AMO Science Protecting the Environment

…Pollution, pollution, wear a gas mask and a veil. Then you can breath as long as you don’t inhale….

— Tom Lehrer from “Pollution” (1965)

Earth is a wonderful place, an exquisite menagerie of truly remarkable features with virtually unlimited possibilities for satisfying the human need for exploration and creativity. Human endeavors can put a strain on the environment, however: Transportation, industry, and activities associated with agriculture all release harmful substances into the atmosphere that can cause illness and damage our environment. These substances can also react among themselves in the presence of sunlight to produce new and potentially more dangerous compounds.

When flying into major metropolitan areas we are not surprised to find ourselves descending into a pall of smog, a reddish-brown haze. Smog consists of the visible products of the reaction of nitrogen dioxide, hydrocarbons, water vapor, and invisible gases such as ozone, as well as airborne particulates. It is corrosive and irritating to the eyes, throat, and lungs. Other forms

EARTH’S ATMOSPHERE

The atmosphere that surrounds Earth protects us from harmful radiation and extreme temperatures of space while providing an environment that sustains life. The diagram shows the layers of the atmosphere and how temperature varies with altitude. Earth’s gravity causes the densities of atmospheric molecules (oxygen, nitrogen, water, etc.) to be greatest near Earth’s surface and to decrease with increasing altitude. The interplay of the molecular densities and the solar radiation penetrating to each layer is one of the most important determinants of the unique physics and chemistry within each layer.

The troposphere, which is closest to Earth, is the most complex region of the atmosphere. It is affected by the oceans and landmasses as well as plant and animal life. This is the region where we experience weather, from the blizzards of winter to the blistering heat of summer. Above the troposphere is the stratosphere, which contains a high concentration of ozone molecules and is sometimes referred to as the ozone layer. These ozone molecules shield Earth’s surface from harmful solar ultraviolet radiation, exposure to which has been linked to skin cancer, eye disorders, and changes in plant growth patterns. Above the stratosphere is the mesosphere and above that, the thermosphere, the layer in which many satellites and the space shuttle orbit. Present within the mesosphere and the thermosphere is a dilute plasma, a collection of ions and electrons created by radiation from the Sun. This plasma, known as the ionosphere, enables radio communications around the globe.



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Atoms, Molecules, and Light: AMO Science Enabling the Future AMO Science Protecting the Environment …Pollution, pollution, wear a gas mask and a veil. Then you can breath as long as you don’t inhale…. — Tom Lehrer from “Pollution” (1965) Earth is a wonderful place, an exquisite menagerie of truly remarkable features with virtually unlimited possibilities for satisfying the human need for exploration and creativity. Human endeavors can put a strain on the environment, however: Transportation, industry, and activities associated with agriculture all release harmful substances into the atmosphere that can cause illness and damage our environment. These substances can also react among themselves in the presence of sunlight to produce new and potentially more dangerous compounds. When flying into major metropolitan areas we are not surprised to find ourselves descending into a pall of smog, a reddish-brown haze. Smog consists of the visible products of the reaction of nitrogen dioxide, hydrocarbons, water vapor, and invisible gases such as ozone, as well as airborne particulates. It is corrosive and irritating to the eyes, throat, and lungs. Other forms EARTH’S ATMOSPHERE The atmosphere that surrounds Earth protects us from harmful radiation and extreme temperatures of space while providing an environment that sustains life. The diagram shows the layers of the atmosphere and how temperature varies with altitude. Earth’s gravity causes the densities of atmospheric molecules (oxygen, nitrogen, water, etc.) to be greatest near Earth’s surface and to decrease with increasing altitude. The interplay of the molecular densities and the solar radiation penetrating to each layer is one of the most important determinants of the unique physics and chemistry within each layer. The troposphere, which is closest to Earth, is the most complex region of the atmosphere. It is affected by the oceans and landmasses as well as plant and animal life. This is the region where we experience weather, from the blizzards of winter to the blistering heat of summer. Above the troposphere is the stratosphere, which contains a high concentration of ozone molecules and is sometimes referred to as the ozone layer. These ozone molecules shield Earth’s surface from harmful solar ultraviolet radiation, exposure to which has been linked to skin cancer, eye disorders, and changes in plant growth patterns. Above the stratosphere is the mesosphere and above that, the thermosphere, the layer in which many satellites and the space shuttle orbit. Present within the mesosphere and the thermosphere is a dilute plasma, a collection of ions and electrons created by radiation from the Sun. This plasma, known as the ionosphere, enables radio communications around the globe.

