College of Marine Studies October 4, 1998 Atmospheric and Oceanic Aerosols and Climate Magdalena Anguelova Ph.D. Student Advisor: Prof. Ferris Webster Duration: 45 min. A sunrise over the China Sea This photograph is taken by the crew of the Space Shuttle. Here the black shadows against the sunlit horizon are high-peaking clouds. The colorful bands above are atmospheric layers and their exceptional brightness is due to concentration of dust in the atmosphere. Dust and other types of particles, called aerosols, and their effect on climate are the subject of this poster. 2 of 52 Outline The big picture - climate elements (18 screens) What are aerosols? (2) Why are aerosols so important? (3) Aerosol properties (3) Aerosol types (3) Aerosol sources and formation (12) Global distribution (5) Summary Hypothesis 3 of 52 The big picture: Sun, Earth, and Atmosphere The climate system on our planet is driven by the energy coming from the sun. The sunlight reaches the Earth through several atmospheric layers. The lowest one, from the Earth surface to about 7miles Thermosphere height, is called troposphere. Mesosphere Next layer, from 7 up to 30 miles above the surface, is called stratosphere. Stratosphere Mesosphere and thermosphere follow above Troposphere up to about 50 miles height. The layers of interest for us are those where the aerosols reside: the troposphere and stratosphere. 4 of 52 The big picture: Sun spectrum Recall: each body with some temperature emits radiation. We feel the radiation emitted from our bodies as heat. This law applies to all objects in the Universe. Solar spectrum The sun is a celestial body with a temperature of 6000 oC. Objects with such high temperature emit energy at the so called Near IR Far IR short wavelengths of the electromagnetic spectrum visualized like this: Visible The Sun emission peaks in the visible range. 5 of 52 The big picture: Earth Spectrum Solar spectrum Earth’s spectrum In contrast, the Earth is colder celestial body with average surface Near IR Far IR temperature of 15oC. short Visible long That is why Earth emits at longer wavelengths, Get oriented in Electromagnetic spectrum ! called infrared (IR), X-rays in medicine Radio broadcasting visualized like this: So, remember: The Sun emits at short wavelengths (SW). The Earth emits at long wavelengths (LW). The big picture: Solar energy at sea level Only a part of the SW solar radiation available at the top of the atmosphere reaches the Earth. Some of it is scattered, absorbed and reflected within the atmosphere by the gases, aerosols, and clouds. The absorbed radiation is re- emitted by the atmospheric constituents back as a LW radiation, i.e., it is converted in heat. 7 of 52 The big picture: Greenhouse effect Similar process takes place at the surface of Earth: from the SW solar radiation (A) left at the sea level, part is absorbed (B) and then re-emitted back (C) to the atmosphere as LW IR radiation. If there were no atmosphere, the IR radiation emitted by Earth would escape to the space and the planet would cool down. But in presence of atmosphere, some IR radiation is trapped and re-emitted back (D) to Earth by naturally occurring gases as CO2 , H2O vapors, and CH4. This is the so called natural greenhouse effect which keeps the Earth’s surface about 33 oC warmer than it would be if greenhouse effect were not present. 8 of 52 The big picture: Climate System How fortunate for all living creatures ! For the natural greenhouse effect makes our planet habitable. Otherwise the Earth would be a frigid and inhospitable place. The atmosphere sets the greenhouse effect at work, and the climate system is created. The solar radiation powers it. The climate machine does not stop if something goes wrong in it. If small perturbation in one of the elements appears, e.g., a change in solar emission, or a change in ocean shapes due to plate tectonics, the system tries to readjust to the new conditions. 9 of 52 The big picture: Climate System Elements The main elements of the climate system are the atmosphere and the oceans. The fast heating and cooling of land, the strong reflection of the sunlight by ice and snow, the clouds and precipitation are Their interaction with the basic the other elements of elements makes the last touches in this machine. this almost perfect harmony. 