Haze over the Mojave Desert from a brush fire in Santa Barbara, California, seen as the Sun descends on the 2016 June solstice, allows the Sun to be photographed without a filter.Bushfire haze in Sydney, AustraliaHaze as smoke pollution over the Mojave from fires in the Inland Empire, June 2016, demonstrates the loss of contrast to the Sun, and the landscape in general.Haze causing red sky, due to the scattering of light on smoke particles, also known as Rayleigh scattering during Mexico's forest fire season Haze in Monterrey, Mexico, during grassland fires
Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates suspended in air obscure visibility and the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of particulates causing horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic ash, dust, sand, and snow.[1] Sources for particles that cause haze include farming (ploughing in dry weather), traffic, industry, windy weather, volcanic activity and wildfires. Seen from afar (e.g. an approaching airplane) and depending on the direction of view with respect to the Sun, haze may appear brownish or bluish, while mist tends to be bluish grey instead. Whereas haze often is considered a phenomenon occurring in dry air, mist formation is a phenomenon in saturated, humid air. However, haze particles may act as condensation nuclei that leads to the subsequent vapor condensation and formation of mist droplets; such forms of haze are known as "wet haze".
In meteorological literature, the word haze is generally used to denote visibility-reducing aerosols of the wet type suspended in the atmosphere. Such aerosols commonly arise from complex chemical reactions that occur as sulfur dioxide gases emitted during combustion are converted into small droplets of sulfuric acid when exposed. The reactions are enhanced in the presence of sunlight, high relative humidity, and an absence of air flow (wind). A small component of wet-haze aerosols appear to be derived from compounds released by trees when burning, such as terpenes. For all these reasons, wet haze tends to be primarily a warm-season phenomenon. Large areas of haze covering many thousands of kilometers may be produced under extensive favorable conditions each summer.
Haze often occurs when suspended dust and smoke particles accumulate in relatively dry air. When weather conditions block the dispersal of smoke and other pollutants they concentrate and form a usually low-hanging shroud that impairs visibility and may become a respiratory health threat if excessively inhaled. Industrial pollution can result in dense haze, which is known as smog.
Since 1991, haze has been a particularly acute problem in Southeast Asia. The main source of the haze has been smoke from fires occurring in Sumatra and Borneo which dispersed over a wide area. In response to the 1997 Southeast Asian haze, the ASEAN countries agreed on a Regional Haze Action Plan (1997) as an attempt to reduce haze. In 2002, all ASEAN countries signed the Agreement on Transboundary Haze Pollution, but the pollution is still a problem there today. Under the agreement, the ASEAN secretariat hosts a co-ordination and support unit.[2] During the 2013 Southeast Asian haze, Singapore experienced a record high pollution level, with the 3-hour Pollutant Standards Index reaching a record high of 401.[3]
In the United States, the Interagency Monitoring of Protected Visual Environments (IMPROVE) program was developed as a collaborative effort between the US EPA and the National Park Service in order to establish the chemical composition of haze in National Parks and establish air pollution control measures in order to restore the visibility of the air to pre-industrial levels.[4] Additionally, the Clean Air Act requires that any current visibility problems be addressed and remedied, and future visibility problems be prevented, in 156 Class I Federal areas located throughout the United States. A full list of these areas is available on EPA's website.[5]
In addition to the severe health issues caused by haze from air pollution, dust storm particles, and bush fire smoke, reduction in irradiance is the most dominant impact of these sources of haze and a growing issue for photovoltaic production as the solar industry grows.[6] Smog also lowers agricultural yield and it has been proposed that pollution controls could increase agricultural production in China.[7] These effects are negative for both sides of agrivoltaics (the combination of photovoltaic electricity production and food from agriculture).
International disputes
Transboundary haze
Haze is no longer just a confined as a domestic problem. It has become one of the causes of international disputes among neighboring countries. Haze can migrate to adjacent countries in the path of wind and thereby pollutes other countries as well, even if haze does not first manifest there. One of the most recent problems occur in Southeast Asia which largely affects the nations of Indonesia, Malaysia and Singapore. In 2013, due to forest fires in Indonesia, Kuala Lumpur and surrounding areas became shrouded in a pall of noxious fumes dispersed from Indonesia, that brings a smell of ash and coal for more than a week, in the country's worst environmental crisis since 1997.
The main sources of the haze are Indonesia's Sumatra Island, Indonesian areas of Borneo, and Riau, where farmers, plantation owners and miners have set hundreds of fires in the forests to clear land during dry weather. Winds blew most of the particulates and fumes across the narrow Strait of Malacca to Malaysia, although parts of Indonesia in the path are also affected.[8] The 2015 Southeast Asian haze was another major crisis of air quality, although there were occasions such as the 2006 and 2019 haze which were less impactful than the three major Southeast Asian haze of 1997, 2013 and 2015.
