What is Haze?

Haze is an atmospheric phenomenon where dust, smoke, and dry particles in the air obscure the sky’s clarity. When seen from afar, and depending upon the direction of view, haze can have a bluish or brownish tint. Various activities such as farming, vehicle traffic, and wildfires can create a pollutant-clouded atmosphere. The World Meteorological Organization (WMO) categorizes atmospheric obscuration by a list of different types. These include: haze, fog, ice fog, steam fog, mist, smoke, volcanic ash, dust, sand, and snow.

Although haze is mostly a dry air phenomenon, some of its particles tend to create condensation, subsequently giving form to mist droplets. This phenomenon is known as wet haze, or visibility-reducing aerosols of the wet type. The process of aerosol creation occurs due to the chemical reactions between the atmosphere and the sulfur dioxide gas emissions, which create small droplets of sulfuric acid. Sunlight, high humidity, and stagnant airflow may enhance this process. 

Wet haze tends to favor warm weather. It can cover large areas and reach thousands of square miles in the favorable summer conditions. It most often occurs when dust and smoke particles accumulate in relatively dry air. When the natural dispersal methods of smoke and other pollutants are hindered, the build-up creates a low-hanging, cloud-like blanket that impairs visibility. The compound isoprene (C5H8), a hydrocarbon emitted from many deciduous trees, share some characteristics with wet haze/aerosols.

What Causes Haze?

Some of the sources of atmospheric haze include:

Volcanic Activity 

In the aftermath of major volcanic eruptions, sulfur dioxide (SO2), hydrochloric acid (HCl), and ashes are distributed into the atmosphere. The haze of sulfur dioxide droplets created by volcanic gases can have a lasting affect; hanging aerosols became a headlining problem in Southeast Asia following the eruption of Mt. Pinatubo in 1991. The Royal Meteorlogical Society notes that the average global temperature dropped as much as 0.5 degrees Celsius for 1992 and 1993. 

Windy Weather

Desert dust blown into the atmosphere by the wind is another form of naturally-occurring haze. The minerals contained in the dust and dirt allow sunlight to be absorbed, warming the area. The dusts can be blown over many miles; dust from the Sahara Desert is sometimes carried over to other parts of the world, including the United States and the Amazon Rainforest. 

Wind also lifts water from the oceans, causing the salts to be evaporated into the air — accounting for some atmospheric haze. The sea salt acts as cloud condensation nuclei, which accelerates cloud formation. Clouds produced in this fashion stay in the sky for long periods of time. Due to the chemical reactions between the air and water molecules and the pollutants, some clouds are unable to release the built-up water they contain and can float around in the sky for any number of years. Some of these pollutant-rich clouds are also responsible for acid rain. 

Human Practices

Human activities like forest burning, factory emissions, automobile traffic pollution, and the burning of coal and other fossil fuels are said to outweigh natural sources in the production of hazes. According to NASA, the industrial revolution (spanning the 18th and 19th centuries) generated a rapid growth in the concentration of sulfate aerosols in the atmosphere. The global temperature is said to have risen by 0.8º C since 1880 — a small change that has a very dramatic influence on the workings of the entire planet. NASA confirms that over 60% of the temperature increase has occurred since 1975 — about 0.15-0.20°C per decade.

Environmental Issues Involving Haze

The greenhouse effect, a theory for global climate change, was first proposed in 1896 by Swedish scientist Svante Arrhenius (1859-1927). Arrhenius suggested that an increase in carbon dioxide (CO2) would in fact lead to a rise in temperature. His theory wasn’t widely accepted until the 1940’s, when the technology to measure long-wave radiation became available. By the 1980’s, the greenhouse gas argument gained popularity with the steep rising of the global mean temperature. In 1988, the greenhouse effect theory was named, and  the Intergovernmental Panel on Climate Change was founded by the United Nations Environmental Programme and the World Meteorological Organization.

The greenhouse gases said to be responsible for much of the global temperature increase, aside from water vapor, are concentrations of four principle gases: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and what are called halocarbons or CFC’s — gases containing fluorine (F), chlorine (Cl) and bromine (Br). While some of these gases are naturally produced by the Earth, over-concentrations can mean drastic changes to the atmosphere. 

Some of the aerosols, however, like those produced by Mt. Pinatubo’s eruption — which blasted 20 million tons of sulfur dioxide into the atmosphere — reflect the sun’s light outward, cooling the area beneath.

Hazy skies can also cause issues with terrestrial photography. The filtering of large quantities of dense atmosphere is sometimes necessary to make accurate detailed images of distant objects. Light scattering from haze particles can cause visual disturbances that interfere with measuring equipment. On hazy days, sunrise and sunset colors appear subdued, and stars may become obscured at night. In some cases, the effects of haze can be so great that, toward sunset, the sun disappears altogether before reaching the horizon.

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