OVERVIEW:

We will discuss the general classes of air pollution, the regulatory response, and introduce numerical modeling of the air pollution plumes. We will also discuss the existing records of the Earth's climate over the past 140,000 years, global ocean circulation, the Younger Dryas Event, and current studies of air-sea interaction in the Gulf of Maine.

OUTLINE:

1. Air pollution phenomenology.

1.1. What is air pollution? Air pollution is defined as "the presence in the outdoor atmosphere of one or more contaminants or combinations thereof in such quantities and of such duration as may be or tend to be injurious to human, plant, or animal life, or property or which unreasonably interferes with the comfortable enjoyment of life or property or the conduct of business." (Wark, K., C.F. Warner, and W.T. Davis, 1998. Air Pollution -- Its Origin and Control. Addison-Wesley, 573 pgs.)

1.2. Air pollution classifications.

Class	Range [kms]	Examples
Local	0 - 30	Automobile exhaust Smokestack exhaust "Bad" low-level ozone
Regional	0 - 1000	Haze Acid deposition
Global	0 - 40,000	Global climate change (a.k.a. "Greenhouse Effect") Destruction of "good" stratospheric ozone

1.3. National Ambient Air Quality Standards (NAAQS). NAAQS originally established in 1970's. Most recently updated in July, 1997. Establishes national standards for air quality with respect to several specific pollutants. Click here to view the National Ambient Air Quality Standards.

1.3.1. Two broad classes of pollutants -- NAAQS sets specific limits on the first and general limits on the second:

1.3.1.1. Criteria -- Those pollutants with a well-known dose/response relationship (DRR). DRR is established via clinical (i.e.controlled) and epidemiological (i.e.hospital record-based) studies. NAAQS mandates Best Available Control Technology (BACT) and sets specific limits of allowable concentrations in the atmosphere. These limits are frequently (often habitually) violated in the country's cities and industrial areas.

1.3.1.2. Hazardous or toxic -- More dangerous than criteria, but exact DRR is unknown. Some examples are benzine and dioxin (both carcinogens). The U.S. Environmental Protection Agency (EPA) eliminated specific concentration limits for these and implemented control technology policy called Maximum Achievable Control Technology (MACT).

1.3.2. Two types of concentration standard for criteria pollutants:

1.3.2.1. Primary standard -- Protect health of human beings.

1.3.2.2. Secondary standard -- Protect health of all the other living things on the planet, and protect the "welfare" of human beings.

1.3.3. Two averaging times for criteria pollutants:

1.3.3.1. Acute -- Short-term average concentration averages. Acute doses are those that may lead to immediate death or illness.

1.3.3.2. Chronic -- Longer-term average concentration averages. Chronic doses are those where the harmful effects materialize later.

1.4. Air pollution meteorology and modeling. The behavior of pollutants in the atmosphere is dependent on several different variables. The background meteorological situation plays a major role in the transport and ultimate fate of pollutants once they are released from the pollution source.

1.4.1. Important meteorological variables are:

1.4.1.1. Wind speed and direction -- Advection (transport of pollutants by wind) is generally more important than diffusion (transport of pollutants by turbulent mixing).

1.4.1.2. Vertical velocity and stability -- Absolutely unstable airmasses are the most conducive to vertical transport and diffusion. Absolutely stable airmasses (especially inversions) inhibit the vertical transport and diffusion of pollutants.

1.4.1.3. Moisture content and cloud cover -- Water vapor reacts chemically with pollutants, and clouds and fog (liquid water) act as scavengers that absorb pollutants from dry air. Falling precipitation deposits pollutants on the Earth's surface.

1.4.1.4. Sun angle (external energy input) -- Incoming Solar energy enhances certain chemical reactions leading to smog. Enough Solar energy may destabilize an airmass enough to break an inversion, resulting in enhanced vertical transport and diffusion (i.e. dillution).

1.4.1.5. Internal chemical reactions within the atmosphere, and chemical reactions between the atmosphere and other major "spheres" of the Earth system -- For example: The ocean's biota uptake excess carbon dioxide (a "pollutant" in terms of the anthropogenic greenhouse effect). Another example is the reaction between anthropogenic NO_x and naturally-formed non-methane hydrocarbons (NMHCs) that result in ozone (O₃) pollution.

1.4.2. In order to better understand the behavior of pollutants within the atmosphere, scientists have developed a suite of numerical models for carrying out computer simulations. Different models are used for different situations and scales.

