Understanding Haze Formation and Its Impact on Air Pollution
Haze is the suspension in dry particles of salt, aerosols, dust, or photochemical smog atmosphere, which are so small (having the diameters 0.1 microns [0.00001 cm]) that they cannot be individually felt or seen with the naked eye. However, the aggregate reduces horizontal visibility and provides the atmosphere with an opalescent appearance. Haze appears as a yellowish or bluish veil based on whether the background is light or dark, respectively. In reference to these colours, the haze is discriminated against by the mist, which gives the sky a greyish cast.
Air Pollution
Haze often takes place, when the smoke and dust particles accumulate in dry air, relatively. Whenever the weather conditions block the dispersal of smoke and other pollutants, they can concentrate and produce a normally low-hanging shroud, that impairs visibility. They can also become a respiratory health threat. Industrial pollution can also result in a dense haze, which is also called smog.
Haze has been a particularly acute problem since 1991 in Southeast Asia. The primary source of haze has been fired occurring in Borneo and Sumatra. In response to the Southeast Asian haze in1997, the ASEAN countries agreed on a Regional Haze Action Plan in the same year. But, in 2002, all the ASEAN countries signed the Transboundary Haze Pollution Agreement. However, pollution is still a problem today. Under this agreement, the ASEAN secretariat hosts a support and coordination unit. During the Southeast Asian haze in 2013, Singapore experienced a record level of high pollution, with the 3-hour Pollution Standards Index reaching a record level as high as 401.
In the United States, the IMPROVE (Interagency Monitoring of Protected Visual Environments) program has been developed as a collaborative effort between the National Park Service and the US EPA to establish the chemical composition of haze in the National Parks and establish air pollution control measures to restore the visibility to the pre-industrial levels. In addition, the Clean Air Act needs that any current visibility problems be remedied, and the problems of future visibility are prevented, in 156 Class I Federal areas located in the entire United States.
International Disputes
Transboundary Haze
Haze is no longer considered a domestic problem. It has become the cause of international disputes among the neighbouring countries. Haze also migrates to the adjacent countries and thereby pollutes other countries too. One of the most recent problems takes place in Southeast Asia, which largely affects the Malaysia, Indonesia, and Singapore nations. In 2013, because of the forest fires in Kuala Lumpur, Indonesia and the surrounding areas became shrouded in a pall of noxious fumes, smelling of coal and ash for more than one whole week, in the country’s worst environmental crisis from 1997.
Sumatra Island of Indonesia, Riau, and the Indonesian regions of Borneo, where plantation owners, fishermen, and miners set hundreds of fires in the forests for land clearing at the time of dry weather, are the primary causes of haze. Also, winds blow the most fumes across the narrow Strait of Malacca to Malaysia, though Indonesia parts are also affected. The 2015 Southeast Asian haze was the other major crisis, although there were occasions like 2006. And, 2019 haze which was less impactful compared to the three major Southeast Asian haze of 1997, 2013, 2015.
Obscuration
Haze causes issues in the terrestrial photography area, where the penetration of huge amounts of dense atmosphere can be necessary to image distant subjects. And, this also makes the visual effect of a contrast loss in the subject, due to the reason, the effect of light scattering through the haze particles. For such reasons, the colours of sunrise and sunset appear subdued on hazy days, and stars can be obscured at night. In some other cases, attenuation by haze is much greater than, toward sunset, the sun has disappeared altogether before reaching the horizon.
Haze is described as an aerial form of the Tyndall effect; thus unlike other atmospheric effects like fog and cloud, haze is spectrally selective: shorter (which are blue) wavelengths are scattered more, and longer (either red or infrared) wavelengths are scattered less. Because of this reason, several super-telephoto lenses often incorporate coatings or yellow filters to enhance image contrast. Infrared (IR) imaging can also be used to penetrate haze over a long distance, with a combination of IR-sensitive detectors and IR-pass optical filters.
Formation of Haze
In general, the industry is a primary source of particulate matter. Industries like steel, cement, and power generation create massive amounts of particulate matter (PM2.5) in the course of their regular operations. These particular emissions are controlled by the use of a dust collection system (which are called baghouse). These specific systems suck up the dust-laden air or fugitive dust and send it through a fabric filter (which is called a baghouse filter), which traps the dust particles, but they allow the air to pass through.
FAQs on What Is Haze in Chemistry?
1. From a chemistry perspective, what exactly is haze?
In chemistry, haze is defined as an aerosol, which is a type of colloidal system. It consists of a suspension of extremely fine, dry solid particles (like dust, salt, or soot) or liquid droplets (like acids) in the atmosphere. These particles are large enough to scatter and absorb sunlight, which reduces visibility and gives the air a characteristic opalescent or milky appearance. The scattering of light by these colloidal particles is known as the Tyndall effect.
