Editorial: Atmospheric chemistry in the urban air

Urban atmospheric chemical processes directly affect air quality of megacities. Owing to the synergistic effect of various pollutants in urban atmosphere, urban air pollution is complex and puzzling. Sustainable urban development is being hindered and urban air quality is deteriorating. Moreover, with the fast urbanization and industrialization in many developing (e.g., (China) and developed countries, more and more people live in cities or city clusters. From the statistics by United Nations Department of Economic and Social Affairs, more than 50% of the world's population are living in urban areas, and the proportion is expected to increase to 68% by 2050. Urban air quality is crucial to human health, especially for people living in urban areas. The recent examples observed in various urban atmospheric environments especially in China have demonstrated the need to broaden the research field of urban atmospheric chemistry and refine more theoretical modeling mechanisms.