1.4.2.1. Local scale "plumes" (0 - 30 km): Models are often based on solutions to the diffusion equation, including the commonly-used Gaussian plume dispersion model (GPDM).

1.4.2.1.1. A superior simulation method, requiring considerably more computer power than the GPDM, utilizes finite-differences rather than Gaussian solutions.

1.4.2.1.2. These models are usually set up to handle "puff" releases and "continuous" releases. Models begin with a simplified picture of the local atmosphere, apply the "source term," and deduce the downwind concentrations of pollutants. If the solution is carried out for many different locations and altitudes, a 3-dimensional picture (contoured) can be generated showing the shape and location of the pollution cloud.

1.4.2.1.3. Models such as these are routinely used to determine the local transport of pollutants from point sources (such as a smokestack), line sources (such as a highway), and area sources (such as a large industrial area).

1.4.2.1.4. The accuracy of Gaussian plume disperson models (and other simplified solutions to the diffusion equation) suffers from the inherently simplified picture of the atmosphere that they begin with.

1.4.2.2. Regional scale (0 - 1000 km): Receptor models have proven to be useful on these scales. These usually begin with measured concentrations of specific pollutants at the downwind location, apply a reverse model, and attempt to determine the source of the pollutants and the relative contribution of each source. Models such as these are used to determine the sources of acid deposition and ozone pollution.

1.4.2.3. Global scale (0 - 40,000 km): These models range from the moderately to the extremely complex, and they continue to evolve toward more and more complex physics as the science continue to gain insight. Models such as these are used to study global climate issues (such as the anthropogenic greenhouse effect) and stratospheric ozone depletion.

2.1. Climate -- A non-linear system. Lutgens and Tarbuck define climate as the "long-term behavior" of "an interactive global environmental system upon which all life depends." Climate is also a highly non-linear system in which physical quantities DO NOT evolve or interact with each other in a simple additive or multiplicative way. The vertical profile of atmospheric pressure change, which changes logarithmically rather than linearly, is an example of the former, and the vertical adiabatic temperature changes (dependent on several variables) are an example of the latter. The climate system includes the atmosphere, the hydrosphere (and cryosphere), the lithosphere, the biosphere, and the astrophysical boundary conditions.

2.1.1. Non-linear systems contain internal positive and negative feedbacks.

2.1.1.1. Positive feedbacks are those that enhance variations from the mean state of the system, whether the variations are internally or externally imposed.

2.1.1.2. Negative feedbacks are those that dampen variations from the mean state of the system, whether the variations are internally or externally imposed.

2.1.2. The climate system's non-linearity causes it to behave chaotically or as a complex deterministic system. It is subject to a fragile form of conditional stability, which is illustrated well by the "Strange Attractor."

2.1.2.1. The system may remain relatively stable for a long period of time, held to a close approximation of its mean state by internal negative feedbacks. For example, minor variations in energy input from the Sun may be compensated for by variations in cloud cover.

2.1.2.2. Should the system be pushed outside some tolerance range, the positive feedbacks will cause a shift (often called catastrophic) to a different stable condition. These shifts can happen over periods of time quite a bit shorter than the stable periods.

2.2. The extinction of the dinosaurs 65 million years ago was brought about by a sudden climate change. The climate change was caused by the collision of an "Earth-crossing" asteroid with the Earth. The asteroid left a crater beneath the Yucatan Peninsula more than 200 miles in diameter, sprayed millions of tons molten rock and dust into the atmosphere, and initiated a global cold, dark period that lasted for decades. Most of the Earth's plant life was dead within a few weeks, and most of the dinosaurs were dead within a few months. (Click here for an artist's impression of this event. Also see the Torino scale.) This has almost certainly occurred more than once in the history of the Earth, and it is likely to occur again. For more about current efforts to understand the asteroid/comet impact hazard, see Harvard University's List of Potentially Dangerous Asteroids, NASA's Asteroid Comet Impact Hazards assessment, and the Australian Spaceguard Survey.

2.3. Utilizing ice cores drilled from the Greenland and Antarctic glaciers, researchers have built a well-documented historical record of the Earth's climate ranging back more than 250,000 years. (Some of this research is being conducted by the University of New Hampshire's Climate Change Research Center.) The record contains evidence of long periods (many millenia) of relative stability, separated by episodes of sudden change (occurring on the scale of decades to centuries). Supporting evidence has come from records of coral growth in the tropics that have been corrected for upward and downward shifting in tectonic plates. Additional evidence comes from calcium carbonate oozes at the bottom of the ocean -- these oozes are the remains of benthic foramonifera shells, which contain oxygen isotope fractions indicative of the seawater temperature and salinity at the time of formation.