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Atoms, Molecules, and Light: AMO Science Enabling the Future of air pollution can be invisible, and while they may not be harmful to breathe, they can have an unacceptable effect on our global atmosphere. Chlorofluorocarbons (CFCs), for instance, released from refrigerants and cleaning agents, are associated with the destruction of ozone in the stratosphere, and carbon dioxide contributes to global warming. To keep Earth safe and habitable, we need to improve our ability to monitor and control human-induced pollution and to understand how various pollutants interact among themselves and how they affect different parts of the atmosphere (see box “Earth’s Atmosphere”). This chapter describes how AMO science provides the essential data for atmospheric models and a variety of innovative techniques that are used to monitor pollution quantitatively. Understanding the Atmosphere and Climate Atmospheric science (and other environmental sciences) is different from many other areas of science in that controlled large-scale experiments are often difficult if not impossible. Nevertheless, microscopic physical and chemical processes that affect the climate occur constantly throughout the atmosphere, the oceans, on land, and in the biosphere. Atmospheric scientists strive to understand these processes in conjunction with sunlight and human activities such as transportation, combustion, industry, and agriculture and to determine how their interaction globally influences the atmosphere and climate. The complexity of the atmospheric system makes it difficult to describe all of its behavior using a small number of theoretical expressions. Therefore, comprehensive numerical models play an essential role (see box “Scientific Models”). Atmospheric scientists must also make extensive use of data provided by SCIENTIFIC MODELS A model is a collection of mathematical expressions that describe what we know about a system. Models allow scientists to carry out controlled virtual experiments on computers to determine how a system will behave under various circumstances. The power of a theoretical model lies in the predictions it makes and its ability to identify key relationships between physical and chemical processes. Differences between the model’s predictions and the measured parameters are used as a guide to further our understanding of the system and to improve the model. Atmospheric models have had a critical impact on national and international policy. The implication of chlorofluorocarbons (CFCs) in the destruction of the ozone layer—which led to the Montreal Protocol, an international treaty banning the manufacturing and use of CFCs—is a success story to which AMO instrumentation and research made a significant contribution. Three chemists were awarded the 1995 Nobel Prize in Chemistry for their work on the formation and decomposition of ozone. laboratory experiments as well as information and parameters obtained from monitoring the atmosphere from the ground, balloons, aircraft, and satellites. AMO science makes an important contribution to instrumentation, data collection, and analysis in atmospheric science. To be a reliable, predictive tool, a model must include as completely as possible the phenomena affecting the system, and it must be based on accurate data. Models of the upper atmosphere (the mesosphere and thermosphere) require knowledge of the intensity of the Sun’s radiation as a function of wavelength, densities of constituent molecules and atoms (primarily N2,O2,

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Atoms, Molecules, and Light: AMO Science Enabling the Future O, and He), the probabilities for atoms and molecules to absorb light of a particular wavelength, and ion-molecule and electron-molecule collision rates. Fundamental AMO research in collision physics, in the spectroscopy of atoms and molecules, and in chemical reactions is providing data central to atmospheric modeling. Generally, atmospheric models become increasingly complicated as the altitude decreases because there are more constituents to include and additional effects to take into account. Modeling the stratosphere and troposphere requires fast supercomputers to run huge computer codes that incorporate thousands of chemical reactions, detailed global wind patterns, and abundances of large numbers of trace species. Sophisticated atmospheric models are used to help guide policy makers in drawing up recommendations governing the burning of fossil fuel and the manufacture and use of ozonedestroying chemicals. Although carbon dioxide is an integral part of plant and animal metabolism and therefore occurs naturally in the troposphere, significant increases in carbon dioxide may cause problematic atmospheric and climatic changes on a global scale. Over the last century, increased burning of fossil fuels—particularly in the generation of electric power, in the heating of homes with oil or natural gas, and in automobile combustion engines—has led to a buildup of carbon dioxide in the atmosphere. Models incorporating data from AMO science predict that as the concentrations of carbon dioxide and other gases such as methane, ozone (from pollution), and the CFCs increase, more of the infrared radiation coming from Earth will be trapped near the surface. Trapped infrared radiation leads to warming of the troposphere and the surface of Earth, known as the greenhouse effect (Figure 8). FIGURE 8 The natural greenhouse effect of Earth’s atmosphere. While the effects of water vapor in our atmosphere produce a natural greenhouse effect—and indeed the effect makes our planet suitable for the type of life that has evolved on it—a rapid increase in heating will affect ecosystems in ways that are at best poorly understood. For instance, evaporation rates would increase substantially, with a resultant increase in rainfall. These rapid changes might also put human populations at risk. In any case, managing their effects will certainly incur costs that are difficult to predict. The warming may already have started, as the mean global temperature has risen by approximately

