The hazy pall is the product of large numbers of fine solid and liquid particles drifting in the atmosphere. These aerosols scatter or absorb sunlight depending on the composition and shape of the particles. Landscape colors that would normally be vibrant and distinct become an opaque shade of gray. The white patches in the image are clouds.
The aerosol particles have a variety of sources, but biomass burning plays an important role in Bangladesh, where a significant portion of the population relies on traditional fuels such as wood, straw, and dung for cooking and for heating homes. These organic fuels deliver a heavy load of particles to the atmosphere because people tend to burn them at relatively cool temperatures; this leads to incomplete combustion.
Vehicle traffic and industry also contribute to the winter haze over Bangladesh. The brick industry, which mainly operates in the winter, is a major source of particles because most brick makers in the region rely on inefficient fixed-chimney kilns rather than cleaner-burning Hoffman or zig-zag kilns. According to one study based on data from ground-based air quality sensors, about one third of the fine particulate pollution comes from motor vehicles, one third from brick kilns, and the rest from winds blowing dry soil and road dust.
While brick production and heating needs increase significantly during the winter, meteorological conditions also play a role in building the haze. During the rest of the year, heavy rains help wash pollution particles from the air; but the winter months tends to be dry. Winds are also weaker in the winter, which leads to less movement and convection in the atmosphere.
Neighboring India has been building coal-fired power plants at a rapid rate in recent years. The Ozone Monitoring Instrument (OMI) on the Aura satellite, for instance, observed 60 percent increases in sulfur dioxide emissions from power plants in India between 2005 and 2012. Though a gas, sulfur dioxide reacts with other substances in the atmosphere to produce sulfate particles that contribute to the gray pall over Bangladesh.