The most recent (140,000-year) record of global sea level illustrates the latest in a long-period cycle of global glaciation and warming.

Since global sea level is inversely related to the amount of ice locked up into the semi-permanent glaciers of the polar regions, it is believed that historical records of global sea level can also be interpreted as records of the Earth's mean surface temperature. (Colder temperatures -> more water locked into ice caps -> lower sea level.)


These records suggest that the following forcings are brought to bear on the global climate system:

2.3.1. Plate tectonics. The location of a landmass with respect to the equator has a considerable influence over the climate of the landmass. The size of the landmass controls its airmass characteristics (large -> dominated by continental airmasses/small -> dominated by maritime airmasses). The topography of the landmass controls the temperature and moisture characteristics of the climate.

2.3.2. Volcanic activity. Periods of extensive volcanism or even a single large eruption can exert influence of the climate. There are several examples of this in history, such as the eruption of Krakatoa in 1883. Krakatoa was an island volcano in the Indonesian archipelago. Its 1883 eruption was so powerful that the entire mountain was destroyed. Because of the all the dust thrown into the atmosphere, the amount of sunlight reaching the Earth's surface was reduced, and the summer of the following year was unusually cold throughout the world.

2.3.3. Variations in the Earth's orbit. These include the eccentricity of the Earth's orbit (a 90,000-year cycle), the change in the angle of inclination of the Earth's axis (a 41,000-year cycle of variation between 22 and 25 degrees), the precession of the Earth's axis (makes a complete circuit once every 26,000 years), and the axial nutation (a smaller wobble in the axial tilt with a 12-year period).

2.3.4. Variations in Solar output. The Sun undergoes an 11-year sunspot cycle, and a 22-year magnetic cycle. There are longer-term variations as well, with their own non-linearities. During high sunspot periods, the total Solar energy output is actually higher because of the prominances that accompany sunspot activity.

2.3.5. "Other astrophysical forcings." This category includes the irregular collision of asteroids and comets with the Earth, resulting in varying degrees of climate change. An object one kilometer in diameter crashing into the ocean will create a tidal wave hundreds of feet high that could inundate coastal regions. A object ten kilometers in diameter hitting the Earth could send billions of tons of pulverized rock into the stratosphere, resulting in long-term climatic disruptions. (See above for more.)

2.3.6. Global ocean circulation. Because of its large heat capacity, the global ocean acts as a general stabilizer for the Earth's surface temperature, preventing wild global-scale fluctuations on the scale of a few days. (Imagine the diurnal temperature range in the desert vs. the diurnal temperature range on a tropical island). The ocean is also a very important transport medium for heat -- moving heat gained by the equatorial ocean to the polar regions by way of the ocean gyres. (The gyres are the basin-scale, anti-cyclonic currents that dominate the North and South Atlantic, the North and South Pacific, and the Indian Oceans.)

2.3.7. Variations in atmospheric gas content, including anthropogenic gasses. The "greenhouse" effect occurs because some gaseous constituents of the Earth's atmosphere are opaque to long-wave (infra-red) radiation. Short-wave radiation heats the Earth's surface, which then re-emits the energy as long-wave. The greenhouse gasses (chiefly carbon dioxide and water vapor) trap energy within the Earth system, holding the mean surface temperature above the freezing point of water.

The carbon dioxide content of the atmosphere varies on an annual basis. In the northern hemisphere spring of each year, the carbon dioxide content plunges as the land plants reinitiate photosynthesis. The CO₂ content reaches a minimum in the late summer. With the onset of the northern hemisphere autumn, photosynthesis begins shutting down again and the atmospheric CO₂ content begins climbing back toward its late-winter maximum. This process is aligned with the northern hemisphere seasons rather than the southern hemisphere seasons, because most of the world's dry land, and therefore most of the world's terrestrial plant life, is in the northern hemisphere.

Records of atmospheric CO₂ content going back several decades indicate a general upward trend superimposed onto the normal annual variation. The scientific consensus is that this is anthropogenic in origin, in other words, resulting from human activities, such as large-scale industrial activity, automobile exhausts (catalytic converters reduce carbon monoxide pollution and replace it with carbon dioxide pollution), and the concurrent reduction in the green cover of the Earth (deforestation), which has some capacity for mitigating the CO₂ buildup.