10 of 52 The big picture: How does it work? The warmth of the Sun is not distributed uniformly over the globe. It is maximum at the equator and the tropics and minimum at the polar regions. The climate machine churns relentlessly attempting to smooth out this temperature imbalance by cooling the tropics and warming the poles. The wind system and ocean currents do the work. 11 of 52 The big picture: Winds and Currents Unequal heating of the atmosphere sets up convection - rising of warm air at the tropics and sinking of cold air at high latitudes. This in turn sets a regular system of winds called Trades (or Easterlies) and Westerlies. These, together with frontal storms, transfer cold air equatorward and warm air poleward. 12 of 52 The big picture: Winds and Currents The winds drag the water in the oceans and form a system of immense ocean currents. They transport cold water toward the Equator and warm water to the poles. As a result of this heat transfer, the average temperature anywhere on the Earth is quite stable over long time period. 13 of 52 The big picture: Radiation Budget So, on a long-time scale, the climate system is in equilibrium. This usually is demonstrated with the radiation budget of the planet. Let see: The SW radiation coming to the Earth is 340 W m-2. Reflected SW = 100 W m-2 About 30% of it is directly reflected back to the space. Incoming SW Emitted LW = 340 W m-2 = 240 W m-2 The remaining 240 W m-2 are absorbed by the Earth- atmosphere system. Climate system As the law requires, the same amount is emitted as LW radiation back to space. Absorbed SW This is the natural and = 240 W m-2 necessary balance ! 14 of 52 The big picture: Troubled Radiation Budget The trouble is... We, humans, are adding more and more greenhouse gases into the atmosphere by burning fossil fuels. Even worse, we increase not only the concentration of the naturally occurring greenhouse gases, but add unnatural greenhouse gases, such as nitrous oxide (N2O) and chlorofluorocarbons (CFCs). In addition, we cut down thousands of trees for lumber, making them unable to take CO2 out of the air. All this waste in the air is letting less and less heat to go back to space. And, the more CO2 and other greenhouse gasses in the atmosphere, the more IR radiation is trapped and re-emitted back to the Earth, the warmer it becomes with possible catastrophic effects. 15 of 52 The big picture: Global Warming This greenhouse effect in excess of the natural one is termed global warming. Scientists try to model and predict the effect of global warming. They recalculate the radiation budget with increased concentration of CO2. If the amount of CO2 doubles, the outgoing LW radiation would decrease by 4 W m-2. This imbalance would induce a gradual change in order to restore the amount of leaving radiation from 236 back to 240 W m-2. This would require an increase in global mean surface temperature by 1.2 K. Instantaneous CO2 Doubling: And this is Emitted LW = 236 W m-2 Warm by 1.2 K to restore 240 W m-2 a trouble ! 16 of 52 The big picture: Modeling Global Warming The current models, however, produce both greater warming and substantial disagreement: from 1.7 to 5.4 K. The main reason for the disagreement stems from the different depiction of the climate feedback mechanisms in the models. These can either amplify or moderate the warming. E.g., a warmer climate means a warmer atmosphere with more water vapor, which itself is a greenhouse gas. So, water vapor provides a positive (or amplifying) feedback mechanism. Different models generally consent on this particular feedback. The feedback associated with cloudiness, however, turns out to be much more difficult matter. 17 of 52 The big picture: Radiation Budget Without Clouds Let track the radiation budget of a hypothetical planet with the same surface temperature but without clouds: The same coming solar radiation; In absence of clouds less SW radiation is reflected back to space, only 50 W m-2 instead of 100 W m-2 ; Earth-atmosphere system absorbs the remaining 290 W m-2, instead of 240 W m-2; At 15 degrees surface temperature Earth-atmosphere system emits only 270 W m-2 and ... There is surplus of 20 W m-2 ! Obviously, the clouds balance the system. How? 18 of 52 The big picture: Cloud Feedback The effect of clouds on the Earth-atmosphere system is termed as Cloud-Radiative Forcing (CRF). We see, the clouds enhance the SW reflection and cool the system by 50 W m-2. In this way the clouds exert negative feedback. But