Obscuration
Haze causes issues in the area of terrestrial photography and imaging, where the penetration of large amounts of dense atmosphere may be necessary to image distant subjects. This results in the visual effect of a loss of contrast in the subject, due to the effect of light scattering and reflection through the haze particles. For these reasons, sunrise and sunset colors and possibly the sun itself appear subdued on hazy days, and stars may be obscured by haze at night. In some cases, attenuation by haze is so great that, toward sunset, the sun disappears altogether before even reaching the horizon.[9]
Haze can be defined as an aerial form of the Tyndall effect therefore unlike other atmospheric effects such as cloud, mist and fog, haze is spectrally selective in accordance to the electromagnetic spectrum: shorter (blue) wavelengths are scattered more, and longer (red/infrared) wavelengths are scattered less. For this reason, many super-telephoto lenses often incorporate yellow light filters or coatings to enhance image contrast.[10] Infrared (IR) imaging may also be used to penetrate haze over long distances, with a combination of IR-pass optical filters and IR-sensitive detectors at the intended destination.
↑ Figure 1. "The setting sun dimmed by dense haze over State College, Pennsylvania on 16 September 1992". "Haze over the Central and Eastern United States". The National Weather Digest. March 1996. Retrieved April 26, 2011.
Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words smoke and fog to refer to smoky fog due to its opacity, and odor. The word was then intended to refer to what was sometimes known as pea soup fog, a familiar and serious problem in London from the 19th century to the mid-20th century, where it was commonly known as a London particular or London fog. This kind of visible air pollution is composed of nitrogen oxides, sulfur oxide, ozone, smoke and other particulates. Man-made smog is derived from coal combustion emissions, vehicular emissions, industrial emissions, forest and agricultural fires and photochemical reactions of these emissions.
Mist is a phenomenon caused by small droplets of water suspended in the cold air, usually by condensation. Physically, it is an example of a dispersion. It is most commonly seen where water vapor in warm, moist air meets sudden cooling, such as in exhaled air in the winter, or when throwing water onto the hot stove of a sauna. It can be created artificially with aerosol canisters if the humidity and temperature conditions are right. It can also occur as part of natural weather, when humid air cools rapidly, notably when the air comes into contact with surfaces that are much cooler than the air.
A nephelometer or aerosol photometer is an instrument for measuring the concentration of suspended particulates in a liquid or gas colloid. A nephelometer measures suspended particulates by employing a light beam and a light detector set to one side of the source beam. Particle density is then a function of the light reflected into the detector from the particles. To some extent, how much light reflects for a given density of particles is dependent upon properties of the particles such as their shape, color, and reflectivity. Nephelometers are calibrated to a known particulate, then use environmental factors (k-factors) to compensate lighter or darker colored dusts accordingly. K-factor is determined by the user by running the nephelometer next to an air sampling pump and comparing results. There are a wide variety of research-grade nephelometers on the market as well as open source varieties.
The Great Smog of London, or Great Smog of 1952, was a severe air pollution event that affected London, England, in December 1952. A period of unusually cold weather, combined with an anticyclone and windless conditions, collected airborne pollutants—mostly arising from the use of coal—to form a thick layer of smog over the city. It lasted from Friday 5 December to Tuesday 9 December 1952, then dispersed quickly when the weather changed.
In meteorology, visibility is the measure of the distance at which an object or light can be clearly discerned. It depends on the transparency of the surrounding air and as such, it is unchanging no matter the ambient light level or time of day. It is reported within surface weather observations and METAR code either in meters or statute miles, depending upon the country. Visibility affects all forms of traffic: roads, railways, sailing and aviation.
The ASEAN Agreement on Transboundary Haze Pollution is a legally binding environmental agreement signed in 2002 by the member states of the Association of Southeast Asian Nations to reduce haze pollution in Southeast Asia. The Agreement recognises that transboundary haze pollution which results from land and/or forest fires should be mitigated through concerted national efforts and international co-operation.
Asian Dust is a meteorological phenomenon that affects much of East Asia year-round and especially during the spring months. The dust originates in the deserts of China, Mongolia, and Kazakhstan, where high-speed surface winds and intense dust storms kick up dense clouds of fine, dry soil particles. These clouds are then carried eastward by prevailing winds and pass over China, North and South Korea, and Japan, as well as parts of the Russian Far East. Sometimes, the airborne particulates are carried much further, in significant concentrations which affect air quality as far east as the United States.
