Biochar

Aerosol Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are smoke, oceanic haze, air pollution, smog and CS gas. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can. The word aerosol derives from the fact that matter "floating" in air is a suspension (a mixture in which solid or liquid or combined solid-liquid particles are suspended in a fluid). To differentiate suspensions from true solutions, the term sol evolved—originally meant to cover dispersions of tiny (submicroscopic) particles in a liquid. With studies of dispersions in air, the term aerosol evolved and now embraces both liquid droplets, solid particles, and combinations of these.

Exposure Concentrated aerosols from substances such as silica, asbestos, and diesel particulate matter are sometimes found in the workplace and have been shown to result in a number of diseases including silicosis and black lung. Respirators can protect workers from harmful aerosol exposure.

Effect on climate Aerosols over the Amazon each September for four burning seasons (2005 through 2008). The aerosol scale (yellow to dark reddish-brown) indicates the relative amount of particles that absorb sunlight. Anthropogenic aerosols, particularly sulfate aerosols from fossil fuel combustion, exert a cooling influence on the climate[3] which partly counteracts the warming induced by greenhouse gases such as carbon dioxide. This effect is accounted for in many climate models.[4] Recent research, as yet unconfirmed, suggests that aerosol diffusion of light may have increased the carbon sink in the Earth's ecosystem.[5] Recent studies of the and major increases since 1967 in rainfall over the Northern Territory, Kimberley, Pilbara and around the Nullarbor Plain have led some scientists to conclude that the aerosol haze over South and East Asia has been steadily shifting tropical rainfall in both hemispheres southward.[7] The latest studies of severe rainfall declines over southern Australia since 1997[8] have led climatologists there to consider the possibility that these Asian aerosols have shifted not only tropical but also midlatitude systems southward. Oxides of nitrogen (NOx) in the atmosphere are a form of pollution which can give rise to smog and act as a greenhouse gas. Their persistence in the atmosphere is affected by aerosol droplets of water. In 1964 long chain fatty acids, either naturally produced from marine organisms dispersed into the atmosphere by wave action or man-made, were found to coat these droplets. In 2006 there was a study of the effect of the LCFA on the persistence of NOx, but the long term implications, although thought to be significant, have yet to be determined. CS gas is the common name for 2-chlorobenzalmalononitrile (also called o-Chlorobenzylidene Malononitrile) (chemical formula: C10H5ClN2), a "tear gas" that is used as a riot control agent. It is generally accepted as being non-lethal. CS was discovered by two Americans, Ben Corson and

Roger Stoughton, at Middlebury College in 1928, and the chemical gets its name from the first letters of the scientists' surnames.[4] The compound is actually a solid at room temperature, though it is used as an aerosol CS was developed and tested secretly at Porton Down in Wiltshire, England, in the 1950s and 1960s. CS was used first on animals, then subsequently on British Army servicemen volunteers. Notably, CS has a limited effect on animals due to "under-developed tear-ducts and protection by fur".

Aerosols:What Are They, and Why Are They So Important? Aerosols are minute particles suspended in the atmosphere. When these particles are sufficiently large, we notice their presence as they scatter and absorb sunlight. Their scattering of sunlight can reduce visibility (haze) and redden sunrises and sunsets. Aerosols interact both directly and indirectly with the Earth's radiation budget and climate. As a direct effect, the aerosols scatter sunlight directly back into space. As an indirect effect, aerosols in the lower atmosphere can modify the size of cloud particles, changing how the clouds reflect and absorb sunlight, thereby affecting the Earth's energy budget. Aerosols also can act as sites for chemical reactions to take place (heterogeneous chemistry). The most significant of these reactions are those that lead to the destruction of stratospheric ozone. During winter in the polar regions, aerosols grow to form polar stratospheric clouds. The large surface areas of these cloud particles provide sites for chemical reactions to take place. These reactions lead to the formation of large amounts of reactive chlorine and, ultimately, to the destruction of ozone in the stratosphere. Evidence now exists that shows similar changes in stratospheric ozone concentrations occur after major volcanic eruptions, like Mt. Pinatubo in 1991, where tons of volcanic aerosols are blown into the atmosphere (Fig. 1). Fig. 1 The dispersal of volcanic aerosols has a drastic effect on the Earth's atmosphere. Following an eruption, large amounts of sulphur dioxide (SO2), hydrochloric acid (HCL) and ash are spewed into the Earth's stratosphere. Hydrochloric acid, in most cases, condenses with water vapor and is rained out of the volcanic cloud formation. Sulphur dioxide from the cloud is transformed into sulphuric acid (H2SO4). The sulphuric acid quickly condenses, producing aerosol particles which linger in the atmosphere for long periods of time. The interaction of chemicals on the surface of aerosols, known as heterogeneous chemistry, and the tendency of aerosols to increase levels of chlorine which can react with nitrogen in the stratosphere, is a prime contributor to stratospheric ozone destruction.

