Aerosol

Air pollution in northern India and Bangladesh.

In environmental engineering, an aerosol is a colloid of solid or liquid particles suspended in a gas. The term aerosol refers to both the particles and the gas in which the particles are suspended The size of the particles can be from 0.002 µm to more than 100 µm, that is, from a few molecules to the size in which said particles cannot remain suspended in the gas for at least a few hours.

The processes that occur at the tropospheric level in direct contact with life on the planet include the emission of particulate matter. The PM₁₀ notation refers to to particles passing through a size-selective head for a 10 μm aerodynamic diameter with 50% cutting efficiency, while for PM_2.5 represents particles less than 2.5 µm in aerodynamic diameter. Other numerical values can be used analogously.

Aerosol generation can be of natural origin or due to human activity. Some particles occur naturally, from volcanic ash, dust storms, soil erosion, forest and grass fires, and seawater spray. Human activities, such as burning fuels and disturbing the Earth's surface, also generate aerosols. In global terms, artificial aerosols generated by human activities represent approximately 10%^{[citation needed]} of the total aerosols in our atmosphere.

Origin and composition

Atmospheric aerosols can originate naturally or anthropogenic. Some of these particles are emitted directly into the atmosphere (primary emissions) and others are emitted as gases that form particles in the atmosphere by coagulation or condensation chemical reactions between reactive molecules (secondary emissions). The composition of the particles of an aerosol depends on the source where they are generated.

The major natural sources are volcanic activity, eroded soils, plants, flowers, microorganisms, the surface of seas and oceans, dust storms, and forest and grass fires. Seawater spray is also a major source of aerosols, although most aerosols fall into the sea close to where they were emitted.

The largest source of aerosols due to human activity is the burning of fuels in thermal engines for transport and in thermoelectric plants for the generation of electricity, the smelting of metals such as copper or zinc, the production of cement, ceramics and bricks, as well as dust generated from construction sites and other land areas where water or vegetation has been disturbed. Some of the main components released are sulfates, nitrates and organic aerosols.

The chemical composition of aerosols directly affects the way the atmosphere interacts with solar radiation and liquid water content. The chemical components of aerosols alter the overall refractive index of the atmosphere. The refractive index determines how much light is scattered and how much is absorbed.

Seawater spray

Seawater spray is considered the second most important source of aerosols globally. Particles from seawater spray have the same composition as seawater: water and substances such as sodium chloride, magnesium, calcium, potassium and sulfate salts. In addition, aerosols of marine origin may contain organic compounds. These sprays do not absorb sunlight.

Mineral powders

Mineral dusts are atmospheric aerosols originated by the erosion of the earth's crust and its subsequent dispersion in the air. They are made up mainly of oxides (SiO 2, Al2O3, FeO, Fe2O3, CaO, and others) and carbonates (CaCO3, MgCO3) that make up the earth's crust. These aerosols absorb sunlight.

Global emissions of mineral dusts are estimated at 1000-5000 million tons per year, of which most is attributed to deserts. The Sahara desert is the main source of mineral dust, which is dispersed in the Mediterranean Sea and the Caribbean to northern South America, Central America, North America and Europe. The Gobi desert is another large source of mineral dust, affecting eastern Asia and western North America.

Although these types of aerosols are generally considered to be of natural origin, it is estimated that around 30% of the mineral dust load in the atmosphere could be attributed to human activities through desertification and land misuse.

Sulfur and Nitrogen Compounds

The generation of matter from which secondary aerosols originate can be of anthropogenic origin (from the combustion of fossil fuels) or of natural biogenic origin. Secondary particles are derived from the reaction of primary gases.

• Sulphur oxides

About half of the sulfur dioxide that reaches the atmosphere is deposited back on the surface and the rest is converted into sulfate ions; another secondary aerosol is formed when sulfur dioxide reacts with oxygen in the atmosphere to form sulfur trioxide, which then reacts with water to form sulfuric acid. Both secondary pollutants contribute to the formation of acid rain. They are produced mainly by combustion of coal, oil, metallurgy and volcanic activity.