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Atoms, Molecules, and Light: AMO Science Enabling the Future 0.5oC during the past 100 years. The consequences of such warming can be very dramatic and might include the thinning of the polar ice caps. Although ozone (O3) near the surface of Earth is a pollutant and contributes to global warming, O3 in the stratosphere screens our planet from harmful ultraviolet sunlight. Ozone is produced naturally in the stratosphere and is destroyed by a complex series of chemical reactions, some of which are fueled by man-made chemicals. FIGURE 9 Measurements of ozone concentrations over Antarctica from natural springtime minimum conditions in 1979 (left) to the extensive ozone hole recorded in 2000 (right). Global concentrations of O3 in the stratosphere are monitored by NASA and the National Oceanic and Atmospheric Administration (NOAA) using satellite-based spectrometers. The data show a dramatic seasonal loss of O3 over Antarctica during the polar spring (October-December). While the formation of the Antarctic ozone hole is cyclic, over the last 21 years the hole has grown larger and is more depleted in O3 and more persistent (Figure 9). Using a variety of measuring techniques based on AMO science, atmospheric scientists have discovered a sequence of chemical reactions occurring in the polar stratosphere during winter, when no sunlight is present. These reactions, which produce chlorine molecules (Cl2), occur on the surface of polar stratospheric clouds of ice crystals. In spring, when sunlight penetrates the region once again, the Cl2 molecules break apart. The Cl atoms then react with O3, destroying it and causing formation of the ozone hole. It has recently been recognized that carbon dioxide (CO2) also plays a role in the growth of the ozone hole. Paradoxically, while CO2 contributes to global warming near Earth’s surface, it causes cooling in the stratosphere. Carbon dioxide molecules generated at ground level can eventually migrate to the upper layers of the atmosphere, where they collide with oxygen atoms. During the collision, the colliding atoms lose energy (i.e., they cool), while the CO2 is transferred to an internal excited state. The excited CO2 then radiates, causing a net cooling of the upper atmosphere. In the stratosphere this cooling contributes to the enhanced formation of polar stratospheric clouds, leading to greater ozone depletion. Models suggest that the doubling of CO2 in the atmosphere, as is predicted to occur over the next century, will result in significant amounts of cooling in the upper atmosphere and, in turn, more O3 depletion.

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Atoms, Molecules, and Light: AMO Science Enabling the Future Laser Altimetry The ability to assess, model, and identify various pollutants and their influence on the environment is a direct result of decades of fundamental AMO research. Autonomous, unattended measurements to monitor pollution over large regions have been made possible through advances in laser spectroscopic tools such as lidar— light detection and ranging (see boxes “Lidar,” “Lidar Mapping of Pollution,” and “Lidar Mapping of Los Angeles Pollution”). The compact nature of a lidar spectrometer allows it to be deployed on satellites and airplanes. A lidar-enabled technique, laser altimetry, is being used to monitor the health of Earth’s polar ice caps. Accurate knowledge LIDAR Lidar can be thought of as radar with laser beams. While radar can image large objects such as aircraft, lidar can be used to map the location and concentrations of atmospheric molecules and aerosols as well as landmasses. In lidar, pulses of light are transmitted into the air, where they are scattered or absorbed and re-emitted by atmospheric constituents. Light returning to the transmitter is collected by an optical receiver and analyzed to extract details about the state of the atmosphere at distant points. The distance to a suspicious plume, for example, can be obtained by measuring the time delay between emission of the laser light and collection of the back-scattered light. The concentration of atmospheric molecules can be obtained by using twin laser beams, one of which is tuned to a wavelength the molecule in question likes to absorb. Advances in laser technology, electro-optical materials, and high-powered, low-cost lasers in the eye-safe 1.5-micron band now permit remote mapping of lower concentrations of atmospheric constituents at increased ranges. LIDAR MAPPING OF POLLUTION The image below shows the striking ability of lidar to measure pollutants in the atmosphere, along with their distribution and circulation. Horizontal lidar scans over El Paso, Texas, provide graphic evidence of pollution transport and how the circulation patterns are affected by the terrain, the transportation system, and political borders. Dense plumes of pollutants (red) moving northwest are coming from identifiable sources to the south. Lidar wind-field measurements in the image are indicated by the position, length, and direction of the arrows, while the altitude of the measurement is indicated by its color. These wind measurements also show atmospheric flows through the Rio Grande Valley basin, which are influenced by the local terrain. These flows are responsible for the transport and dilution of pollutants in the area. Wind measurements with aerosol lidars can capture even small-scale atmospheric flows, such as the turbulence downwind of the Sierra de Cristo Rey peak and the channeling of the low-level flow through the valley basin.

