Air Pollution: How It Causes Low Visibility

Visibility is a measure of how well an observer can see a scene, including how far they can see and how well they can distinguish colours, forms, and textures. Haze, which consists of fine particles and gaseous air pollution in the atmosphere, impairs visibility by scattering and absorbing light. The more particles there are in the air, the lower the visibility. Haze can be caused by natural sources such as dust and wildfire smoke, but it is primarily caused by man-made air pollution from sources such as motor vehicles, industrial fuel burning, and manufacturing operations. In certain parts of the world, haze episodes have caused widespread public concern due to their frequent occurrence and impact on visibility.

Particulate matter in the air

Particulate matter (PM) is a mixture of solid particles and liquid droplets found in the air. These particles vary in size, shape, and chemical composition and may contain inorganic ions, metallic compounds, elemental carbon, and organic compounds. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be visible to the naked eye. Others are so minuscule that they can only be observed under an electron microscope.

PM2.5, or fine inhalable particles, have diameters of 2.5 micrometers or less. To put this into perspective, a human hair is about 70 micrometers in diameter, making it 30 times larger than the largest fine particle. These fine particles can be composed of hundreds of different chemicals and are emitted directly from sources such as construction sites, unpaved roads, fields, smokestacks, and fires. They can also form in the atmosphere through complex chemical reactions involving pollutants like sulfur dioxide and nitrogen oxides.

PM10 particles, with diameters of 10 micrometers or less, are considered inhalable and can induce adverse health effects. PM2.5 particles, being smaller, pose an even greater risk to health. They can penetrate deep into the lungs and may even enter the bloodstream. Short-term exposure to PM2.5 has been linked to premature mortality, increased hospital admissions for heart and lung issues, acute and chronic bronchitis, asthma attacks, and respiratory symptoms. Long-term exposure to PM2.5 has been associated with premature death, especially in infants, children, and older adults with pre-existing heart or lung conditions.

Particulate matter pollution is a significant cause of reduced visibility, often referred to as haze. This haze is created when sunlight interacts with the tiny pollution particles in the air, causing some light to be absorbed and scattered before it reaches an observer. As the concentration of fine particles increases, more light is scattered and absorbed, resulting in reduced clarity, colour, and visual range. This phenomenon is particularly notable in scenic areas like national parks, where air pollution has substantially decreased visual range.

The scattering of light by particulates is the primary driver of reduced visibility. However, light absorption by gases and particles can also contribute to discolourations in the atmosphere, although it usually has a lesser impact on visibility degradation. Particles with diameters between 0.1 and 1.0 µm are the most effective at reducing visibility per unit aerosol mass. Additionally, certain particles, such as sulfates, scatter more light, especially in humid conditions.

Photochemical smog

Air pollution is associated with reduced visibility due to the presence of particulate matter and haze in the atmosphere. This haze is caused by the scattering and absorption of sunlight by tiny pollution particles, resulting in a reduction in the clarity and colour of what we see.

One significant contributor to air pollution and reduced visibility is photochemical smog, which is a complex mixture of pollutants that forms over heavily populated cities and urban areas. Photochemical smog is a type of air pollution characterised by the presence of nitrogen oxides, volatile organic compounds (VOCs), and ozone, among other pollutants. It forms when nitrogen oxides, primarily emitted from internal combustion engines, react with sunlight to produce free atoms of oxygen. These oxygen atoms then combine with molecular oxygen to form ozone.

The formation of photochemical smog is influenced by various factors. Firstly, there must be a substantial source of emissions, typically from vehicular traffic, to release sufficient nitrogen oxides, hydrocarbons, and other VOCs into the air. Secondly, warmth, ample sunlight, and relatively low air movement are necessary for the chemical reactions to occur without diluting the reactants.

The major components of photochemical smog include nitrogen dioxide (NO2), ozone (O3), peroxyacetyl nitrate (PAN), aldehydes, and other secondary pollutants. These compounds have adverse effects on human health, including eye irritation, respiratory ailments, and damage to the membranes of the respiratory system. They can also interfere with the nitrogen cycle and cause damage to crops, trees, and plants.

