"SimCity 4" enables you to build a realistic city from the ground up, with all the growth and vibrancy of a real city -- and all the smog too, if you aren't careful. Like real cities, your city can end up plagued by air pollution, which makes your city a less desirable place to live. Through careful management of your city's industries, education, green spaces and laws, however, you can reduce your air pollution.
Isolating "Dirty Industries"
"Dirty industries" are an eventuality of your city as it grows; some industries are simply required to get the city on its feet, even those that make use of heavy smokestacks and contribute to pollution. Eventually these industrial buildings can be replaced with high-tech ones that produce dramatically less pollution, but until then you'll have to make do. The best way to deal with dirty industries is to cluster them in an isolated area, somewhere near the outskirts of your city. This will reduce the amount of air pollution reaching the general populace, at least until high-tech industry is available.
Investing in Education
Investing heavily in education is one of the best ways to reduce pollution. A well-educated public will produce high-tech industries faster, and the sooner high-tech industries are deployed to replace mid-tech or dirty industries, the better. A good education system also makes your city more attractive to outside Sims, which can increase your population and growth. Investing in education is generally win-win, but you may need to have higher taxes in order to see faster results.
Increasing Parks and Trees
Green spaces in your city can reduce pollution while also increasing the happiness of the population, beautifying your city and making it a more desirable place to live. The more parks and trees your city has, the lower its air pollution. However, green spaces do reduce the amount of space available for commercial, residential and industrial areas, which may damage your city's growth. Plan your parks according to what your city can reasonably sustain without incurring too many losses.
Reducing Traffic and Waste
Reducing both traffic and landfill use can also have positive effects on your city's air pollution. City-wide recycling centers and recycling programs can greatly reduce waste, meaning you need to maintain fewer landfills and have less pollution, while also generating some income. By employing mass transit systems, you can reduce the number of cars on the road and thus the amount of pollution they put out.
Issuing City Ordinances
Part of governing your city is the ability to issue ordinances, which can have both positive and negative effects on your city. There are a number of ordinances that can help reduce air pollution both directly and indirectly. The "Clean Air Act" directly reduces pollution, but also reduces industrial growth. "Energy Conservation" causes your city to use less energy, which means less strain on energy-producing industries like coal, but it also reduces commercial and industrial growth. The "Pro-Reading Campaign" increases the education level of Sims beyond school, which increases the development rate of high-tech industry.