The 2006 Southeast Asian haze was an air pollution event caused by continuous, uncontrolled burning from "slash and burn" cultivation in Indonesia, which affected several countries in the Southeast Asian region and beyond, including Malaysia, Singapore, southern Thailand, and as far away as Saipan; the effects of the haze may have even spread to South Korea. Local sources of industrial pollution also, inadvertently, contributed to increases in air toxicity ; notably at-risk areas included communities close to textile factories, fertilizer plants, meat-packing plants, industrialised dairy farms, shipping ports, and oil refineries. Air quality was lower, overall, for residents of more densely-populated cities. In the highly urban and industrialised Klang Valley of Malaysia, in particular, the surrounding elevated terrain acted as a natural retainer of polluted air, aggravating the situation as the haze set in.
Arctic haze is the phenomenon of a visible reddish-brown springtime haze in the atmosphere at high latitudes in the Arctic due to anthropogenic air pollution. A major distinguishing factor of Arctic haze is the ability of its chemical ingredients to persist in the atmosphere for significantly longer than other pollutants. Due to limited amounts of snow, rain, or turbulent air to displace pollutants from the polar air mass in spring, Arctic haze can linger for more than a month in the northern atmosphere.
The 1997 Southeast Asian haze was an international air pollution disaster that occurred during the second half of 1997, its after-effects causing widespread atmospheric visibility and health problems within Southeast Asia. Considered the most severe Southeast Asian haze event of all time, the total costs of the 1997 haze are estimated at US$9 billion, due mainly to health care and disruption of air travel and business activities.
The Southeast Asian haze is a fire-related recurrent transboundary air pollution issue. Haze events, where air quality reaches hazardous levels due to high concentrations of airborne particulate matter from burning biomass, have caused adverse health, environmental and economic impacts in several countries in Southeast Asia. Caused primarily by slash-and-burn land clearing, the problem flares up every dry season to varying degrees and generally is worst between July and October and during El Niño events. Transboundary haze in Southeast Asia has been recorded since 1972 with the 1997 and 2015 events being particularly severe.
Air pollution has been an ongoing problem in many countries in the Southeast Asia region, and Malaysia is one of the worst affected. Haze has been a major issue for the country, driven by slash and burn practices by farmers and peat fires blown by the wind from Indonesia.
Particulates or atmospheric particulate matter are microscopic particles of solid or liquid matter suspended in the air. The term aerosol commonly refers to the particulate/air mixture, as opposed to the particulate matter alone. Sources of particulate matter can be natural or anthropogenic. They have impacts on climate and precipitation that adversely affect human health, in ways additional to direct inhalation.
The 2013 Eastern China smog was a severe air pollution episode that affected East China, including all or parts of the municipalities of Shanghai and Tianjin, and the provinces of Hebei, Shandong, Jiangsu, Anhui, Henan, and Zhejiang, during December 2013. A lack of cold air flow, combined with slow-moving air masses carrying industrial emissions, collected airborne pollutants to form a thick layer of smog over the region. Levels of PM2.5 particulate matter averaged over 150 micrograms per cubic metre; in some areas, they were 300 to 500 micrograms per cubic metre.
The air quality in Delhi, the capital territory of India, according to a WHO survey of 1,650 world cities, and a survey of 7,000 world cities by the US-based Health Effects Institute in August 2022, is the worst of any major city in the world. It also affects the districts around Delhi. Air pollution in India is estimated to kill about 2 million people every year; it is the fifth largest killer in India. India has the world's highest death rate from chronic respiratory diseases and asthma, according to the WHO. In Delhi, poor quality air irreversibly damages the lungs of 2.2 million or 50 percent of all children.
The 2015 Southeast Asian haze was an air pollution crisis affecting several countries in Southeast Asia, including Brunei, Indonesia, Malaysia, Singapore, southern Thailand, Vietnam, Cambodia and the Philippines.
The 2016 Southeast Asian Haze was a transnational haze crisis which is a recurring problem with transboundary air pollution brought on by fires. The 2016 haze that took place affected several countries in Southeast Asia, including Indonesia, Malaysia and Singapore. It brought negative effects towards the environment and brought an impact on people's health and the economy.
The 2010 Southeast Asian haze was an air pollution crisis which affected many Southeast Asia countries such as Indonesia, Malaysia and Singapore during the month of October in 2010.
A trans-national air pollution crisis affected several countries in Southeast Asia from February to September 2019, including Brunei, Indonesia, Malaysia, the Philippines, Singapore, Thailand, and Vietnam.
Particulate pollution is pollution of an environment that consists of particles suspended in some medium. There are three primary forms: atmospheric particulate matter, marine debris, and space debris. Some particles are released directly from a specific source, while others form in chemical reactions in the atmosphere. Particulate pollution can be derived from either natural sources or anthropogenic processes.
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