Nowadays, it can be seen very rare news on discussion about air pollution. Very high levels of air pollution have been observed in many major cities across both developed and developing economies of the world cities with varying degrees of regularity. In particular, the chemistry aspects has a major responsibility to play both of informing the development of cost-effective mitigation strategies and understanding the causes of high air pollutant concentrations. The chemical reactions of air pollutants can transform from one to another (secondary pollutants), which have a main parameter on measured levels and the influence of emerging emission sources and changing climate on air quality and health. We should understand the relative contributions of local emissions and the pollutants transported to the city from more distant sources. Figure 1. Evolution of megacities, showing percentage urban and urban agglomeration by size (adapted from the UN World Urbanization Prospect, 2018 rev...