Polluted air, in contrast, usually contains much higher concentrations of water-soluble particles, which means pollution-rich clouds tend to have more numerous, but smaller, droplets. Natural aerosols-often sulfates, sea salt or ammonium salts-are the most common condensation nuclei in pristine environments. In fact, most clouds owe their existence to aerosols that serve as the tiny “seeds,” called cloud condensation nuclei. That’s true, but aerosols play a critical role in the process. Most elementary school students learn that clouds form when enough water vapor condenses. While greenhouse gases disperse widely and have a fairly consistent impact from region to region, aerosol effects are less consistent, partly because of how the particles affect clouds. On a global scale, these aerosol “indirect effects” typically work in opposition to greenhouse gases and cause cooling. Whereas aerosols can influence climate by scattering light and changing Earth’s reflectivity, they can also alter the climate via clouds. Salt particles tend to reflect all the sunlight they encounter. 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. Organic carbon, sometimes called brown carbon or organic matter, has a warming influence on the atmosphere depending on the brightness of the underlying ground. Black carbon, in contrast, absorbs radiation readily, warming the atmosphere but also shading the surface. Pure sulfates and nitrates reflect nearly all radiation they encounter, cooling the atmosphere. Darker aerosols can absorb significant amounts of light. Broadly speaking, bright-colored or translucent particles tend to reflect radiation in all directions and back towards space. An aerosol’s effect on light depends primarily on the composition and color of the particles. However, since aerosols comprise such a broad collection of particles with different properties, the overall effect is anything but simple.Īlthough most aerosols reflect sunlight, some also absorb it. Climatologists describe these scattering and absorbing properties as the “direct effect” of aerosols on Earth’s radiation field. 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.ĭifferent aerosols scatter or absorb sunlight to varying degrees, depending on their physical properties. In the Southern Hemisphere, slash-and-burn agriculture in the Amazon and Central Africa releases large amounts of smoke and soot. In the Northern Hemisphere, plumes of mineral dust swirl over deserts and arid regions. In eastern China, fast-growing cities such as Beijing also produce heavy blankets of aerosol.ĭepending on the season and weather conditions, surges of aerosols can make their way into the atmosphere almost anywhere on Earth. The aerosol layer is comprised of complex mixtures of dust blowing from the Thar Desert and pollution from the densely populated plain. 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. However, the most aerosol-laden air in the United States today pales in comparison to Asia.
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