How Convection and Advection Moves Pollution

Pollutants are dispersed through a variety of processes involving the motions of air. These processes may be divided into the general categories of diffusion, turbulence, convection, and advection.



The various transport and dispersion processes for pollutants.

The following material was taken from a series of online lectures at Georgia Tech and slightly modified

Diffusion and Turbulence.
Diffusion (molecular diffusion) is the average motion of a molecule (or particle) as a result of its collisions with other molecules (or particles). Molecules always move from a region of high concentration to a region of lower concentration due to the "desire" to be in an area where maximum movement is permitted. A small volume of particles will spread out to maximze the distance between them. This is seen, or more correctly smelled when you and your classmates are in a room and someone sprays air freshener in the corner. Those closest to the source of thr spray smell the fragrance first, and progressively those farther and farther away smell is later until the room is filled with the nice scent. The movement of the aerosols from the spray (yes ... a form of pollution) follows thus rule: "The average distance that each molecule moves is constrained by the frequency of its collisions with other molecules."
Diffusion is not a very effective mechanism of air molecule transport because of high frequency of air molecule collisions. For instance, at STP the frequency of air molecule collisions is about 1 billion collisions per second. A pollutant tracer will be diffused from one point to another if the pollutant is not uniformly mixed in the atmosphere. In other words, in the case of molecular diffusion, the transport of a tracer occurs as a result of gradients in its concentration from place to place.
Turbulence (or turbulent diffusion) is the irregular air movement in which the wind constantly varies in speed and direction. This complex motion can be usually decomposed into individual vortices, or eddies. In the air that is agitated, all sizes all eddies can exist. The combination of all eddies acting simultaneously produces the effect called turbulence. Turbulence can be generated by any process that induces motion in a fluid like the atmosphere. Atmospheric turbulence near the Earth's surface differs from that at higher levels. When solar radiation heats up the surface, which in turn warms the air, the warm, light air rises, and cooler, denser air descends to replace it. That movement of air, together with disturbances around surface obstacles, makes low-level winds extremely irregular.
At altitudes of several thousand meters or more, the frictional effect of surface topography on the wind is greatly reduced, and the small-scale turbulence that is characteristic of the lower atmosphere is absent. Although upper-level winds usually are relatively regular, they sometimes become turbulent enough to have an effect on aviation.
Importance of turbulence:
Turbulence is important because it churns and mixes the atmosphere and causes water vapor, air pollutants, and other substances, as well as energy, to become distributed at all elevations.

Convection.
Convection is vertical motion driven by buoyancy. Convection is a localized phenomenon, driven primarily by surface heating during the day. The convective eddies and the smaller eddies comprising turbulence work together to create a mixed layer of air at the ground during the day. Height (or depth) of the mixed layer depends on the amount of heating at the surface and on the temperature stability. The more heating there is, and the less stability there is, the higher layer will be. Mixed layer tends to be shallow at night, in cold weather, and over the oceans because the buoyancy generated at the surface, which drives convection, turbulence, and mixing is relatively weak.
Importance of convective transport of air pollutants:
1. Convection lofts pollutants away from the surface. Where they otherwise could be in contact with people, plans, and animals.
2. In the rising convective column, precipitation may form and wash out the soluble pollutants.
3. Convection transports some pollutants into upper air levels with stronger prevailing winds; these winds are strong enough to disperse the pollutants over great distances.

Advection and long-range transport.
Advection is a horizontal motion of the atmosphere, and the prevailing winds are known as advective winds. The fastest winds are find aloft, in the jet streams. For example: velocities of advective winds aloft may be up to 400 km/h and velocities of winds near the surface are about 10 to 20 km/h.
Importance of advective transport of pollutants:
1. Advection removes the pollutants to a distance from the source.
2. Advection acts to dilute the pollutants.
3. Advection is the process responsible for the long-range transport of pollutants downwind from sources.

Okay ... let's make some connections here to Earth Pollution Problems

In each case listed below, pollution of one form or another was lofted into the atmosphere in a vertical direction. Advection of winds was the responsible for the transfer of the mass of pollution from the source to distance locations. The higher the loft of the pollutants the farther the spread. The lighter the pollutants, the longer they remained aloft in the atmosphere. It is therefore possible for some forms of pollution to remain in the global atmosphere for days to years, and the results can be horrific, as you will learn.

1) Volcanic Eruptions: Pinatubo

On June 15, 1991, the second largest volcanic eruption of the past 100 years took place on the Philippine Islands. Mt Pinatubo erupted. Nearly 20 million tons of sulfur dioxide were injected into the stratosphere in Pinatubo's 1991 eruptions, and dispersal of this gas cloud around the world caused global temperatures to drop temporarily (1991 through 1993) by about 1°F (0.5°C). At one time, the ash cloud was 200 km wide in all directions over the caldera. Advection of high air currents carried the ash cloud to the west where is spread out and eventually blanketed a wide swath that encircled the globe. The effects of a single volcano carried a significantly greater impact that all of the carbon dioxide emissions from cars, as seen in the drop in temperatures.

To learn more details on Pinatubo, you are directed to the USGS: Pinatubo where an extensive collection of articles exists for your study.

