Air Pollution: Visible Signs And Their Impact

Air pollution is a mix of hazardous substances from human-made and natural sources. It is a major threat to health and climate, causing respiratory and other diseases, and contributing to climate change. While some air pollution is invisible, there are visible signs that can indicate poor air quality. For example, when sunlight beams through a window, larger particles like dust become more visible. A thick layer of smog covering the sky in big cities is another example of a visible sign of air pollution. Other signs of poor air quality include coughing, sneezing, breathing difficulties, itchy and watery eyes, headaches, and fatigue.

Haze, smog, and smoke

Smog is a type of air pollution formed by the emission of primary and secondary pollutants. Primary pollutants are emitted directly from a source, such as sulfur dioxide from coal combustion or vehicle exhaust systems. Secondary pollutants, such as ozone, are formed when primary pollutants undergo chemical reactions in the atmosphere. Photochemical smog, a common type of smog found in cities during the summer, is the result of the chemical reaction between sunlight, nitrogen oxides, and volatile organic compounds. This type of smog is particularly associated with vehicular emissions and industrial fumes.

Winter smog, on the other hand, is characterized by increased coal and other fossil fuel usage for heating, as well as the lack of dispersion of pollutants under atmospheric inversions. Coal fires, used for heating or energy production, can emit significant clouds of smoke that contribute to winter smog. London, for example, was notorious for its coal-caused smogs, nicknamed "pea-soupers," up through the mid-20th century. Today, air pollution from coal combustion remains a significant problem in China, where it sometimes forces the closure of roads, schools, or airports in certain cities during the autumn and winter months.

Smoke, often a byproduct of combustion processes, is another visible sign of air pollution. It consists of a collection of gases and fine particles released into the air. Smoke from wildfires, for example, can release particulate matter, carbon monoxide, and other pollutants into the atmosphere, contributing to air pollution and negatively impacting air quality.

Vehicle exhaust

One of the primary concerns regarding vehicle exhaust is the release of particulate matter. Diesel engines, in particular, emit black soot and metal particles, known as diesel particulate matter. These particles can be inhaled, leading to respiratory issues and other health problems. Modern cars are now equipped with diesel particulate filters (DPFs) to mitigate the release of these harmful substances.

Furthermore, vehicle exhaust contributes to the presence of hydrocarbons and carbon monoxide in the atmosphere. Hydrocarbons, in combination with NOx, produce ozone. While ozone in the upper atmosphere protects us from ultraviolet rays, its presence at lower levels contributes to smog and exacerbates respiratory conditions. Carbon monoxide, an odourless and colourless gas, can be harmful to human health, particularly in enclosed spaces.

It is important to note that advancements in technology and policy implementations have helped reduce vehicle emissions. Manufacturers have improved engine and exhaust designs, and catalytic converters and particulate filters are now standard in new petrol and diesel cars. Additionally, cities worldwide have established clean air zones to discourage highly polluting vehicles from entering congested areas. These combined efforts aim to mitigate the visible and invisible signs of air pollution caused by vehicle exhaust emissions.

Industrial emissions

One of the most common visible signs of industrial air pollution is haze or smog, which obscures shapes and colours in the environment. This haze is caused by a mixture of pollutants, including ground-level ozone, created when pollutants from industrial sources react chemically in the presence of sunlight. Other visible signs of industrial air pollution may include smoke and dust from industrial processes, as well as increased haze or smog reducing visibility.

Refineries, for example, emit a range of airborne pollutants, including fine particulate matter (PM2.5), which is particularly harmful as it can be inhaled deeply into the lungs, leading to respiratory and cardiovascular issues. Other pollutants released by refineries include sulfur dioxide and nitrogen oxides, which contribute to smog, acid rain, and respiratory problems.

In addition to refineries, petrochemical plants, which process hydrocarbons from crude oil and natural gas, emit pollutants such as PM2.5, sulfur dioxide, nitrogen oxides, and hazardous air pollutants (HAPs) like formaldehyde and acetaldehyde. Commercial transportation also contributes to industrial emissions, releasing pollutants such as PM2.5, nitrogen oxides, and greenhouse gases like carbon dioxide and methane.

