Global Sources of Local Pollution An Assessment of Long-Range Transport of Key Air Pollutants to and from the United States (2010) / Chapter Skim
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Appendix B: Technical Discussion of Atmospheric Transport Mechanisms
Appendix B: Technical Discussion of Atmospheric Transport Mechanisms
Pages 193-212
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From page 193... ...
The winds transport air both horizontally and vertically. Vertical transport is important when considering long-range pollutant transport because pollutants distributed to higher altitudes usually encounter stronger winds that provide rapid transport to distant locations.
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From page 194... ...
The atmosphere's vertical temperature profile plays the dominant role in controlling whether and how quickly an air pollutant will be dispersed upward from its point of emission. The change of temperature with height or lapse rate is used to quantify vertical temperature profiles.
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From page 195... ...
As a result of this stability pollutants injected into the stratosphere tend to remain there for much longer periods than in the troposphere. An important characteristic of free tropospheric air movement is that air parcels experiencing no exchange of heat energy conserve their potential temperature and thus move along surfaces of constant potential temperature (isentropic surfaces)
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From page 196... ...
is dominated by rising air near 60° and sinking air over the poles, sometimes denoted the Polar Cell. The simplified view of the global circulation described above becomes more complex when the effects of continents and oceans are included.
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From page 197... ...
The Bermuda and Pacific high-pressure regions are evident but weak. Conversely, the Icelandic and Aleutian Lows represent the average of transient synoptic-scale low-pressure systems that form near the east coasts of Asia and North America and then move eastward, reaching maximum intensity near the location of lowest pressure in the figure.
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From page 198... ...
July. The Intertropical Convergence Zone (ITCZ)
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From page 199... ...
The quasi-permanent Bermuda and Pacific high-pressure regions are larger and better defined during summer than winter. Their southern extents produce the northeasterly trade winds, which combined with their Southern Hemisphere counterpart, produce the ITCZ that now is located north of the equator.
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From page 200... ...
The jet streams in the upper troposphere are regions of strongest winds. • Wind speeds in the troposphere generally increase with altitude.
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From page 201... ...
As cold air approaches the center of the cyclone the air begins to ascend into the middle troposphere while making a clockwise loop, eventually reversing direction and combining with the WCB in the upper troposphere. The role of the cold conveyor belt in transporting pollutants aloft has received relatively little attention.
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From page 202... ...
, cold conveyor belt (CCB) , and dry intrusion (DI)
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From page 203... ...
These storms can rapidly move boundary layer pollutants to the upper troposphere where they can be transported great distances by the stronger horizontal winds aloft (Dickerson et al., 1987; Lelieveld and Crutzen, 1994)
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From page 204... ...
These strong updrafts can transport boundary layer air to the upper troposphere or lower stratosphere on the order of minutes, compared with hours or days for synoptic systems. This is an important consideration for short-lived chemical species.
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From page 205... ...
The leading edge of the advancing low level sea breeze is a region of strong ascent that often produces thunderstorms in humid regions of the world. At night the temperature gradient reverses, causing offshore flow near the surface (the land breeze)
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From page 206... ...
They concluded that dry convection appears to dominate vertical transport, with warm conveyor belts first coming into play as the cyclonic systems move off the coast. As the PBL collapses with the onset of evening, pollutants that were transported aloft by turbulence remain, forming a residual layer (Stull, 1988)
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From page 207... ...
The kinematic method utilizes the three dimensional wind components at an initial location to advect air parcels over an interval of time. Once at the new location and time the wind components at that location and time are used to advect the parcel.
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From page 208... ...
Trajectories and particle dispersion models require a perfect depiction of the atmosphere at every time step to be totally accurate. This is a corollary to the famous statement that every time a butterfly flaps its wings, its motion will ultimately affect the weather (Lorenz, 1963)
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From page 209... ...
Strong Asian plumes have been observed by aircraft over the eastern North Pacific Ocean (Heald et al., 2003; Nowak et al., 2004) and the west coast of the United States (Jaffe et al., 1999, 2003a; Jaeglé et al., 2003; Cooper et al., 2004)
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From page 210... ...
First, the pollutants must be transported vertically out of the boundary layer where winds are relatively light and into the free troposphere where winds are stronger, especially near the jet stream. Once in the free troposphere the pollutants are transported quasi-horizontally by larger wind systems such as the prevailing westerlies.
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From page 211... ...
Transport from the Sahara to the far southeastern United States occurs at lower latitudes and is due to quasi-permanent subtropical high pressure located over the Atlantic Ocean (Bermuda and Azores Highs)
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