To prevent or reduce health problems associated with air pollutants, many countries set air quality standards. These standards prescribe the maximum average concentration levels in the ambient air, as allowed by law. Failure to satisfy these standards can result in fines, penalties, and increased government regulation.
In the USA, the standards are called National Ambient Air Quality Standards (NAAQS). In Canada, they are called Canadian Ambient Air Quality Standards (CAAQS). The European Union (EU) also sets Air Quality Standards. Other countries have similar names for such goals. Table 19–1 lists standards for a few countries. Governments can change these standards.
|Table 19-1. Air quality concentration standards for the USA (US), Canada (CAN), and The European Union (EU) for some of the commonly-regulated chemicals, as of Sep 2017. Concentrations represent averages over the time periods listed. For Canada, the CAAQS are changing over years 2015 -> 2020. Older Canadian National Ambient Air Quality Objectives (acceptable levels) are in grey.|
|Sulfur Dioxide (SO2)|
|1 yr||->5 ppb|
|1 day||125 µg m–3|
|3 h||1300 µg m–3 or 0.5 ppm|
|1 h||75 ppb||->70 ppb||350 µg m–3|
|Nitrogen Dioxide (NO2)|
|1 yr||100 µg m–3 or 53 ppb||53 ppb||40 µg m–3|
|1 h||100 ppb||213 ppb||200 µg m–3|
|Carbon Monoxide (CO)|
|8 h||10,000 µg m–3 or 9 ppm||13 ppm||10,000 µg m–3|
|1 h||40,000 µg m–3 or 35 ppm||31 ppm|
|8 h||0.070 ppm||63 -> 62 ppb||120 µg m–3|
|Particulates, diameter < 10 µm (PM10)|
|1 yr||70 µg m–3||40 µg m–3|
|1 day||150 µg m–3||120 µg m–3||50 µg m–3|
|Fine Particulates, diam. < 2.5 µm (PM2.5)|
|1 yr||12 µg m–3||10->8.8 µg m–3||25 µg m–3|
|1 day||35 µg m–3||28->27 µg m–3|
|1 yr||0.5 µg m–3|
|3 mo||0.15 µg m–3|
|1 yr||5 µg m–3|
|1 yr||6 ng m–3|
|1 yr||5 ng m–3|
|1 yr||20 ng m–3|
|PAH (Polycyclic Aromatic Hydrocarbons)|
|1 yr||1 ng m–3|
In theory, these average concentrations are not to be exceeded anywhere at ground level. In practice, meteorological events sometimes occur, such as light winds and shallow ABLs during anticyclonic conditions, that trap pollutants near the ground and cause concentration values to become undesirably large.
Also, temporary failures of air-pollution control measures at the source can cause excessive amounts of pollutants to be emitted. Regulations in some of the countries allow for a small number of concentration exceedances without penalty.
To avoid expensive errors during the design of new factories, smelters, or power plants, air pollution modeling is performed to determine the likely pollution concentration based on expected emission rates. Usually, the greatest concentrations happen near the source of pollutants. The procedures presented in this chapter illustrate how concentrations at receptors can be calculated from known emissions and weather conditions.
By comparing the predicted concentrations against the air quality standards of Table 18–1, engineers can modify the factory design as needed to ensure compliance with the law. Such modifications can include building taller smoke stacks, removing the pollutant from the stack effluent, changing fuels or raw materials, or utilizing different manufacturing or chemical processes.