In the rapidly industrializing countries, the major air pollution problem has typically been high levels of smoke and SO2 arising from the combustion of fossil fuels for domestic and industrial purposes. In both developed and developing countries, the major threat to clean air is now posed by traffic emissions, especially in the swiftly motorizing megacities of East Asia and Southeast Asia (cities with population of 10 million or more). Petrol and diesel based motor vehicles, coal combustion in the industrial boilers and power plants emit a wide variety of pollutants, viz., Sulfur Dioxide (SO2), Carbon Monoxide (CO), Nitrogen Oxides (NOx), Volatile Organic Compounds (VOC’s) and particulates, which have an alarming impact on urban and regional air quality causing considerable damage to human health and environmental balance in and around the cities. Enhanced amounts of acid rain, and photochemical activity are observed in the urban centers with substantial potential to impact both local and regional climate. Significant efforts have been made during recent years to improve the urban air quality in Asia through collective efforts between government and non-government agencies, and international organizations. Curtailing the already substantial damage to environment and human health and avoiding much heavier damages in the future will require a through understanding of the urban pollution characteristics. Since, much of the improvements are likely to effect the urban environments first, understanding their influence on overall air quality is of utmost significance. Presently, the urban air pollution problems are continuing to increase and air pollutants originating from urban regions are recognized to be significantly influencing the regional and global air quality. Increasing density of industries and vehicle population in urban Asia is constantly contributing to the long-range transport of pollutants such as ozone (O3) and its precursors; especially, the mobile sources rich in NOx and VOC emissions. Understanding the sensitivity of O3 production to NOx and VOC mixing ratios originating from megacities is important, because future choices need to be made as to whether the focus of pollution control efforts should be on reducing NOx, or reducing VOC emissions. Not only NOx and VOC, but the interactive nature of various pollutants and their combined effect on urban and regional air quality needs to be studied before any choices can be made. Measurements from the TRACE-P and AceAsia field experiment obtained during the period of March-April, 2001 provided means to evaluate the impact of megacity emissions on the regional air quality at various levels. Although, the measurements themselves weren’t enough to distinguish between the urban and regional signals, with the help of sensitivity runs performed using a three-dimensional atmospheric chemistry model, we were able to identify and evaluate urban characteristics. In this paper, we exclusively focus on (1) understanding the impact of large urban sources in Asia on photochemistry in the cities and at downwind locations, (2) evaluating the contribution of megacities to the long range transport of O3 and its precursors, and (3) explaining the chemical and physical evolution of Asian megacity pollution during TRACE-P and AceAsia field experiment. This is achieved through quantification of megacity pollution and its influence on regional photochemistry, and characterization of the megacity pollution using observations and atmospheric modeling results.