What makes aerosols

Figure 4. Sea Spray Aerosols. When waves break in the ocean, aerosol particles are formed. Image credit: Haim Weizman. Another natural source of particulate matter is sea spray from the ocean. Think about walking along the beach—does the crisp ocean breeze feel different from the wind when you are further inland? When you are near the ocean, you are probably feeling the sea spray—small droplets of water and other particles that are suspended in the air.

The ocean water contains salt and other organic compounds that are released by the algae, bacteria, and other life forms that live in the ocean. When waves break, droplets of sea water containing these salts and organic matter are entrained in the atmosphere.

The water in these small droplets can evaporate, which leaves a solid particle made up of the sea salt and organic compounds. Human sources of aerosol particles include smoke from fires, vehicle exhaust, and factories. When hydrocarbon fuels such as the gasoline in our cars are burned, what happens?

On a cold day, you may see smoke coming out of the exhaust pipe in your car, and maybe even some drops of water. When fuel is burned, it undergoes combustion. Combustion is a chemical reaction where the fuel reacts with oxygen O 2 in the air to break down into smaller compounds—the main products are carbon dioxide CO 2 and water H 2 O.

However, these fuels can also react through incomplete combustion, which forms carbon monoxide CO instead of carbon dioxide. In addition to these natural and human sources of aerosol particles, there are also secondary sources of aerosol particles that come from chemical reactions in our atmosphere. Gases, such as ozone, can react with organic gases in the air to form solid products—which form aerosol particles!

Based on these sources of aerosols, can you think of some potential sources of aerosol particles in your community? Which areas in your community might have high levels of particulate matter? Figure 5. Direct aerosol effect. Figure 6. Light Scattering. A Light scattering causes an incoming light wave yellow arrow to be scattered in all directions green arrows through its interaction with a particle blue circle B Light scattering is based on the fact that light is an electromagnetic wave.

The electromagnetic wave exerts a force on a charged particle, such as an electron, or a polar molecule.

When this charged particle moves as a result of the force, a new light wave is emitted. This is the scattered light. C Particles of different sizes cause the light to be scattered in different directions. Figure 7. Formation of Cloud Droplets. The water vapor in the atmosphere needs a particle, called a cloud condensation nucleus, to condense on.

When a particle is present, water can condense onto it, forming a cloud droplet. Figure 8. You need to add a pressurised gas to produce the desired effect. Aerosol cans contain two different substances. The liquid product you're interested in releasing, i. The gas helps to push the liquid into the air and turn it into an aerosol cloud, resulting in the fine mist we see when we press down on the nozzle of the aerosol can. Nowadays, the chemicals used are more environmentally friendly and do not damage the ozone layer.

However, though they may be better for the environments, there are other risks. Some of them are harmful to inhale, which can be highly flammable.

That is why it is essential to adhere to all safety instructions and make sure you are using the correct PPE. Aerosol cans would be useless if it weren't for their effectiveness at delivering measured quantities of product. So what makes them able to do that precisely? It is the small valve at the top of the can. When you press the nozzle, it opens the valve, reducing the amount of pressure allowing the contents to escape in a controlled and measured manner.

Fossil fuel combustion produces large amounts of sulfur dioxide, which reacts with water vapor and other gases in the atmosphere to create sulfate aerosols. Desert dust, volatile organic compounds from vegetation, smoke from forest fires, and volcanic ash are natural sources of aerosols. Automobiles, incinerators, smelters, and power plants are prolific producers of sulfates, nitrates, black carbon, and other particles.

Deforestation, overgrazing, drought, and excessive irrigation can alter the land surface, increasing the rate at which dust aerosols enter the atmosphere. Even indoors, cigarettes, cooking stoves, fireplaces, and candles are sources of aerosols.

Nature generates broad swaths of particles—detectable by satellites—over both water and land. Over land, massive plumes of dust blow above deserts.

This map shows the global distribution of aerosols and the proportion of those aerosols that are large or small. Intense colors indicate a thick layer of aerosols. Yellow areas are predominantly coarse particles, like dust, and red areas are mainly fine aerosols, like smoke or pollution.

Gray indicates areas with no data. Meanwhile, the eastern portion of the United States and urban areas in Europe are hotspots for the production of human-made aerosols. Plumes of industrial aerosols — typically sulfates from coal power plants and black and organic carbon from vehicle traffic — rise from cities such as New York, Pittsburgh, London, and Berlin.

The western portion of the United States is comparatively clear, though some areas experience aerosol loads that rival the worst conditions in the East. Industrial aerosols, dust, and wildfire smoke frequently pollute the air in the Los Angeles Basin. Likewise, the port of Houston has some of the most aerosol-laden air in the world. Beijing was completely obscured by air pollution on October 9, However, the most aerosol-laden air in the United States today pales in comparison to Asia.

Satellites can detect a visible pall of aerosol clouds over Bangladesh, northern India, and northern Pakistan—an area called the Indo-Gangetic plain, especially during the pre-Monsoon season. The aerosol layer is comprised of complex mixtures of dust blowing from the Thar Desert and pollution from the densely populated plain. In eastern China, fast-growing cities such as Beijing also produce heavy blankets of aerosol. Depending on the season and weather conditions, surges of aerosols can make their way into the atmosphere almost anywhere on Earth.

