollution levels. It focuses on health effects that can take place within a few hours or days after breathing polluted air.
An AQI value is calculated for each of the individual pollutants in an area (ground-level particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and ozone). The pollutant with the highest AQI is used as the overall AQI reading for that day and is listed as the principal pollutant.
What are the main sources of emission?
Most air pollution comes from human activities: burning either fossil fuels (coal, oil or natural gas) to power motor vehicles and industrial processes, or burning of biomass to heat houses or produce energy. The burning processes release particulate matter, carbon monoxide, carbon dioxide (CO2) and partially burnt hydrocarbon compounds into the atmosphere. Depending on the origin of the fuel, SO2 concentrations released in combustion vary considerably. Technologies to remove it in energy production are available and already in common use in most developed countries. (Note: carbon dioxide is not counted as a pollutant but the main greenhouse gas.)
Pollution from traffic consists of particulate matter, NOx, CO, volatile organic compounds (VOCs), and also other compounds in small amounts, like polyaromatic hydrocarbons (PAH). Lead emission from traffic has reduced dramatically after moving to unleaded fuels. Particulates come from the exhaust emissions, especially from diesel engines, but also from dust and dirt from roads and tires. Other fine particulates are formed by chemical reactions in the atmosphere. Formation of ozone (O3) in urban areas is mainly caused by traffic pollutants in a photochemical reaction with UV radiation from the sun.
Some of the pollutants also come from natural sources. For example, forest fires emit particulates and VOCs into the atmosphere. Volcanoes spew out sulfur dioxide and large amounts of volcanic ash. Sand storms can also mix particulates into the air from the underlying ground surface. Vegetation emits VOCs that are different from those formed in burning processes.
How does weather affect air quality?
Changes in day-to-day weather are the greatest factor in controlling the changes in air quality. Weather determines how quickly pollutants are dispersed away from an area and also the thickness of the atmospheric layer (called the mixing height) where the emissions are diluted in a vertical direction. Normally, pollutants are mixed with the help of convection or blown away by wind from their sources without building up to unsafe amounts. But the pollutants can also be trapped close to the surface. They cannot escape, and after a while they may build up to unhealthy levels. This trapping can happen when winds are weak or calm, and when the air close to the ground cools down and warm air moves over, creating a stable condition that keeps pollutants close to the surface. These so-called inversions frequently occur on clear nights.
Poor air quality can also result from summer heat. In hot, sunny weather photochemical smog can form through complex chemical reactions involving NOx, O3 and VOCs. Photochemical smog has a light brown color and can reduce visibility and affect breathing.
Wind can also transport pollution over long distances. For example, in the Helsinki metropolitan area, more than half of the fine particulates (PM2.5) are transported from outside the country, mainly from Central and Eastern Europe.
Air pollutions are finally washed out of the sky by rain, fog or snow. This is called “wet deposition”. When air pollutions deposit without the benefit of rain, it is called “dry deposition”.
How can air quality be estimated and forecast?
Air quality can today be forecast for a few days based on the weather forecast and a corresponding “chemical transport” model. Unfortunately the normal weather forecasts do not fully serve the needs of air quality forecasting. Measuring and forecasting atmospheric conditions is critical for understanding the formation, transformation, dispersion, transport, and removal of the pollutants. More specific data on atmospheric conditions is needed for better forecasts. One of the tasks is to get more dense and accurate observation data on the mixing height across urban areas to better determine the air volume where the surface pollutants are mixed.
Modern air quality models can take a variety of data as input: inventory and models of emission sources, local dispersion and mixing, weather model and forecast, long-range transportation of pollution, chemical reactions and deposition.
What does Vaisala offer?
The major needs for atmospheric weather parameters for air quality forecasting include both surface and vertical profile measurements. The main parameters are wind speed and direction, turbulence, vertical wind and temperature profiles (to determine the layered atmospheric air structure), visibility, cloud height and cover, rain, and solar radiation.
Vaisala can offer the latest technology for the measurement of atmospheric weather conditions relevant to the air quality estimation, dispersion and forecasting. The instruments include: radiosondes, boundary layer profilers, ceilometers, wind sensors and a number of surface weather sensors and stations.
Vaisala radiosondes offer the highest level of temperature, humidity and wind measurement performance for synoptic soundings all over the world. There are also special sensor options to measure ozone and radioactivity profiles in the atmosphere.
Vaisala’s boundary layer profiler uses radio frequency (RF) Doppler technique to provide continuous data on wind speed and direction at different altitudes up to several kilometers. With an optional acoustic system (Radio Acoustic Sounding System, RASS), it can also measure the virtual temperature profile that can be used to determine stability conditions. The RF signal (intensity and spectrum) can also be used to determine the mixing height as turbulent structure at the top of the mixed layer provides higher reflectivity and a broader spectrum for the RF signal.
Measurements near the surface level are also essential to estimate, for example, mixing and transport of surface emissions. Vaisala offers a number of instruments to measure the principal weather parameters, like wind, temperature, humidity, pressure and rain. All these parameters are also packed into a single instrument, the Vaisala Weather Transmitter WXT510, which offers a compact solution for the purpose. In some areas, visibility is used as an indicator for the air quality. Vaisala has an optical visibility sensor suitable for this purpose.
Ceilometers are designed for the measurement of cloud height and cover utilizing optical lidar technique (light detection and ranging). These devices, however, have also turned out to be outstanding for air quality measurement purposes due to the principle of measurement. They measure optical scattering of aerosols and ice crystals to see the clouds. As the performance and sensitivity has increased over the years, they can also measure the low levels of pollutant aerosols - particulate matter. This can be used to determine the real profile and mixing height of the aerosols. The Vaisala Ceilometer CL31 has been successfully used for this purpose to provide important continuous data on the hour-to-hour and day-to-day variation of the aerosol profile. In addition to supplying a variety of instruments, Vaisala can also offer weather observation network solutions and services to support the needs for air quality monitoring and forecasting.
Vaisala has built a dense observation network in the Helsinki metropolitan area in collaboration with the Finnish Meteorological Institute. Its goal is to serve the purpose of air quality measurement, among many other applications. This Helsinki Testbed observation network (http:\\testbed.fmi.fi) is based on a number of mast measurements using Vaisala Weather Transmitters WXT510 at several altitudes. In addition to wind profile information near the surface, the vertical mixing conditions of the atmosphere can also be calculated from the vertical temperature gradient. The dense network also includes five Vaisala ceilometers that measure the aerosol profile and mixing height. The aim is to use this dense data set for air quality modeling to improve the real-time estimation and forecasting of spatial and temporal variations in air quality.
Author: Hannu Talvitie, Vaisala, Helsinki, Finland