Haze formation in China: Importance of secondary aerosol

Air pollution is the world's largest single environmental hazard that causes more than a few million premature deaths in 2012 (World Health Organization, 2014), particularly in developing countries with rapid industrialization and urbanization. Rapid economic growth of China in the last three decades has resulted in serious air pollution problems on both local and regional scales. Megacities in China such as Beijing and Shanghai have suffered from haze episodes frequently with the daily mass concentrations of fine particulate matter (PM2.5, fine particulates with aerodynamic diameter less than 2.5 μm) over the Chinese air pollution standard of 75 μg/m3 (China National Environmental Monitoring Centre, 2013), which is three times higher than the air quality guideline of 25 μg/m3 recommended by the World Health Organization, highlighting the urgency of urban PM mitigation in China.