they also absorb LW radiation coming from the earth and re-emit it back. So that simultaneously with the negative the clouds provide also positive feedback and warm the Earth with 30 W m-2. The net result of these two opposite processes is cooling by 20 W m-2. So, Cloud Radiative Forcing (CRF) SW CRF = -50 W m-2 closes the balance of absorbed LW CRF = 30 W m-2 and emitted radiation. NET CRF = -20 W m-2 19 of 52 The big picture: Modeling Cloud Feedback How exciting ! The cooling by clouds would mitigate the global warming ! However, the cooling by clouds may change as the climate changes due to global warming. Scientists constructed and ran 2.0 models again to see how the cloud 1.5 radiative forcing would change. Cloud forcing 1.0 Here 19 (!) different models 0.5 show quite different results for cloud feedback : 0.0 from modest cooling -0.5 MRI CCM0 CCM1 BMRC DNM LMD CCC CNRM CSU GICC OSU/IAP ECMWF MGO ECHAM GFDL II UKMO GFDL I OSU/LLNL CCM/LLNL through almost missing to strong positive. Model name 20 of 52 The big picture: What about Aerosols? The discrepancies in modeling the cloud feedback pointed out that we need to know well the cloud properties and their global pattern. So, since the beginning of 90s studies of clouds have priority and many programs for measuring the global cloud coverage and properties have been initiated. You probably ask yourself already impatiently: Where in this long story are the aerosols? Well, the findings about clouds gradually showed that the cloud properties and lifetime are significantly affected by aerosols. How exactly? It turned out we do not have enough knowledge about aerosols in order to know how they do that. So, for the last 2 years the aerosols, not the clouds, are the Gordian knot of the climatic studies. 21 of 52 What are Aerosols? Aerosols are minute stable particles, solid or liquid, suspended in the atmosphere. Samples of clean (rural) and polluted rural site (urban) air under microscope show different particle shapes (spherical or arbitrary) and concentrations. For the scale: 1 m = 10-6 m urban site 22 of 52 We see Aerosols as... Aerosols are too small to be observed by naked eye. We do not see the air molecules too. But we see the result of scattering of the sunlight by them as blue sky. Similarly, the red sunsets and sunrises are result of scattering and absorbing of the sunlight by aerosols. The red color comes from the fact that the aerosols are larger than the air molecules and scatter more effectively the red light than the blue one. Another manifestation of the presence of aerosols is haze. 23 of 52 Why are Aerosols so Important? Atmospheric aerosols influence the climate in two ways: directly - through the reflection and absorption of solar radiation. The mechanisms are well understood: scattering of coming SW solar radiation back to space; absorption of coming SW solar radiation. In both cases less radiation reaches and heats the Earth, i.e., the aerosols cool the Earth-atmosphere system. indirectly - through modifying the optical properties and lifetime of clouds. Aerosols act as cloud condensation nuclei (CCN) on which H2O vapors in the atmosphere condense and form cloud droplets. Two scenarios are at work: 24 of 52 Cloud/Aerosol Scenarios When there are more aerosols (i.e., more CCN), more droplets form in the cloud. We observe: 1) more surface available to reflect the light. Net result: cloud albedo (reflection) increases. 2) inhibition of the growth of the existing droplets, hence condensation and rain are delayed. Net result: prolonged cloud lifetime. 25 of 52 More Roles for Aerosols Aerosols act as sites for chemical reactions to take place. The most significant example: destruction of stratospheric ozone. During winter in the polar regions, aerosols grow to form polar stratospheric clouds. The cloud particles provide huge surface area for chemical reactions. These reactions lead to the formation of large amount of reactive chlorine, which ultimately leads to destruction of the ozone in the stratosphere. Increased aerosol pollution from 1979 to 1989 resulted in ozone hole over Antarctic. 