Helena Joyce
The Earth's atmosphere is divided into several layers, each with its own unique characteristics. These layers are primarily defined by variations in temperature as altitude increases. Air pollution, caused by human activities and natural sources, can reach different altitudes depending on various factors such as wind speed and the type of pollution. Light pollution, for example, occurs when light reflects off atoms in the atmosphere, and its intensity decreases as altitude increases. Wildfires can produce massive amounts of smoke that reach great altitudes, sometimes resulting in fire tornadoes or fire whirls. Dust from the Saharan Desert can also be carried across the Atlantic Ocean by winds, affecting air quality in North and South America. While the dispersion of pollution depends on several variables, it's important to understand the impact of human activities on the Earth's atmosphere and the potential consequences for our planet and human health.
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What You'll Learn
The troposphere and how it cleans itself
The troposphere is the lowest layer of the Earth's atmosphere, containing 80% of the total mass of the planetary atmosphere. It is the site of all weather phenomena on Earth. The troposphere is wider at the equator (10mi) than at the poles (5mi), with an average height of 13 km (8.1 mi; 43,000 ft). The name troposphere means "region of mixing", referring to the constant convective overturn of material due to hot air rising and cold air falling. This mixing results in the dispersion and eventual washout of dust and pollutants by rainfall, making the troposphere self-cleaning.
The troposphere contains 99% of the water vapour in the atmosphere, with concentrations varying latitudinally. Water vapour plays a crucial role in the self-cleaning process of the troposphere. As water evaporates from the Earth's surface, it rises into the cooler upper regions of the troposphere, where it condenses and falls back to the surface as rain. This condensation process helps to remove pollutants from the air, as they become incorporated into the water droplets and are then washed out of the atmosphere.
The troposphere also contains gases such as nitrogen (78%), oxygen (21%), carbon dioxide, and ozone. While carbon dioxide acts as a greenhouse gas, trapping heat and contributing to climate change, it is present in relatively small amounts in the troposphere. The concentration of carbon dioxide has nearly doubled since 1900, raising concerns about potential temperature increases and significant changes to worldwide weather patterns.
Additionally, the troposphere is affected by combustion by-products such as carbon, soot, ash, nitrites, and sulphites, which can contribute to air pollution and acid rain. However, these negative effects can be mitigated through physical means such as scrubber towers, which capture pollutants and process them into valuable by-products.
Recent studies have also revealed the role of chemical transformations in the troposphere, particularly during nighttime when photochemical reactions cease. The production of OH at the air-water interface, for instance, has shed light on previously observed interfacial reactions involving phenolic compounds and gaseous ozone or nitrate radicals. These findings contribute to our understanding of the complex chemistry occurring within the troposphere and its role in maintaining air quality.
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The mesosphere and its negligible water vapour
The mesosphere is the third layer in Earth's atmosphere, extending from the stratopause (about 50 kilometres) to roughly 85 kilometres above the Earth's surface. It is warmed from below by the stratosphere, the second layer in Earth's atmosphere. The mesosphere has very little water vapour and ozone, which are necessary for generating heat. As a result, the temperature drops across this layer. The air in this region is extremely thin, with a density of about 1/1000 that of the surface. As altitude increases, this layer becomes increasingly dominated by lighter gases, and the remaining gases become stratified by molecular weight.
Water vapour (H2O) is a critical trace gas in the Earth's atmosphere. It is responsible for regulating the weather and climate of our planet. Water vapour in the stratosphere comes from the transport of tropospheric air through the tropical tropopause and in situ methane oxidation in the middle to upper stratosphere. While the mesosphere has negligible water vapour, there have been reports of "mysterious moisture" in this layer. Noctilucent clouds, for example, form when wisps of water vapour rise to the top of the atmosphere during the summer. Water molecules stick to specks of meteor smoke, forming icy clouds that glow electric blue when struck by high-altitude sunlight.
The presence of water vapour in the mesosphere is influenced by various factors, including the solar cycle. During a deep solar minimum, the ultraviolet radiation that typically destroys water vapour in the mesosphere is reduced. This can lead to an increase in water vapour concentrations, as observed in some regions. However, the overall trend in the mesosphere shows a rapid global increase in water vapour of 5-6% per decade in the lower stratosphere after 2002. This increase has been measured by instruments such as SABER and MLS.
The impact of increasing water vapour in the mesosphere can have significant consequences. For instance, in the upper mesosphere, more abundant water vapour from increasing methane contributes to more frequent polar mesospheric cloud occurrences. The long-term fluctuations in water vapour levels in the mesosphere can influence global and regional climate patterns. Additionally, the presence of water vapour in the mesosphere can affect the formation of noctilucent clouds, leading to sightings at lower latitudes than usual.
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The stratosphere and its upper boundary
The stratosphere is the second-lowest layer of the Earth's atmosphere, located above the troposphere and below the mesosphere. It extends from the top of the troposphere to about 50 kilometres (31 miles) above the ground. The stratosphere is composed of stratified temperature zones, with warmer layers of air located higher (closer to outer space) and cooler layers lower (closer to the surface of the Earth). This is known as temperature inversion, which is the opposite of the troposphere, where temperature decreases with altitude. The tropopause border between the troposphere and stratosphere marks the beginning of this temperature inversion.
