Fig 1. Singapore’s Marina Bay shrouded in haze.

Haze is a concentration of smoke, dust, moisture, and vapour suspended in air that is enough to impair visibility. Haze pollution can be said to be “transboundary” if its density and extent is so great at source that it remains at measurable levels after crossing into another country’s air space.

Singapore has been experiencing severe transboundary haze pollution from as far back as the 1960’s due to forest fires in Indonesia. More recently in 2015, prolonged dry weather conditions led to escalations in hotspot activities and the region experienced unprecedented severity and massive geographical spread of the smoke haze affecting various countries, including Singapore, Malaysia, Thailand, the southern part of Philippines and the northern part of Laos. Millions of people were affected by the haze, which included 19 deaths in Indonesia.

Fig 2. Fireman putting out forest fire in Indonesia

The forest fires in Indonesia are caused by corporations as well as small-scale farmers that practice slash-and-burn to clear vegetation for palm oil, pulp and paper plantations. Once lit, the fires often spiral out of control and spread into protected forested areas and peat. Peat is accumulation of partially decayed vegetation or organic matter found in natural areas such as peatlands.

Fig 3. Smouldering peatland fire.

Peat consists of high carbon content and once ignited by a heat source it smoulder, which means it burns but with no flame. These smouldering fires may be undetected for very long periods of time, creeping through the underground peat layer. These fires are especially hard to put out and are a significant contribution to haze.

The annual haze at its largest can reach up to hundreds of kilometres across. The haze can be made worse by dry seasons, changes in wind direction and poor precipitation. Haze is also more pronounced under more sunlight, which is abundant in the region, as sunlight accelerates photochemical reactions.

Haze pollution originating from large-scale forest and land fires is characterised by a high concentration of particulate matter, which, among other effects, reduces visibility. Due to the specific emission characteristics of land and forest fires, haze is predominately made of very fine particles with a diameter of less than 10 mm. While coarse particles flush out of the atmosphere within several hours up to a day, fine particles have the longest residence time (up to weeks) in the atmosphere and travel extensive distances (hundreds to thousands of kilometres). Their elimination out of the atmosphere is mainly due to rain.