Thermal inversions

Topography plays a role in thermal inversions, as cold air can sink into low-lying areas like valleys, settling below layers of warm air. This effect is more pronounced during the evening when the land begins to cool. The Earth's surface no longer radiates as much heat, causing the air near the surface to cool faster than the air above, forming the inversion. Certain weather conditions can also contribute to thermal inversions. For example, moderate to strong winds and rainfall help mix layers of cold and warm air, preventing the formation of inversions. On the other hand, weak winds and snow can create ideal conditions for thermal inversions to develop.

The strength, duration, and height of a thermal inversion directly impact the severity of the associated pollution event. A stronger inversion, with a greater temperature difference between the inversion and mixing layers, prevents pollution from dispersing into higher atmospheric levels. As the duration of an inversion increases, pollution accumulates in the mixing layer, leading to a significant decline in air quality. The inversion acts as a boundary, trapping pollutants near the Earth's surface and causing a build-up of smog and haze that reduces visibility.

Some of the most notable pollution episodes in history have been linked to thermal inversions. For instance, in December 1952, London experienced a thermal inversion due to anticyclonic conditions and a lack of wind. The cold weather led to increased burning of cheap, sulfurous coal for warmth, releasing particulates, sulfur oxides, and hydrochloric acid into the air. The thermal inversion trapped these pollutants, resulting in a thick layer of deadly smog that lasted for days and is estimated to have caused up to 12,000 deaths.

Haze

The impact of haze on visibility can be significant. In eastern US national parks, the average visual range has decreased from 90 miles to just 15-25 miles due to air pollution. Similarly, in the western parks, the visual range has decreased from 140 miles to 35-90 miles. This reduction in visibility not only affects the experience of visitors to these scenic areas but also has environmental and health implications.

Some of the pollutants that contribute to haze have been linked to serious health issues and environmental damage. Exposure to fine particles in the air has been associated with increased respiratory illnesses, decreased lung function, and even premature death. Additionally, haze can exacerbate the occurrence of sea fogs in certain regions.

To address the issue of haze and its impact on visibility, the US Environmental Protection Agency (EPA) has been monitoring visibility in national parks and wilderness areas since 1988. In 1999, the EPA announced a significant effort to improve air quality in these areas. The agency continues to work on implementing plans and regulations, such as the Regional Haze State Implementation Plans, to reduce haze and improve visibility across the country.

Humidity

Firstly, humidity can affect the dispersion of pollutants. High humidity can cause pollutants to stick to water droplets and other particles in the air, removing them from the atmosphere and reducing pollutant concentrations. However, this can also lead to the accumulation of pollution on surfaces, which can be harmful to the environment.

Secondly, humidity can speed up chemical reactions that produce air pollutants. Water molecules can react with pollutants and other chemicals in the air to create new compounds. For example, in the presence of sunlight, nitrogen oxides and volatile organic compounds may react to form ground-level ozone, which is harmful to the environment.

Thirdly, humidity can impact the movement of pollutants. Air temperature affects the movement of air and, therefore, the movement of air pollution. Warm air near the ground rises, carrying pollution to higher altitudes. In contrast, during cold weather, warm air acts as a lid, trapping cold air and pollution close to the ground, which is known as a thermal inversion. Thermal inversions are more common in cities and can trap pollutants from vehicles, chimneys, and smokestacks.

High humidity can increase the rate of harmful or toxic chemicals in the air, negatively impacting respiratory health, especially for individuals with weakened respiratory systems or pre-existing conditions like asthma. It can also cause the growth of harmful moulds and bacteria that trigger negative respiratory responses. Additionally, humidity increases the negative effects of certain pollutants like smog.

In summary, humidity plays a significant role in determining air quality and can influence the dispersion, chemical reactions, and movement of air pollutants. While high humidity can help reduce certain pollutants in the air, it can also lead to the formation of other harmful compounds and negatively impact human health.

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