Volcanic Aerosol Three types of aerosols significantly affect the Earth's climate. The first is the volcanic aerosol layer which forms in the stratosphere after major volcanic eruptions like Mt. Pinatubo. The dominant aerosol layer is actually formed by sulfur dioxide gas which is converted to droplets of sulfuric acid in the stratosphere over the course of a week to several months after the eruption (Fig. 1). Winds in the stratosphere spread the aerosols until they practically cover the globe. Once formed, these aerosols stay in the stratosphere for about two years. They reflect sunlight,

reducing the amount of energy reaching the lower atmosphere and the Earth's surface, cooling them. The relative coolness of 1993 is thought to have been a response to the stratospheric aerosol layer that was produced by the Mt. Pinatubo eruption. In 1995, though several years had passed since the Mt. Pinatubo eruption, remnants of the layer remained in the atmosphere. Data from satellites such as the NASA Langley Stratospheric Aerosol and Gas Experiment II (SAGE II) have enabled scientists to better understand the effects of volcanic aerosols on our atmosphere.

Desert Dust The second type of aerosol that may have a significant effect on climate is desert dust. Pictures from weather satellites often reveal dust veils streaming out over the Atlantic Ocean from the deserts of North Africa. Fallout from these layers has been observed at various locations on the American continent. Similar veils of dust stream off deserts on the Asian continent. The September 1994 Lidar In-space Technology Experiment (LITE), aboard the space shuttle Discovery (STS-64), measured large quantities of desert dust in the lower atmosphere over Africa (Fig. 2). The particles in these dust plumes are minute grains of dirt blown from the desert surface. They are relatively large for atmospheric aerosols and would normally fall out of the atmosphere after a short flight if they were not blown to relatively high altitudes (15,000 ft. and higher) by intense dust storms. Because the dust is composed of minerals, the particles absorb sunlight as well as scatter it. Through absorption of sunlight, the dust particles warm the layer of the atmosphere where they reside. This warmer air is believed to inhibit the formation of storm clouds. Through the suppression of storm clouds and their consequent rain, the dust veil is believed to further desert expansion. Recent observations of some clouds indicate that they may be absorbing more sunlight than was thought possible. Because of their ability to absorb sunlight, and their transport over large distances, desert aerosols may be the culprit for this additional absorption of sunlight by some clouds.

Human-Made Aerosol The third type of aerosol comes from human activities. While a large fraction of human-made aerosols come in the form of smoke from burning tropical forests, the major component comes in the form of sulfate aerosols created by the burning of coal and oil. The concentration of humanmade sulfate aerosols in the atmosphere has grown rapidly since the start of the industrial revolution. At current production levels, human-made sulfate aerosols are thought to outweigh the naturally produced sulfate aerosols. The concentration of aerosols is highest in the northern hemisphere where industrial activity is centered. The sulfate aerosols absorb no sunlight but they reflect it, thereby reducing the amount of sunlight reaching the Earth's surface. Sulfate aerosols are believed to survive in the atmosphere for about 3-5 days. The sulfate aerosols also enter clouds where they cause the number of cloud droplets to increase but make the droplet sizes smaller. The net effect is to make the clouds reflect more sunlight than they would without the presence of the sulfate aerosols. Pollution from the stacks of ships at sea has been seen to modify the low-lying clouds above them. These changes in the cloud droplets,

due to the sulfate aerosols from the ships, have been seen in pictures from weather satellites as a track through a layer of clouds. In addition to making the clouds more reflective, it is also believed that the additional aerosols cause polluted clouds to last longer and reflect more sunlight than non-polluted clouds.