2. How is haze different from fog and smog?
While often used interchangeably, haze, fog, and smog have distinct chemical and physical differences:
- Haze: Consists of fine, dry solid particles like dust, smoke, or pollutants suspended in the air. It forms in less humid conditions and reduces visibility by scattering light, often giving the sky a brownish or bluish tint.
- Fog: Is essentially a cloud at ground level, composed of tiny liquid water droplets or ice crystals. It forms when the air is saturated with moisture (relative humidity is near 100%) and significantly reduces visibility, appearing grey or white.
- Smog: Is a severe form of air pollution, a mixture of smoke and fog. It contains haze particles along with harmful gases like sulphur dioxide (SO₂) and nitrogen oxides (NOx). Photochemical smog, common in cities, is formed when sunlight reacts with these pollutants to create ozone (O₃) and other secondary pollutants.
3. What are the primary chemical pollutants that cause haze?
Haze is primarily caused by both natural sources and human-made chemical pollutants. The key anthropogenic pollutants include:
- Sulphur Dioxide (SO₂): Released from burning fossil fuels like coal and oil. In the atmosphere, it oxidises to form sulphuric acid droplets.
- Nitrogen Oxides (NOx): Emitted from vehicle exhaust and industrial processes. These contribute to the formation of nitric acid and particulate matter.
- Particulate Matter (PM2.5): These are incredibly fine particles (less than 2.5 micrometres in diameter) from combustion, dust, and industrial activities that can remain suspended in the air for long periods.
- Volatile Organic Compounds (VOCs): Released from paints, solvents, and fuels, they participate in atmospheric reactions that form secondary pollutants contributing to haze.
4. How is the concept of haze relevant to the CBSE Chemistry syllabus?
The concept of haze is directly relevant to several topics in the CBSE Chemistry syllabus for the 2025-26 session, particularly for Classes 11 and 12. It serves as a practical, real-world example for:
- Chapter on Surface Chemistry: Haze is a classic example of an aerosol, which is a type of colloid (a solid or liquid dispersed in a gas). It demonstrates properties like the Tyndall effect.
- Chapter on Environmental Chemistry: Haze is a key aspect of air pollution. The study of its causes (SO₂, NOx) and its more severe form, smog (both classical and photochemical), is a core part of this unit. It connects directly to concepts like acid rain and particulate pollutants.
5. Why is prolonged exposure to haze considered a health risk?
The health risk from haze is not due to poor visibility itself, but because of the chemical nature of its constituent particles. The primary danger comes from fine particulate matter (PM2.5) and associated pollutants like ozone (O₃) and SO₂. Because these particles are extremely small, they can bypass the body's natural defences, penetrate deep into the lungs, and even enter the bloodstream. This can lead to a range of health issues, including respiratory problems like asthma, cardiovascular diseases, and irritation of the eyes, nose, and throat.
6. What chemical reactions in the atmosphere lead to the formation of haze?
Haze often forms from secondary pollutants created by chemical reactions in the atmosphere. A prime example is the formation of sulphate haze from industrial emissions. The process involves:
- Oxidation of SO₂: Sulphur dioxide (SO₂) gas from industrial sources is oxidised in the atmosphere to form sulphur trioxide (SO₃). This can be catalysed by particulate matter.
2SO₂(g) + O₂(g) → 2SO₃(g) - Formation of Sulphuric Acid: The sulphur trioxide then reacts rapidly with atmospheric water vapour (H₂O) to form droplets of sulphuric acid (H₂SO₄).
SO₃(g) + H₂O(l) → H₂SO₄(aq)
These tiny, corrosive droplets of sulphuric acid are a major component of industrial haze and contribute significantly to acid rain.
7. Why does haze sometimes appear brownish or bluish, and what does the colour indicate?
The colour of haze provides clues about its size and chemical composition, which is explained by the physics of light scattering:
- Bluish Haze: Is typically caused by the scattering of sunlight by extremely small particles (smaller than the wavelength of light), a phenomenon known as Rayleigh scattering. This is common in natural, relatively clean environments, like the blue haze seen over distant mountains.
- Brownish Haze: Indicates the presence of larger particles and specific pollutants. A distinct brown colour is often caused by nitrogen dioxide (NO₂) gas, a key component of photochemical smog from traffic pollution. NO₂ gas is inherently brown and absorbs blue light, making the haze appear reddish-brown. Therefore, a brown haze is a strong indicator of heavy air pollution.