• Nitrogen oxides

They have as primary aerosols the name NO_x (combined nitric oxide and nitrogen dioxide) that oxidize rapidly in the atmosphere, giving rise to nitrate or nitric acid, they are important in the formation of photochemical smog and influences reactions of formation and destruction of tropospheric and stratospheric ozone. They come from combustion at high temperatures. Nitrous oxide in the troposphere is inert, it disappears from the stratosphere in photochemical reactions and also has a greenhouse effect, its origin is natural sources such as microbiological processes in soil and oceans.

Sulphate and nitrate aerosols are strong scatterers of light.

N₂O, which transmits high-frequency radiation but reflects low-frequency radiation, deserves special attention.

Organic matter

Organic matter aerosols can be primary or secondary. The secondaries are derived from the oxidation of volatile organic compounds (VOCs). Organic material in the atmosphere can be of biogenic or anthropogenic origin. Organic matter aerosols influence the behavior of the atmosphere in the face of radiation, some scattering it and others absorbing it.

Another important type of aerosol is elemental carbon (also known as carbon black). These types of aerosols have a high absorbance of light and are believed to favor the greenhouse effect.

Methane (CH₄) is the most abundant and important of the atmospheric hydrocarbons, it is a primary pollutant formed naturally by anaerobic metabolism reactions, it disappears from the atmosphere by reacting with OH radicals, forming mainly ozone, is one of the greenhouse gases considered in the Kyoto protocol. In some statistics, emissions of volatile organic compounds excluding methane are distinguished as NMVOCs (from English Non-Methane Volatile Organic Compounds).

Permanence of particles in suspension

Size is an important parameter for the behavior of particles, so the smallest ones (less than 2.5 μm in diameter) have lifetimes in the atmosphere from days to weeks, travel distances of 100 kilometers or more, and tend to be homogeneous in urban areas, thus undergoing transformations. In contrast, coarse particles (around 10 μm in diameter) generally settle out faster, with a half-life in the atmosphere of just minutes to hours, therefore, they present greater spatial variability.

Radiative changes of aerosols

Reduction of solar radiation due to volcanic eruptions.

Aerosols, both natural and anthropogenic, can affect climate by changing the way electromagnetic radiation is transmitted into the atmosphere. Direct observations of the effects of aerosols are quite limited, so any attempt to estimate their global effect necessarily involves the use of computer models. The Intergovernmental Panel on Climate Change, IPCC, says: While radiative changes due to greenhouse gases can be determined with a high degree of precision [...] the related uncertainties with radiative changes due to aerosols remain large, and are highly dependent on estimates from global model studies, which are currently difficult to verify.

Here a graph is available showing the contributions (for 2000, compared to 1750) and the uncertainties of various changes.

Currently the MPLNET network, managed by NASA, is capable of continuously measuring the distribution of aerosols and clouds in the atmosphere in different locations. The objective pursued is to characterize the long-term evolution of aerosols and thus refine the models of evolution of the terrestrial climate.

Sulfate Spray

A sulfate spray has two main effects, one direct and one indirect. The direct effect, through the albedo, tends to cool the planet: the most expected value of the radiative forcing according to the IPCC is -0.4 W/m², with a confidence interval of -0.2 to -0.8 W/m². However, there are important uncertainties.

The effect varies significantly with geographic location, with most of the cooling effect possibly located downwind of major industrial centers. Modern climate models that try to calculate the attribution of recent climate changes must include sulfate effects, which seem to contribute, at least in part, to the slight drop in global temperature in the middle of the century XX. The indirect effect (due to the action of aerosols as cloud condensation nuclei, thus modifying cloud properties) is more uncertain, but is believed to be a cooling effect.

Carbon Black

Black carbon is made up of clumps of carbon that form small spheres and is one of the most important types of aerosol absorption in the atmosphere. It must be distinguished from organic carbon that is part of organic molecules. The contribution of black carbon from fossil fuels has been estimated by the IPCC in the IPCC Fourth Assessment Report with an expected global radiative forcing of +0.2 W/m² (it was +0.1 W/m² in the second report assessment The IPCC), with a confidence interval of +0.1 to +0.4 W/m².