Climatic Effects of Aerosols The additional reflection caused by pollution aerosols is expected to have an effect on the climate comparable in magnitude to that of increasing concentrations of atmospheric gases. The effect of the aerosols, however, will be opposite to the effect of the increasing atmospheric trace gases - cooling instead of warming the atmosphere. The warming effect of the greenhouse gases is expected to take place everywhere, but the cooling effect of the pollution aerosols will be somewhat regionally dependent, near and downwind of industrial areas. No one knows what the outcome will be of atmospheric warming in some regions and cooling in others. Climate models are still too primitive to provide reliable insight into the possible outcome. Current observations of the buildup are available only for a few locations around the globe and these observations are fragmentary. Understanding how much sulfur-based pollution is present in the atmosphere is important for understanding the effectiveness of current sulfur dioxide pollution control strategies.

The Removal of Aerosols It is believed that much of the removal of atmospheric aerosols occurs in the vicinity of large weather systems and high altitude jet streams, where the stratosphere and the lower atmosphere become intertwined and exchange air with each other. In such regions, many pollutant gases in the troposphere can be injected in the stratosphere, affecting the chemistry of the stratosphere. Likewise, in such regions, the ozone in the stratosphere is brought down to the lower atmosphere where it reacts with the pollutant rich air, possibly forming new types of pollution aerosols.

Aerosols As Atmospheric Tracers Aerosol measurements can also be used as tracers to study how the Earth's atmosphere moves. Because aerosols change their characteristics very slowly, they make much better tracers for atmospheric motions than a chemical species that may vary its concentration through chemical reactions. Aerosols have been used to study the dynamics of the polar regions, stratospheric transport from low to high latitudes, and the exchange of air between the troposphere and stratosphere.

Future NASA Aerosol Studies NASA's ongoing Atmospheric Effects of Aviation Project (AEAP) has measured emissions from the engines of several commercial and research aircraft. Jet engine emissions have been shown to affect the concentrations of atmospheric water vapor and aerosols, and they may affect how clouds form and the concentrations of atmospheric ozone. Few actual measurements of their effects have been made, however. In the spring of 1996, the Subsonic Aircraft Contrail and Cloud Effects Special Study

(SUCCESS) focused on subsonic aircraft contrails and the impact of the aerosols in those contrails on cirrus clouds and atmospheric chemistry. Researchers have determined that aircraft contrails can prolong the presence of high altitude cirrus clouds while also decreasing the size of the ice crystals that make up the clouds.Studies like SUCCESS and AEAP will be ongoing as scientists continue to try to understand how aerosols affect our atmosphere and climate. What are aerosols? Aerosols are fine, airborne particles consisting at least in part of solid material. Density of the basic materials of aerosols range from 1.0 g/cm3 (for soot) to 2.6 (for minerals). The ocean is a major source of natural aerosols. Air-sea exchange of particulate matter contributes to the global cycles of carbon, nitrogen, and sulfur aerosols, such as dimethylsulfide (DMS) produced by phytoplankton. Ocean water and sea salt are transferred to the atmosphere through air bubbles at the sea surface. As this water evaporates, the salt is left suspended in the atmosphere. Four other significant sources of aerosols are terrestrial biomass burning, volcanic eruptions, windblown dust from arid and semi-arid regions, and pollution from industrial emissions (Fig 1). Clean continental air often contains less than 3,000 particles per cubic centimetre (of which half are water-soluble), polluted continental air typically 50,000/cm3 (of which two-thirds are soot, and the rest mostly water-soluble). Urban air typically contains 160,000/cm 3, mostly soot, and only 20% is water-soluble. Desert air has about 2,300/cm 3 on average, almost all water-soluble. Clean marine air generally has about 1,500/cm3, about all water-soluble. The lowest sea-level values occur over the oceans near the subtropical highs (600/cm3 on average, but occasionally below 300/cm3). Arctic air has about 6600/cm3 (including 5,300 soot) and on the Antarctic plateau only 43/cm3 occur (about all sulphate) ( where N(z) is the concentration at height z (km) above ground level. The scale height H typically is 10 km for continental (incl desert) air in summer, 6 km for urban air, 2 km for continental air in winter, and 1 km over the marine subtropical high pressure regions. The higher values during the summer imply more deep-tropospheric stirring of the aerosols by thunderstorms. In places with a well-defined, long-lived inversion on top of a well-mixed planetary boundary layer, the aerosol concentration tends to be homogenous in the PBL and perhaps 10x lower above the inversion. How aerosols affect climate Aerosols play an important role in the global climate balance, and therefore they could be important in climate change. Natural variations of aerosols, especially due to episodic large eruptions of volcanoes, such as Mt. Pinatubo in 1991, are recognized as a significant climate forcing, that is, a factor that alters the Earth's radiation balance and thus tends to cause a global temperature change. In addition, there are several ways in which humans are altering atmospheric aerosols, not only near the ground (e.g. industrial emissions) but as high as the lower stratosphere (where they are continuously emitted by aircraft), and thus possibly affecting climate (e.g. through contrails) (3). Aerosols force climate in two ways (4): •direct radiative forcing: the scattering of solar radiation and the absorption/emission of terrestrial radiation.