26 of 52 Aerosol Properties The effect of aerosols on climate is termed aerosol radiative forcing. To estimate aerosol radiative forcing we need to know which aerosol properties control the different processes. Aerosols represent only a small part of the mass of the atmosphere. Yet, they have the potential to influence the heat budget of the planet. The reason is that most processes involving aerosols are controlled by the aerosol surface, not by the aerosol mass. That is, many small particles do better than few large. Thus, the most important aerosol properties are: size and shape concentration and lifetime 27 of 52 Aerosol properties: Size and Shape Typical distribution of aerosol mass and number by size. We see 2 peaks over coarse and fine particles. The size controls the physical and chemical processes ! Particles with diameter: • 0.01 - 0.05 m act as cloud nuclei; • 0.08 to 0.5 m accumulate mass; • 0.1 - 2 m efficiently scatter the light; • above 1 m provide medium for chemical reactions. 28 of 52 Aerosol properties: Concentration and Lifetime Concentration is defined as the total number of particles per unit volume; Concentration changes with height and site, being higher where the aerosols form: • look - there are more aerosols close to the Earth surface, and their number decreases with height; • there are more aerosols close to the continents, and their number decreases in remote oceanic places. Lifetime is the time aerosols reside in the atmosphere before being removed by precipitation or conversion in something else; Aerosol lifetime ranges from 2-3 years to 3-5 days; Most aerosols are short-lived. 29 of 52 Aerosol Types There are many types of aerosols classified by different criteria. Depending on their size aerosols are (we already know this): coarse and fine; Depending on their source aerosols are: • natural - produced by volcanic emission or oceans; • anthropogenic - result of the human activities; Depending on their mechanism of formation aerosols are: • primary - delivered to the atmosphere directly as particles; • secondary - formed within the atmosphere from gases; Depending on their residence site aerosols are: tropospheric and stratospheric 30 of 52 Aerosol Types: Natural Aerosols Examples of natural aerosols are: Primary (formed directly as particles) Most numerous are Soil dust (mineral aerosol) Sea salt Secondary Volcanic dust (formed in the atmosphere from gases) Organic aerosols Sulfates from biogenic gases Sulfates from volcanic SO2 Organic matter from biogenic C Nitrates from NOx Let talk about some of these ! 31 of 52 Natural Aerosols: Soil Dust Sources Major sources of soil dust are arid regions such as deserts of Northern Africa and Asia. One of the largest source is Sahara desert. This photograph is a good example: We see a dust storm north of Arabian Sea - a basin surrounded by arid terrain. The area joining Iran, Afghanistan and Pakistan experiences the highest frequency of dust storms in the world: Imagine, this over 30 dust storms per year. As a result, village endures the deposition rate of such a mess mineral aerosols in the twice, sometimes Indian Ocean is more more, a month ! than 5 times any other region of the world oceans. 32 of 52 Natural Aerosols Formation: Soil Dust Lifting The number of aerosols delivered by extreme events as dust storms dominates the number of aerosols created by continuous lower wind. Though the particles produced in this way are relatively large, they are found all over the globe: the strong winds lift them at high altitudes and the atmospheric circulation transports them over thousands of kilometers. There are 4 mechanisms of detachment and lifting of soil particles by wind: (a) creeping - one large particle bounces several times creating many smaller particles; (b) turbulent lifting - strong wind projects particles directly in air; (c) surface collision; (d) soil splashing. 33 of 52 Natural Aerosol: Source of Sea Salt Aerosols On a windy day, when even a skillful Hawaiian surfer may flip, the ocean is covered with whitecaps. Whitecaps are the major source for sea salt particles. They produce numerous drops, which evaporate, shrink to a smaller size, and form sea salt aerosols. Interestingly enough, the vast oceans produce a bit less aerosols than deserts produce dust. The reason is that the formation of sea salt drops, parenting the sea salt aerosols, includes several processes requiring more energy than mere lifting of a dust particle. Let see these processes. 