The stratosphere is heated by the rapid photolysis and reformation of ozone, which is abundant in this layer. Ozone absorbs high-energy ultraviolet (UV) radiation from the Sun, converting the UV energy into heat. This temperature inversion, along with its resistance to vertical mixing, makes the stratosphere dynamically stable. There is no regular convection and associated turbulence in this part of the atmosphere, which is why commercial jet aircraft fly in the lower stratosphere to avoid the turbulent weather of the troposphere.
However, exceptionally energetic convection processes, such as volcanic eruption columns and overshooting tops in severe supercell thunderstorms, can carry convection into the stratosphere on a local and temporary basis. Volcanic eruptions can fling aerosol particles into the stratosphere, where they may linger for months or years, sometimes altering Earth's global climate. Rocket launches can also inject exhaust gases into the stratosphere, with uncertain consequences.
The upper boundary of the stratosphere is known as the stratopause, which is marked by a sudden decrease in temperature. The stratopause extends from about 50 kilometres to the mesosphere, which begins at roughly 80-85 kilometres above the Earth's surface.
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The impact of wind speed and direction
Wind speed and direction have a significant impact on the spread and dispersion of air pollution. In urban areas, the wind interacts with various structural components such as building roofs, facades, and other surfaces, influencing the dynamics of heat and pollution transfer. The wind field plays a crucial role in air infiltration by impacting both forced and natural convection triggered by temperature differences. The mechanical turbulence induced by wind facilitates the exchange of heat, substances, and energy, helping to mitigate the Urban Heat Island (UHI) effect.
In hilly or mountainous regions, changing wind directions, often associated with uphill or downhill thermal flows, can profoundly impact the dispersion of heat-related pollution, creating unique natural ventilation patterns. For example, in the Lausanne/Pully urban area in Switzerland, the presence of tall and compacted buildings in Lausanne significantly influences wind speed and direction, resulting in a more erratic pattern. Conversely, the wind direction in the residential areas of Pully is less affected due to smaller and less densely packed buildings.
Wind speed and direction also play a role in the spread of viral diseases, such as COVID-19. Studies have found a correlation between air pollution and COVID-19 cases, suggesting that air pollution may be associated with increased morbidity and mortality due to the virus. However, the influence of confounding factors like population density requires further investigation to establish a definitive causal relationship.
On a larger scale, strong winds associated with thunderstorms, tornadoes, and blizzards can have powerful effects on the dispersion of pollution. Thunderstorms can cause downdrafts, leading to straight-line winds that spread out horizontally when they reach the ground. Tornadoes, some of nature's strongest winds, form when the winds surrounding a thunderstorm change speed and direction, resulting in intense pressure gradients and wind speeds that can exceed 300 mph.
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The health effects of air pollution
Air pollution is the presence of contaminants in the atmosphere, such as dust, fumes, gas, mist, odour, smoke or vapour, in quantities that can be harmful to human health. The health effects of air pollution are wide-ranging and impact people from all walks of life, although it is clear that certain demographics are more vulnerable to the negative consequences.
The main pathway of exposure from air pollution is through the respiratory tract. Pollutants cause inflammation, oxidative stress, immunosuppression, and mutagenicity in cells throughout the body, impacting the lungs, heart, and brain, among other organs. Short-term exposure to high levels of particulate matter can lead to reduced lung function, respiratory infections, and aggravated asthma. Long-term exposure increases the risk of noncommunicable diseases, including stroke, heart disease, chronic obstructive pulmonary disease, and cancer. Ozone, a powerful lung irritant, is another form of air pollution that can cause serious respiratory issues and even shorten lives.
Children are particularly vulnerable to the health effects of air pollution. Higher levels of air pollution increase short-term respiratory infections, causing more school absences. Children who play outdoor sports and live in high-ozone areas are more likely to develop asthma. Maternal exposure to air pollution is associated with adverse birth outcomes, such as low birth weight, pre-term birth, and small gestational age births. A growing body of evidence also suggests that air pollution may affect diabetes and neurological development in children.
Racial and ethnic disparities also play a significant role in the health effects of air pollution. Historically, racist zoning policies and discriminatory lending practices have turned communities of colour into "sacrifice zones," where residents are forced to breathe dirty air and suffer the associated health problems. Research has shown that people of colour are more likely to be exposed to air pollution and experience greater harm to their health as a result. Low-income areas are also more vulnerable to the negative health consequences of air pollution, as residents are often in close proximity to sources of pollution and have fewer resources to relocate or access quality healthcare.
Additionally, climate change induced by air pollution contributes to rising sea levels, extreme weather events, heat-related deaths, and the increased transmission of infectious diseases. Climate change also exacerbates pollen allergies, with plants producing more pollen and extending the pollen production season, leading to adverse health effects for those with allergies and asthma.
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Frequently asked questions
Pollution can reach varying heights in the atmosphere, depending on the type of pollution and the conditions that aid its spread. Some types of pollution, like dust from the Saharan Desert, can be carried by winds and reach as far as North and South America.
Light pollution does weaken with height. At 5500m, light pollution is halved compared to ground level.
The highest layer of the Earth's atmosphere is the exosphere, which begins at an altitude of 500-1000km. This layer contains many artificial satellites.
The troposphere is the first layer of the atmosphere and is where weather phenomena occur. It is also where dust and pollutants are mixed and eventually washed out by rainfall.
Smoke from wildfires can be lifted high above the surface by upper-level winds, sometimes even spanning an entire continent.