•indirect radiative forcing, mainly by effects of aerosols on cloud properties. (A minor indirect effect involves the heterogeneous chemistry of greenhouse gases: these gases may react at the surface of an aerosol and therefore change radiative properties.) Greenhouse gases have a well-understood effect on the global radiative balance and surface temperatures, their concentration has little variability (except water vapour and ozone), and their long-term trends are well-known. Therefore, there is much confidence in the greenhouse gas component of the anticipated climate change during the next few decades. However climate forcings due to aerosols are not determined well, especially the indirect radiative forcing. Indeed, aerosols are one of the greatest sources of uncertainty in interpretation of climate change of the past century and in projection of future climate change. The effect of aerosols on clouds is highly speculative. The theory is that the more aerosol, the smaller the cloud droplets tend to be, and clouds with more but smaller drops have a higher albedo. This would increase the planetary albedo, i.e. have a cooling effect. Twomey (5) proposed the first step in this theory, by showing empirically that the mean droplet mass in a cloud decreases in proportion with the number concentration of aerosols (N). In other words, the mean droplet radius (r) is proportional to: r ~ N-0.33 or Dr/r = -(DN/N)/3 So if the aerosol concentration increases by 30% (DN/N = 0.3), the droplet radius decreases by 10%, and for the same total cloud water content, the number of droplets will increase by 30%. While this relation has been corroborated surprisingly well in various field experiments, it is uncertain whether the cloud water content will be conserved. And the radiative forcing of clouds depends strongly on the heights of their bases and tops, which are unknown. Also, a variation in the drop size of a cloud will affect the way in which the cloud will evolve, produce rain or evaporate. In some circumstances aerosols may create clouds where none existed before, because they act as cloud condensation nuclei. Satellite imagery over ocean areas prone to stratus clouds (especially over high-pressure regions and/or low sea surface temperatures) reveal that ships often trigger lines of stratus along their track. These 'ship tracks' increase the albedo and cause net cooling. Another example is the production of DMS by phytoplankton (3). Some of this DMS seeps into the atmosphere where the sulphate aerosol enhances cloud formation. Accurate measurements at Cape Grim in northwest Tasmania have shown that DMS is the main agent of the nucleation of clouds over the southern oceans (6). The surface waters within the large ocean gyres are generally depleted of nutrients, especially iron. An experiment has shown that the seeding of iron dramatically increases the DMS production in the ocean. The fractions of aerosols that are manmade, biogenic, volcanic, or soil-based, and the chemical reactions of some aerosols, are poorly understood. The inclusion of the ice phase adds an extra step of complexity. In short, the effect of aerosols on clouds and thus on climate is very uncertain. The climate forcing of greenhouse gases, aerosols, and other variants, is commonly expressed in terms of the resulting net change to the radiation balance, expressed in W m-2. What really matters of course is how much warming or cooling a variant bears. For instance, the net warming of the Earth's atmosphere (as compared to an atmosphere without greenhouse gases, aerosols or clouds) is 33K at the surface (from -18? C to +15? C, Section 2.8). The reason for the use of radiation units rather than temperature units is that the effect of some perturbation (such as a

volcanic eruption) on surface temperatures involves a cornucopia of complex feedback effects, such as atmospheric and ocean circulations. The radiative forcing is more 'raw', and allows more direct comparisons, e.g. between general circulation models (GCMs). The aerosol concentrations depend on the wind, the land surface conditions (vegetation cover), sea surface temperature, and other climate factors. During an Ice Age the aerosol types and distributions were very different from those during an interglacial. It is possible that aerosol acts as a positive feedback in enhancing the difference between glacials and interglacials. For instance, during a glacial, the stronger winds over denuded periglacial plains might pick up more dust, contributinging to a higher planetary albedo, maintaining the cold. In other words, climate and aerosol are inter-related in a complex way.