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Atoms, Molecules, and Light: AMO Science Enabling the Future LIDAR MAPPING OF LOS ANGELES POLLUTION Sunlight reacts with emissions from cars, trucks, and smokestacks to create smog. Lidar helps us to identify where smog is created and to see how pollution is transported by the wind. This image shows a series of vertical scans taken by an eye-safe lidar located in metropolitan Los Angeles. Purple areas indicate high levels of airborne particulates; there are several such patches as the lidar looks east into the densely packed streets of Beverly Hills. One particularly prominent plume of pollution appears at the inter-section of Wilshire and Santa Monica boulevards, two arteries with a lot of stop-and-go traffic. Westward at the 405 Freeway, the particulate level actually drops. This is because winds from the Pacific Ocean help to mix and disperse the pollutants and because the traffic was moving smoothly on the freeway when the measurement was taken. Notice also the layer-cake structure of the atmosphere. The data were acquired within a few minutes of one another and represent a snapshot of the atmosphere. Successive scans can be combined to create movies of how the air around us behaves and ultimately can be used for urban planning and decision making. of whether or not the polar ice caps are changing in size has been a scientific goal for decades. Changes in the thickness of the polar ice cap would also be an unavoidable consequence of any large-scale climate changes that may be occurring in our atmosphere. While accurate measurement of polar ice cap thickness is very difficult, the Greenland ice sheet, second in size only to the Antarctic ice sheet, lies on land above sea level and provides a stable measuring platform (Figure 10). Laser altimetry exploits the FIGURE 10 This map shows that the Greenland ice sheet is thinning, primarily along coastal regions, and thickening in some inland areas.

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Atoms, Molecules, and Light: AMO Science Enabling the Future Global Positioning System to precisely assess, every few years, the rate at which the elevation of the entire ice sheet is changing. These measurements indicate that the sheet is thinning along some coastal regions. Internal combustion engine exhaust is a major contributor to ground-level air pollution and smog. One of the most important factors limiting the design of modern, cleaner-running diesel engines is the amount of nitrogen oxides that these engines emit. LASER-INDUCED FLUORESCENCE AMO scientists developed laser-induced fluorescence several decades ago to investigate atoms and molecules that were hard to detect or otherwise invisible. In planar laser-induced fluorescence (PLIF), the output of a laser tuned to a wavelength absorbed by an atom or molecule of interest is formed into a thin sheet of light and sent through the combustion region. Fluorescence emitted by the atoms or molecules in the path of the sheet of light is detected at right angles with a very sensitive camera to produce the spatial images shown above. This pair of PLIF images shows the formation of NO in a diesel engine near the beginning (left) and near the end (right) of the combustion cycle. The combustion environment inside a diesel engine can vary dramatically in both the location of the combustion and when in the engine cycle it occurs. The chemistry is complex, involving mixtures of hydrocarbons, molecular oxygen, molecular nitrogen, and water vapor. Sensitive techniques such as laser-induced fluorescence are needed to study the formation of these and other chemical species during combustion processes (see box “Laser-Induced Fluorescence” and Figure 11). FIGURE 11 An actual diesel engine modified for planar laser-induced fluorescence imaging. Planar laser-induced fluorescence (PLIF) is a technique being used to determine temporal and spatial maps of nitric oxide (NO) in an operating diesel engine. These images, along with detailed knowledge of collision cross sections of the molecules in the combustion chamber, are used to extract NO concentrations from PLIF images like those shown in the box. What we have learned from these images reveals that NO forms progressively during combustion, with approximately one-half to one-third forming at the end of the cycle. Such results have already impacted research directions of engine manufacturers as they work to produce engines with reduced emissions.

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Atoms, Molecules, and Light: AMO Science Enabling the Future The night sky illuminated in a Guide Star demonstration. Inelastic scattering of the outgoing laser beams by the atmosphere makes them strikingly visible.