In the Northern Hemisphere, plumes of mineral dust swirl over deserts and arid regions. In the Southern Hemisphere, slash-and-burn agriculture in the Amazon and Central Africa releases large amounts of smoke and soot. Fires, some sparked by lightning and some by human activity, leave large patches of forest ablaze during summers in Canada, Russia, and the United States.

Dust storms top and wildfires bottom are significant, if transient, sources of aerosols. Although most aerosols remain suspended in the atmosphere for short periods—typically between four days and a week—they can travel vast distances. Particles moving with the atmosphere at 5 meters Dust plumes from the Sahara frequently cross the Atlantic and reach the Caribbean.

Winds sweep a mixture of Asian aerosols—particularly dust from the Gobi desert and pollution from China—east over Japan and toward the central Pacific Ocean. Smoke from wildfires in Siberia and Canada can find its way to the Arctic ice cap. Dust from North Africa spreads over the Atlantic Ocean on July 1, from to Universal Time in this series of images derived from a computer model of aerosol movement.

Click for animation. Over time, aerosol emissions have changed significantly. In Asia, anthropogenic emissions have increased in recent decades as urbanization and industrialization has proceeded at a breakneck pace.

By contrast, aerosols have declined in North America and Europe as factories have moved to developing countries and Western nations have adopted more stringent clean air regulations. Most aerosols are brighter than land or ocean, and cool the Earth by reflecting sunlight back to space. Different aerosols scatter or absorb sunlight to varying degrees, depending on their physical properties.

However, since aerosols comprise such a broad collection of particles with different properties, the overall effect is anything but simple. Although most aerosols reflect sunlight, some also absorb it. Broadly speaking, bright-colored or translucent particles tend to reflect radiation in all directions and back towards space. Darker aerosols can absorb significant amounts of light. Pure sulfates and nitrates reflect nearly all radiation they encounter, cooling the atmosphere.

Black carbon, in contrast, absorbs radiation readily, warming the atmosphere but also shading the surface. Organic carbon, sometimes called brown carbon or organic matter, has a warming influence on the atmosphere depending on the brightness of the underlying ground. Dust impacts radiation to varying degrees, depending on the composition of the minerals that comprise the dust grains, and whether they are coated with black or brown carbon.

Salt particles tend to reflect all the sunlight they encounter. Black carbon aerosols, similar to the soot in a chimney, absorb sunlight rather than reflecting it. A dark surface low albedo will already absorb a large portion of the solar radiation, and absorbing aerosols will thus have a small effect. Scattering aerosols will instead amplify the total reflectance of solar radiation, since the solar radiation would otherwise be absorbed at the surface.

Over a bright surface high albedo scattering aerosols have a reduced effect. Absorbing aerosols may, however, substantially reduce the outgoing radiation and thus have a warming effect. Aerosols are vital for cloud formation because a subset of them may serve as cloud condensation nuclei CCN and ice nuclei IN. An increased amount of aerosols may increase the CCN number concentration and lead to more, but smaller, cloud droplets for fixed liquid water content.

This increases the albedo of the cloud, resulting in enhanced reflection and a cooling effect, termed the cloud albedo effect Twomey ; Figure 3b. Smaller drops require longer growth times to reach sizes at which they easily fall as precipitation. This effect, called the cloud lifetime effect, may enhance the cloud cover see illustration in Figure 3b and thus impose an additional cooling effect Albrecht However, the life cycles of clouds are controlled by an intimate interplay between meteorology and aerosol-and-cloud microphysics, including complex feedback processes, and it has proven difficult to identify the traditional lifetime effect put forth by Albrecht in observational data sets.

Absorbing aerosols also have the potential to modify clouds properties, without directly acting as CCN and IN, by: 1 heating the air surrounding them while reducing the amount of solar radiation reaching the ground, which stabilizes the atmosphere and diminishes the convection and thus the potential for cloud formation, 2 increasing the atmospheric temperature, which reduces the relative humidity, inhibits cloud formation, and enhances evaporation of existing clouds.

This is collectively termed the semi-direct aerosol effect Hansen et al. Radiative forcing RF is often used to quantify and compare the potential climate impact of the various aerosol effects. RF is defined as a change in the Earth's radiation balance due to a perturbation of anthropogenic or natural origin.. The total aerosol forcing probability density function PDF , in addition to individual aerosol components, indicating both the magnitudes and uncertainty of the effects, is shown in Figure 4a.

The wider a PDF, the larger is the uncertainty. Combining all aerosol effects blue dashed curve in Figure 4a enhances the uncertainty compared to considering only the direct aerosol effect and cloud albedo effect.

Figure 4: Aerosol functions. Radiative imbalances of 0. Industrial era temperature change is taken as 0. If we assume a total aerosol RF and a current energy imbalance, we can compute the resulting climate sensitivity using Equation 1 Figure 4b.

This can then be compared with the PDFs for the current aerosol RF to get an indication of the range in climate sensitivities allowed by the present knowledge red and blue lines in figure 4b. A similar figure has previously been presented in Andreae et al. The allowed climate sensitivity ranges from about 2 to 8 Kelvin K for a doubling of CO 2 using the known industrial age warming of around 0.

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