34 of 52 Natural Aerosol Formation: Sea Salt Air entrained into the water after wave breaking creates bubble clouds. The large bubbles rise to the surface and burst. Their caps shatter in thousands small droplets called film drops. Upon bursting, bubble cavity collapses, a water jet rises from its bottom, and several small drops, called jet drops, are pinched from the tip. Under high winds so called spume drops are torn from the crests of breaking waves and blown directly into the air. Least drops are formed by splashing mechanism, when some small unstable projections of water form drops. 35 of 52 Natural Aerosol: Volcanic dust This photograph of the eruption of Mt. St. Helens in 1980 is a good example for the huge clouds of ash particles and gases, including sulfur dioxide, that volcanoes blast into the atmosphere as they erupt. Short-term global cooling often has been linked with such events. The year 1816 has been referred to as “the year without a summer.” It was a time of significant weather disruption in New England and in Western Europe with killing summer frosts in the United States and Canada. The unusual weather was attributed to a major eruption of the Tambora volcano in 1815 in Indonesia. The volcano threw sulfur dioxide gas into the stratosphere, and the aerosol layer that formed led to brilliant sunsets seen around the world for several years. 36 of 52 Natural Aerosol: Cooling by Volcanic Dust Aerosols in atmosphere increase after major eruptions The relative global cooling of 1993 is ascribed to the eruption of Mount Pinatubo in 1991. Several weeks after spreading of volcanic dust across the Pacific, the sulfur dioxide had spread all over the world. The red color shows maximum aerosol Not all large volcanic eruptions produce global- concentration scale cooling. Mount Agung in 1963 caused a considerable decrease in temperatures around much of the world, whereas El Chichón in 1982 seemed to have little effect. It is believed the 1982 El Niño cancelled out the effect of the El Chichón eruption. SO2 cloud from Mt. Pinatubo, September 23, 1991 37 of 52 Natural Aerosol Formation: Volcanic dust Millions of tons of ash and SO2 gas can reach the stratosphere from a major volcano. The ash is soon washed out by rain. SO2 stays and under the action of light converts to tiny aerosols of sulfuric acid. These aerosols are persistent, and after the stratospheric winds spread them over the globe, they These particles reflect the sunlight, stay there for several thereby cooling the Earth. years. They grow slowly and are regularly removed by rain for a long time. 38 of 52 Natural Aerosols: Sulfates and organic matter The living creatures in the oceans and on land are involved in the creation of organic aerosols and sulfates. Bubble bursting in oceans and burning of terrestrial vegetation deliver organic carbon and other particles. Phytoplankton in the oceans emits gas called Dimethylsulphide (DMS). DMS is transferred into the atmosphere where organic aerosols form by gas-to-particle The biosphere from satellites. conversion. Emissions of natural organic aerosols from oceans dominate the terrestrial sources. 39 of 52 Aerosol types: Anthropogenic Aerosols Examples of anthropogenic aerosols are: Primary (formed directly as particles) Most numerous are Industrial aerosols Biomass burning Secondary Soot (formed in the atmosphere from gases) Sulfates from industrial SO2 Organic matter from biogenic C Nitrates from NOx Let talk about some of these ! 40 of 52 Anthropogenic Aerosols: Sources of Industrial Pollution The primary industrial aerosols originate from inorganic these sources increase or Allimpurities in the fuel we use : from incomplete fuel combustion. • carbon dioxide (CO ) 2 • methane (CH4) • nitrous oxide (N2 O) • halocarbons (CFCs) Airplanes and factories Most dangerous are the sulfate aerosols release water vapor formed from these gases, because they are forming additional persistent. clouds and reflecting the incoming sunlight. About 90% of the sulfur emissions are Recent estimates show that industrial able to change Plants, cars, fromcontrails are regions in the Northern and aircraft hemisphere. locally in regions emit directly soot, and the climate with heavy airplane traffic. nitrogen oxides. 