2) Emissions from Coal-burning Plants

For years, coal-burning plants and other forms of fossil fuel burning plants in the US sent pollution into the air. When sulphur dioxide (SO2) and nitrogen oxides (NO) combines with water in the atmosphere, it is converted into sulphuric acid (H2SO4), ammonium nitrate (NH4NO3) and nitric acid (HNO3). The transformation of SO2 and NOx to acidic particles and vapours occurs as these pollutants are transported in the atmosphere over distances of hundreds to thousands of kilometers. The transfer of these pollutants is the result of warm air advection. Pollution is emitted from US plants and carried north across the border into Canada. There, the chemicals reacted with water to form the various acids described above. This acid is a solid precipitate or a liquid compound, and it drops to the ground with the rain, creating a mix known as "acid rain." The pH (a measure of the amount of acid in water) of Canadian lakes dropped and the fish began to die. While the Canadian government cried "foul," the US government was very slow to react to the problem. Eventually, an agreement was reached and US plants are forced to equip their smoke stacks with scrubbers that clear the outgoing gas of the pollutants.

While Canada was being affected by warm air advection, the US land and lakes were similarly affected by cold air advection. As the pollution was lofted into the air, cold air masses carried the chemicals to the south, and acid rain affected the country in the opposite direction of Canada ... onto our own lakes. To learn what the Canadian government is suggesting to its people to reduce acid rain, check out "what you can do" and try it at home!

link to Activities relating to Acid Rain and Canada.

3) Pollution from Texas refineries

A British Petroleum refinery in Texas is reviewing its pollution numbers after reporting different data to federal and state officials. Experts say this highlights the need for more oversight, the Houston Chronicle said. The Texas City refinery isn't new to regulatory trouble. In 2005 it was fined $31.3 million for 300 violations found by the U.S. Occupational Safety and Health Administration after 15 workers died in an explosion. The Chronicle reports the refinery is now recalculating numbers and retesting its air pollutant levels. It reported to U.S. the Environmental Protection Agency that it released 10.25 million pounds of pollutants into the air in 2004, up from 3.3 million in 2003. Copied from Washington Post, May 7, 2006

Now 10.25 million pounds of pollutants is a lot! It has been reported that some of the refineries in Texas are so large (as everything else is claimed to be in Texas) that small weather systems exist over the area. As the pollution is lofted into the air, warm air convection carries the pollutant as far north as Minnesota. When the pollution was tagged with a "marker" the Texas pollution was found to have traveled as far north as Minnesota. It is hard to imagine acid rain here inour state whose pollution originated over 1000 miles south of here, but this highlights the power of warm air advection as well as the need to regulate refineries.

link to Refinery Reform to give you an example what what is being done and how perhaps you can be involved.

4) Chernobyl

On April 25th -26th, 1986 the World's worst nuclear power accident occurred at Chernobyl in the former USSR (now Ukraine). The Chernobyl nuclear power plant located 80 miles north of Kiev had 4 reactors and whilst testing reactor number 4 numerous safety procedures were disregarded. At 1:23am the chain reaction in the reactor became out of control creating explosions and a fireball which blew off the reactor's heavy steel and concrete lid. The Chernobyl accident killed more than 30 people immediately, and as a result of the high radiation levels in the surrounding 20-mile radius, 135,00 people had to be evacuated. It has beem 20 years since the accident, and still reports are coming in of children born with genetic defects from the radiation fallout. Here is an example of a pollution that is nuclear! The cloud of radioactive gas spread over the community and advection pushed the cloud away from the local site. Those who were closest to the accident site were most affected, while those farther away less so due to diffusion of the radiation the farther away the cloud moved and dispersed. However, cancer rates have increased 10 times since the accident.

Link to the March, 2006 report.

5) Chlorofluorocarbons and the Montreal Protocol

In 1997, I attended the Nobel Conference at Gustavus College in St Peter, Minnesota. Dr. Sherwood Rowland was a primary presenter and the recipient of the Nobel Prize for his work on the effects of chlorofluorocarbons on ozone. These molecules (known as CFCs) were used in refrigerators and air conditioners. What was not known was the ability of these CFCs to remain a stable molecule for a very long time, and the additional ability to float under the upward convection of warm air, well up into the upper levels of the atmosphere. Some forms of CFCs can remain in the atmosphere for 50 to 140 years! The CFC reacts with ozone, and causes destruction of this important atmospheric molecule. Ozone is of central value in blocking harmful forms of ultraviolet radiation from the Sun.

Whereas CFCs are safe to use in most applications and are inert in the lower atmosphere, they do undergo significant reaction in the upper atmosphere or stratosphere. In 1974, two University of California chemists, Professor F. Sherwood Rowland and Dr. Mario Molina, showed that the CFCs could be a major source of inorganic chlorine in the stratosphere following their photolytic decomposition by UV radiation. In addition, some of the released chlorine would become active in destroying ozone in the stratosphere2. Ozone is a trace gas located primarily in the stratosphere (see ozone). Ozone absorbs harmful ultraviolet radiation in the wavelengths between 280 and 320 nm of the UV-B band which can cause biological damage in plants and animals. A loss of stratospheric ozone results in more harmful UV-B radiation reaching the Earth's surface. Chlorine released from CFCs destroys ozone in catalytic reactions where 100,000 molecules of ozone can be destroyed per chlorine atom. from NOAA publication

As the global ozone layer was depleted by the long-lived CFCs, alarm was raised all over the world. Since ozone is restored by lightning storms at a far slower rate that it was being depleted, some form of agreement had to be reached to stop the release of CCFs into the air, and this meant using a different cooling mechanism! Nations gathered to sign an agreement to halt the use of CFCs. The agreement, called the Montreal Protocol, was signed in 1987.

Link to the report on CFCs. Link to a more interesting problem (CFC usage in asthmatic inhalers) provides an issue for ethical argument.

Okay, time to move back to Convection/Advection or somewhere else in the course ... Earth-Science Introduction or Earth Introduction.


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