Understanding the Southeast Asian haze

Abstract

The Southeast Asian region had been subjected to a drastic reduction in air quality from the biomass burnings that occurred in 2013 and 2015. The smoke from the biomass burnings covered the entire region including Brunei, Indonesia, Malaysia, Singapore and Thailand, with haze particulate matter (PM) reducing the air quality to hazardous levels. Here we report a comprehensive size–composition–morphology characterization of the PM collected from an urban site in Singapore during the two haze events. The two haze events were a result of biomass burning and occurred in two different geographical source regions. We show the similarities and variations of particle size distribution during hazy and clear days during the two haze events. Sub-micron particles (<1 μm) dominate (∼50%) the aerosols in the atmosphere during clear and hazy days. Using electron microscopy, we also categorize the PM, namely soot, organic–inorganic clusters and hybrid particles. The composition and morphology were similar in both the haze events. The majority of the PM is composed of carbon (∼51%) and other elements pertaining to the earth’s crust. The complexity of the mixing state of the PM is highlighted and the role of the capture mode is addressed. We also present the morphological characterization of all the classified PM. The box counting method is used to determine the fractal dimensions of the PM, and the dimensionality varied for every classification from 1.79 to 1.88. We also report the complexities of particles and inconsistencies in the existing approaches to understand them.