A Review of Characteristics, Causes, and Formation Mechanisms of Haze in Southeast Asia

Haze is a well-known air pollution phenomenon linked to the severe and persistent particulate matter (PM) episodes in Southeast Asia (SEA), which significantly impacts the environment, health, and economy. This work reviewed for the first time the characteristics of haze episodes in terms of PM concentrations, chemical compositions, and the causes of haze in both Lower (Maritime) and Upper (Mainland) SEA. In addition, we carried out a systematic comparison of the frequency and intensity of haze events through SEA regions in recent years. Our finding indicated that the different trend of haze frequency and intensity between SEA cities are not only due to local air pollution sources such as biomass burning (BB) but also meteorology and long-range transport. Other sources such as secondary aerosols also play an important role in haze  formation, but they have not been comprehensively investigated in previous studies. Due to the complicated formation mechanisms and the transportations of haze and its impacts on SEA’s human health and economy, more sophisticated and specific policies are needed to deal with haze issues not only for individual countries but also on a regional scale.