Health Effects

Pollution measurement station in Emden, Germany.

The health effects of inhaling suspended particles have been extensively studied in animals and in humans. Those that manifest first are lung pain, headaches, throat discomfort, irritation and tearing of the eyes, while for chronic exposures some are asthma, lung cancer, cardiovascular problems, and premature death. The effects are also related to health disorders such as increased frequency of chronic respiratory diseases, decreased respiratory capacity and increased heart disease. The size of the particles is one of the main determinants of whether they enter the respiratory tract by inhalation. Larger particles generally filter into the nose and throat and do not cause problems, but particles smaller than about 10 micrometers (PM₁₀) can settle in the bronchi and lungs and cause health problems. Similarly, particles smaller than 2.5 micrometers (PM_2.5) tend to penetrate the gas exchange regions of the lung, and particles very small (<100 nanometers) can pass through the lungs and affect other organs. In particular, a study published in the Journal of the American Medical Association indicates that PM_2.5 they tend to form deposits in the arteries, causing vascular inflammation and arteriosclerosis, a hardening of the arteries that reduces their elasticity, which can lead to heart attacks and other cardiovascular problems Researchers suggest that even short-term exposure to high concentrations it can contribute considerably to the development of heart disease.

There is also evidence that particles smaller than 100 nanometers can pass through cell membranes. For example, the particles can migrate in the brain. It has been suggested that the particles may cause brain damage similar to that found in Alzheimer's patients. Particles emitted by modern diesel engines (commonly known as diesel exhaust) are commonly around 100 nanometers (0.1 micrometers) in size. In addition, these soot particles can carry potentially carcinogenic components, such as benzopyrenes, adsorbed on their It is becoming increasingly apparent that legislative limits for engines that are set in terms of mass emissions are not an adequate measure of health hazards A 10 µm diameter particle has about the same mass as 1 million of particles 100 nm in diameter, but it is clearly much less dangerous as it is unlikely to enter the respiratory tract of a human body and if it did it would be quickly removed There are proposals for new regulations in some countries, with Proposals to limit the surface area of the particles or the number of particles.

The relationship between increased deaths and illnesses with particulate pollution was first demonstrated in the early 1970s and has been demonstrated several times since. Particle pollution is estimated to cause between 22,000 and 52,000 deaths per year in the United States (as of 2000).

There are various types of studies to find out the relationship between the exposure or condition and the effect on the body, including toxicological studies and epidemiological studies.

Toxicological studies

They involve the evaluation of the dose-response relationship of a given organism under controlled conditions, exposing it to different doses. They are more useful to determine effects from acute exposures or effects caused by chronic exposures, they are generally carried out in laboratories. It has found significant data between the association of exposure to particles indicative of harmful effects, providing information on mortality and morbidity, however, it has not given the specific properties responsible for its toxicity, such as size, number, shape, composition or reactivity. One of its main problems is that when it is carried out in laboratories, the properties of the particles are known, but they are not representative of the real mixture to which the population is exposed.

Pariemiological studies

They focus on illnesses, causes of death, relationships between these events or what could be dispatched in a group of people are evaluated, so focused on environmental studies, they evaluate exposure to some agent (atmospheric pollution) and if this It is associated with illness or premature death. The effects have been documented for more than 70 years, where, from the first evidence (Osa Valley, Belgium in 1931, Sonora, Pennsylvania, Mexico in 1948 and Atolondres, England in 1952) the association between exposures to particulate matter was found. and gastrovascular mortality and morbidity, which triggered the creation and application of pollution control and prevention measures and programs.

At world level, these studies have been carried out in two types to evaluate the association between indicators of morbidity and premature mortality with air pollution, the first being time series studies, which are used to assess acute particulate exposure due to the relationship between changes in pollution levels and daily fluctuations in the number of incidences of illnesses, hospital admissions, or deaths. While the second type are longitudinal studies that follow up a previously selected group of people over many years to assess the relationship between chronic particle exposure and the incidence of morbidity indicators or the mortality rate.