2.4. Air-Sea Interaction and the Global Thermohaline Conveyor Belt.

2.4.1. The Global Thermohaline Conveyor Belt (GTCB) is a density-driven current that connects all of the world's major bodies of water. Thermo = temperature, haline = refers to salt; the density of seawater is nonlinearly dependent on its temperature and salinity. Warm, fresh water is the least dense. Cold, salty water is the most dense.

2.4.1.1. On time scales of about one thousand years, a drop of water makes a complete circuit through the GTCB. It begins when relatively salty waters in the North Atlantic, the Labrador Sea, and the Greenland Sea are cooled by continental polar airmasses (cP) originating in Canada and Greenland. Cooling the surface water makes it denser than the water immediately below, so it sinks until finding its equilibrium point -- about 2000 meters down. This new watermass is called North Atlantic Deep Water (NADW). The GTCB is driven by NADW formation.

2.4.1.2. After sinking, NADW migrates southward along the western edge of the Atlantic basin until reaching the Southern Ocean (the body of water circumnavigating Antarctica). From there, it travels eastward into the southern Indian and South Pacific oceans, migrates northward, and eventually rises to the surface north of the equator in both basins. It takes about 500 years for a drop of water originating in the North Atlantic to reach this point. Over the next 500 years, it makes the return journey near the ocean's surface -- taking part in more commonly known surface currents such as Brazillian Current and the Gulf Stream. The water parcel eventually reaches the region of the Atlantic south of Greenland (the North Atlantic Drift), where it is again cooled by cP outbreaks, sinks into the abyss, and repeats the journey.

2.4.2. The ocean's surface water absorbs gasses from the atmosphere. When NADW is formed, the sunken water mass sequesters atmospheric carbon dioxide for 500 years. Natural records indicate that as atmospheric carbon increases, the ocean absorbs more of it -- so the GTCB is a mitigation mechanism for the anthropogenic greenhouse effect.

2.4.3. The surface portion of the GTCB is also a mechanism for transporting heat from the equator to the poles, and thus for moderating the climate of northern Europe. While cP airmasses cool the surface watermass of the North Atlantic, the airmasses are in turn warmed by the watermass. By the time the airmasses reach Europe, they are considerably warmer and moister than they were when they left their source region.

2.4.3.1. The amount of heat transported northward from the equator is a function of the speed and volume of the surface currents (such as the Gulf Stream). Slower/smaller Gulf Stream = less heat transported northward.

2.4.3.2. The historical records indicate that during the major glaciation periods, the GTCB ran at approximately half its current speed, and the sinking motions that drove it occurred considerable further south that they do currently.

2.4.3.3. The evidence also suggests that the GTCB (a complex, deterministic system) has two stable modes: A fast mode corresponding to warm periods (such as the present), and a slow mode corresponding to ice ages.

2.4.3.4. During the general warming that followed the end of the last major ice age (about 14,000 years ago), a large amount of fresh water from melting glaciers was injected into the North Atlantic. Since fresh water is less dense than salt water, this inhibited the sinking that accompanies NADW formation, and the GTCB shifted back to its slower, more southerly ice-age configuration. This event, known as the "Younger Dryas Event," occurred between 11,000 and 10,000 years ago, and was marked by a return to an ice-age climate worldwide, falling sea levels, and the reinitiation of glaciation in northern Europe.

2.4.3.5. The anthropogenic greenhouse effect may cause "global warming" in the immediate future. But if enough fresh water (from melting glaciers in Greenland) is injected into the North Atlantic to surpress NADW formation, another event similar to the Younger Dryas could occur -- an ice age.

2.4.4. Understanding the interaction between the atmosphere and the ocean in the North Atlantic is critical to understanding how NADW is formed and how the GTCB is driven. Similar air-sea interactions occur in the Gulf of Maine (as well as other inland seas), and the University of New Hampshire's Ocean Process Analysis Laboratory took advantage of this by studying data recorded in the central Gulf during the winter of 1997-1998.

LAB:

No lab this week.

HOMEWORK:

1. Skim Lutgens and Tarbuck chapter 13.

2. Study notes and labs from meetings 1 through 10.

3. Prepare questions for review session next week.