41 of 52 Anthropogenic Aerosols: Biomass Burning - natural and... Biomass burning refers to the burning of the world's forests, grasslands, and agricultural lands. It releases significant quantities of gases and particles into the atmosphere. There are natural fires like this one in Arizona, but it is generally believed that most biomass burning is human-initiated. The oil fires in Kuwait is one such example. The Hochderffer fire, Coconino National Forest, AR 42 of 52 Anthropogenic Aerosols: Man-initiated Biomass Burning The biomass burning has increased significantly over the last century. Regular measurements and monitoring from space helped in the last few years to understand that biomass burning is much more widespread than previously thought. Biomass burning is a widespread practice for land clearing and land use change such as conversion of forest regions to grazing and agriculture areas. Roughly 175 million acres of forest and grassland are burned each year world-wide. 43 of 52 Anthropogenic Aerosols Formation: Biomass Burning Combustion gases include CO2, CO, hydrocarbons, NxO, etc. CO2 and CH4 are direct addition to the greenhouse gases. The other gases are chemically active and impact the composition and chemistry of the troposphere, leading to destruction of ozone. 80% of the total biomass burning occurs in tropical rain forests 2/3 of the and savanna grasslands Earth's savannas are located in Africa, recognized as the "burning center" of the planet. Biomass burning extends to fire-free regions as smoke and aerosol particles rise high into the troposphere and are carried long distances by winds. 44 of 52 Aerosol global distribution Satellite observations reveal that there is no "global aerosol" that fills the troposphere with a uniform background aerosol. • The global aerosol distribution is a collection of independent aerosol regions each having its own source and unique spatial temporal pattern. • Marine aerosols dominate large areas, but continental aerosol plumes show more intense reflection of sunlight. Hence, the aerosol impact over the continents is likely to be much higher than over the oceans. • The aerosol reflection is strongest in the Tropics where most of the solar radiation is absorbed and aerosol-cloud interactions are intense. • There is a pronounced seasonality in each aerosol region; the higher aerosol levels appear in the summer. 45 of 52 Aerosol global distribution: Oceanic aerosol summer winter Indeed, aerosols are concentrated in the Tropics and their reflection is higher in summer than in winter. Even more aerosols are present during phytoplankton bloom in spring. spring 46 of 52 Aerosol global distribution: Oceanic CloudCondensationNuclei Recall: the more aerosols, the more nuclei for forming cloud drops (CCN). We see, most CCN are around the continents where the aerosols produced by human activity are most. 47 of 52 Aerosol global distribution: Continental aerosol over oceans Once again the same seasonal pattern: more aerosols in summer than in winter. Note 2 places we considered: winter • The pronounced plume from Africa - Sahara and savanna fires produce enormous quantity of aerosols; • Indian ocean - the arid areas around Arabian sea with strong summer dust storms. 48 of 52 Aerosol global distribution: Volcanic aerosols This is animation showing the spreading of aerosols after 3 volcano eruptions in period 1985 - 1997; rate - every 3 months. Red: high aerosol reflection. Eruptions : • Nevado del Ruiz, Columbia, 1985 Most of the volcanic aerosols were high in the stratosphere and remained obvious for several years. • Kelut, Indonesia, February, 1990 small increase; • Mt. Pinatubo, 1991: the dominant event in this animation, aerosols in stratosphere increased by a factor of 30. 49 of 52 Aerosol global distribution: Sulfur emissions As expected, the industrial regions are the major sources of anthropogenic sulfate aerosols. 49 of 52 Summary • Aerosols influence the climate directly via scattering of sunlight indirectly via changing clouds’ optical properties • Aerosols provide medium for chemical reactions in the atmosphere • Aerosols are unevenly distributed over the globe • Aerosols are short lived with exception of the volcanic dust 50 of 52 Hypothesis There is a hypothesis: aerosol cooling, mainly due to man-produced sulfates, may cancel the effect of global warming. Calming but not yet proven idea… While uniformly distributed greenhouse gases over the globe may cause global warming, the uneven aerosol distribution may only cool places here and there. This may still be not enough to outweigh the warming. We have much more work to do ... 51 of 52 Still here? Reading? That’s it! Puzzled? Then please, questions?
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