The definition of an aerosol is a suspension of tiny particles or droplets in the air. It includes dust which is solid particles formed by physical weathering, bioaerosol such as virus and bacteria, and fumes which are formed by compression of steam produced by gas or combustion. Since the size of the particles is about 0.01 to 100 μm, the speed of sinking in the atmosphere is relatively slow and the particles show significant stability in the air. Thus, aerosols are easy to spread and stay in the atmosphere. |
Aerosols are combined with reaction gas and cause acid rain, smog, and visibility reduction. Aerosols also have radiative forcing which means aerosols could float in the air and lead to climate change by blocking or absorbing the solar radiation energy. Radiative forcing is a measure of influence of a factor which maintains or changes energy balance by affecting the earth-atmosphere system. This forcing is a major indication of potential climate fluctuation mechanism. Positive radiative forcing tends to increase the surface temperature and negative radiative forcing tends to decrease the surface temperature. In other words, if radiative forcing is positive value, surface warming occurs, and if radiacive forcing is negative value, surface cooling occurs. The radiative forcing of aerosols is known as –0.9(–1.9∼–0.1) W/m2(1750~2011) on average which is negative value.
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There are several criteria to sort the types of aerosols. Methods of formation, constituents, and correlation of AOD and FF could be an example. Methods of formation could be divied into natural causes and artificial causes. Yellow dust, volcanic ash, sea salt aerosol, dust, smog and so on belong to natural causes and black carbon, sulfur compounds, organic compounds and so on by human activities belong to artificial causes. Constituents could be classified by carbon compounds, sulfates, nitrates and so on. Those three figures are most of Seoul fine dust components.
Considering both Aerosol Optical Depth(AOD) and Fine mode Fraction(FF) could be another way to sort. Aerosol optical depth is a measure of the extinction of the solar beam by medium along the direction of progress. In other words, particles in the atmosphere can block sunlight by absorbing or by scattering light. AOD tells how much direct sunlight is prevented from reaching the ground by those aerosol particles. The more aerosols exist in the atmosphere, the higher AOD value it has and if AOD value is 0.7, it means only the half of the sunlight penetrates the contaminated area.
Through the correlation of AOD and FF, aerosols could be divided into three major aerosols, which are dust, pollution, and sea-salt aerosols. One of major component of Northeast Asia aerosol is dust aerosol. Since physical and optical properties of dust aerosol is clearly distinguished from other aerosols, it is important to identify the existence and proportion of dust. For dust and pollution aerosols, it is difficult to distinguish both of them because they both have high AOD value. Naturally originated aerosols and artificially generated aerosols could be distinguished by using Fine mode Fraction(FF) from remote sensing satellite. Naturally originated aerosols are mainly distributed from 1 to 2.5 μm and artificially generated aerosols are mostly distributed from 0.1 to 0.25 μm. This criteria enables to classify types and origins of aerosols which is hard to distinguish when only AOD is used.
Dust aerosols usually occur in spring and are mainly formed by natural and physical processes. They travel great distances to Korea. Dust aerosols have frequently observed with relatively high AOD value(>0.3) of large particles(FF<0.65) and their contribution to total AOD in spring was up to 24.0% which was greater than other seasons.
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Pollution aerosols usually flow from Northeast Pacific coastal regions generated both artificially and naturally. Pollution aerosols originated from anthropogenic sources as well as natural processes are observed in the regime of high FF(>0.65) with wide AOD variation. Average pollution AOD was 0.31±0.05 and its contribution to total AOD was 79.8% in summer, which means pollution aerosols have great impact on total AOD in summer. Particulate contaminants are divided by primary pollutants(SO2, NOX, NH3) which is emitted directly from sources like cars, factories or chimneys and secondary pollutants(sulfate, nitrate, ammonium) which are formed as a result of chemical reactions and photochemical reactions of the gas pollutants and VOCs(Volatile Organic Compounds) in the atmosphere.
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Sea-salt aerosols are formed at the sea surface by wind. Sea-salt aerosols are observed at low AOD(<0.3), almost below 0.1, and slightly higher FF than dust and lower FF than pollution. They show maximum AOT(0.33±0.11) and FF(0.66±0.21) in spring and minimum AOD(0.19±0.05) and FF(0.60±0.14) in fall. Spatial property was that AOD increase is observed as closing to the eastern part of China due to trasport of aerosols from China by prevailing westerlies. |