The burning of fossil fuels in power plants and factories is a significant source of industrial air pollution. This includes the combustion of coal, which releases harmful chemicals and gases, contributing to both air pollution and greenhouse gas emissions. The reliance on fossil fuels for energy production is a major driver of air pollution, and the transition to cleaner and more sustainable practices is essential to improving air quality.

Wildfires and burning organic matter

Wildfires and the burning of organic matter are significant contributors to air pollution. The smoke released during wildfires contains a mixture of gaseous pollutants, hazardous air pollutants (HAPs), water vapour, and particle pollution. These particles can be composed of different components, including acids, inorganic compounds, organic chemicals, soot, metals, soil, dust, and biological materials. While some particles are too small to be seen with the naked eye, others can become visible when the concentration of particles in the air is high, appearing as haze or smog. This haze can reduce visibility, giving the air an opalescent appearance that subdues colours and makes it difficult to distinguish physical features, depth, and texture.

The particulate matter (PM) emitted by wildfires and burning organic matter can be classified as coarse particles (PM10-2.5) or fine particles (PM2.5). Coarse particles, which are less common in wildfire smoke, have diameters larger than 2.5 micrometres and up to 10 micrometres. On the other hand, fine particles, which make up about 90% of the particle mass in wildfire smoke, are generally 2.5 micrometres in diameter or smaller. These fine particles can penetrate deep into the lungs and may even enter the bloodstream, posing a significant health risk to humans and animals.

The composition of emissions from wildfires and burning organic matter can vary depending on factors such as fuel type, moisture content, fire conditions, temperature, and weather. Wildfires emit a range of gaseous compounds, including nitrogen oxides (NOx), carbon monoxide (CO), methane (CH4), and volatile organic compounds (VOCs). These VOCs can further react through oxidation to form secondary organic aerosols (SOAs), which contribute to the complex mixture of pollutants in the air.

The impact of wildfires and burning organic matter on air quality can be assessed through emission inventories (EIs) and satellite data, which provide detailed information about the hectares burned and the types and amounts of pollutants emitted. For example, the National Emission Inventory (NEI) in the United States showed that wildland fire emissions represented approximately 32% of total primary PM2.5 emissions in the country. Additionally, satellite data, such as the GOES-16 data for continental US fire detection, helps track fire progression and understand the diurnal patterns of large wildfires.

The smoke and pollutants released from wildfires and burning organic matter have significant environmental and health impacts. The toxic chemicals released during burning, including nitrogen oxides, sulfur dioxide, VOCs, and polycyclic organic matter (POMs), can cause long-term health problems, especially for individuals with pre-existing respiratory conditions. Burning trash, plastics, and treated wood can release additional hazardous chemicals, heavy metals, and carcinogens, further exacerbating the health risks associated with air pollution.

Chemical odours

While air pollution is often associated with visible signs such as haze, smog, and exhaust fumes, it can also manifest as chemical odours that are not always noticeable to everyone. These odours can be a result of a variety of sources, including vehicle emissions, industrial processes, and even natural sources such as wildfires.

The impact of chemical odours on individuals can vary. Some people may be more sensitive to environmental odours and experience symptoms such as headaches and nausea even at low concentrations. The length of exposure also plays a role, with prolonged exposure potentially leading to more severe effects. Certain groups, such as people with asthma, children, and older adults, may be more vulnerable to the effects of these odours.

It is important to note that the concentration of pollutants and the frequency and duration of exposure can influence the level of harm caused by chemical odours. While a strong odour may indicate the presence of pollutants, it does not always correlate with the level of toxicity. However, in some cases, the accumulation of a substance in the air over time can lead to an increase in toxicity.

To address concerns related to chemical odours and air quality, individuals can refer to tools provided by environmental protection agencies, such as the "What is that odour?" tool, which helps identify the source of an odour, or the EPA's Toxic Release Inventory (TRI), which allows users to search for chemicals released in their area. Additionally, during periods of high odour intensity, individuals can consider modifying their activities, such as exercising indoors, to minimise potential exposure.

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