**5. Conclusion**

In the chapter, we described the current and emerging technologies and strategies that are being used or proposed to control air pollutants. Control technologies and strategies could be classified into upstream and downstream controls. Upstream technologies and strategies are usually being used to control air pollution emitted from mobile sources such as transportation, agriculture, and construction activities. Downstream technologies are usually applied to control particulate matter and gaseous pollutants emitted from stationary sources. The upstream control exhibited more advantages concerning to cost and efficiency than the downstream control.

The air pollutants including SOx and Pb emitted from transportation activities could be significantly reduced by controlling the precursor pollutants in raw materials. The NOx, CO, and hydrocarbon emissions from transportation activity could be reduced by the application of catalytic converters and the control of lubricant consumption. The strategies to reduce motorized transportation demand and improve the road quality and traffic flow decrease the energy consumption demand, leading to decrease in air pollution emission. The strategies to control livestock feeding, animal housing systems, manure storage systems, application of manure for crops and application of fertilizers and pesticides significantly reduced the emission of methane (CH4), nitrous oxide (N2O), and ammonia (NH3). The strategies to control construction activities including control of site planning, construction traffic, demolition works, and site activities significantly reduced the emission of particulate matter.

To control particulate matter pollutants, the technologies including cyclone, wet scrubber, electrostatic precipitators, and fabric filter have been efficiency applied. Among them, electrostatic precipitator is the most advantageous method and widely applied in the United States. The traditional methods such as adsorption, absorption, condensation, incineration to control air pollutants emitted from stationary sources exhibited many certain disadvantages including high cost and efficiency. Therefore, they should to be modified, enhanced, or replaced by modern technologies such as biological and photocatalytic technologies.
