27 Transport Processes in Atmosphere

Lalu Das

1. Learning outcomes
2. Introduction
3. Transport processes
3.1. Horizontal transport
3.1.1. Geostrophic flow
3.1.2. Coriolis force
3.1.3. Geostrophic balance
3.1.4. Effect of friction
3.2. Vertical transport
3.2.1. Buoyancy
3.2.2. Turbulence
3.2.3. Time scales for vertical transport
4. Various scale atmospheric transport processes and air pollutants
4.1. Global circulation
4.2. Synoptic systems
4.3. Mesoscale systems
5. Distribution of air pollutants into the atmosphere
6. Summary

1. Learning outcomes

• After studying this module, you shall be able to know
• know different types forces responsible for transport of mass, momentum and energy know various types of horizontal and vertical transport processes
• know how various scale atmospheric transport processes involve to move air pollutants in different layers of the atmosphere and intercontinental transport of air pollutants

2. Introduction

In the disciplines of physics and chemistry, the exchange or movement of mass, momentum and heat between the system and environment is normally termed as transport phenomena. The mass, heat, and momentum transfer are often assumed in the simple principle that the sum total of the quantities being studied must be conserved by the system and its environment. Thus, the different phenomena that lead to transport are each considered individually with the knowledge that the sum of their contributions must equal zero. In physics, transport phenomena are all irreversible processes of statistical nature steaming from random continuous motion of molecules, mostly observed in fluids. Almost all physical phenomena ultimately involve systems seeking their lowest energy state in keeping with the principle of minimum energy. As they approach this state, they tend to achieve true thermodynamic equilibrium, at which point there are no longer any driving forces in the system and transport ceases. The various aspects of such equilibrium are directly connected to a specific transport. As for example, the heat transfer referred to achieve thermal equilibrium with its environment whereas mass and momentum transport move the system towards chemical and mechanical equilibrium, respectively. Transport processes include heat conduction implying energy transfer; fluid flow indicates momentum transfer and molecular diffusion refers to mass transfer. The transport of mass, energy and momentum can be affected by the presence of external sources. Some examples are: an odor dissipates more slowly when the source of the odor remains present; the rate of cooling of a solid that is conducting heat depends on whether a heat source is applied and the gravitational force acting on a rain drop counteracts the resistance or drag imparted by the surrounding air. The mass, momentum and energy can be transported by the process of diffusion. The molecular transfer equations of Newton’s law for fluid momentum, Fourier’s law for heat, and Fick’s law for mass are very similar. One can convert from one transfer coefficient to another in order to compare all three different transport phenomena. In fluid systems described in terms of temperature, matter density and pressure, it is known that temperature differences lead to heat flows from the warmer to the colder parts of the system; similarly, pressure differences will lead to matter flow from high-pressure to low-pressure regions. When both pressure and temperature vary, temperature differences at constant pressure can cause matter flow (as in convection) and pressure differences at constant temperature can cause heat flow. Perhaps surprisingly, the heat flow per unit of pressure difference and the density (matter) flow per unit of temperature difference are equal. Transport phenomena have wide applications in many branches of science. In the present note, we are concentration on atmospheric transport processes.

3. Transport processes

• 1. Horizontal transport

The horizontal transport of the atmosphere involves a balance between the pressure-gradient force and the Coriolis force. The resulting steady flow is called the geostrophic flow. Below 1 km altitude, the horizontal flow is modified by friction with the surface. All these basic flows including the forces are defined and described below.

3.1.1.   Geostrophic flow

Large-scale movement of air in the atmosphere is driven by horizontal pressure gradients originating from differential heating of the Earth’s surface. As air moves from high to low pressure on the surface of the rotating Earth, the Coriolis force deflects it.

3.1.2.   Coriolis force

An object moving horizontally in any direction on the surface of the Earth experiences a Coriolis force perpendicular to the direction of motion, to the right in the northern hemisphere and to the left in the southern hemisphere. Convince yourself that the Coriolis force in the southern hemisphere indeed acts to deflect moving objects to the left. One can derive the Coriolis acceleration γc applied to horizontal motions: (1)where w is the angular velocity of the Earth and v is the speed of the moving object in the rotating frame of reference. The Coriolis force is zero at the equator and increases with latitude

3.1.3.   Geostrophic balance

A pressure gradient in the atmosphere generates a pressure-gradient force oriented along the gradient from high to low pressure. In three dimensions the acceleration γp from the pressure-gradient force is (2) Where, ▽ = (∂/∂x, ∂/∂y, ∂/∂z) is the gradient vector. Consider an air parcel initially at rest in a pressure-gradient field in the northern hemisphere (Figure 1). There is no Coriolis force applied to the air parcel since it is at rest. Under the effect of the pressure-gradient force, the air parcel begins to flow along the gradient from high to low pressure, i.e., perpendicularly to the isobars (lines of constant pressure). As the air parcel acquires speed, the increasing Coriolis acceleration causes it to curve to the right. Eventually, equilibrium is reached when the Coriolis force balances the pressure-gradient force, resulting in a steady flow (zero acceleration). This steady flow is called the geostrophic flow. It must be parallel to the isobars, as only then is the Coriolis force exerted in the direction directly opposite of the pressure-gradient force (Figure1). In the northern hemisphere, the geostrophic flow is such that the higher pressure is to the right of the flow; air flows clockwise around a center of high pressure and counterclockwise around a center of low pressure (Figure 2). The direction of flow is reversed in the southern hemisphere. A center of high pressure is called an anticyclone or simply a High. A center of low pressure is called a cyclone or simply a Low.

3.1.4.   Effect of friction

Near the surface of the Earth, an additional horizontal force exerted on the atmosphere is the friction force. As air travels near the surface, it loses momentum to obstacles such as trees, buildings, or ocean waves. The friction acceleration γf representing this loss of momentum is exerted in the direction opposite to the direction of motion. The resulting slowdown of the flow decreases the Coriolis acceleration (equation 1), so that the air is deflected towards the region of low pressure. This effect is illustrated by the triangle of forces in Figure 3. The flow around a region of high pressure is deflected away from the High while the flow around a region of low pressure is deflected towards the Low. This result is the same in both hemispheres. In a high-pressure region, sinking motions are required to compensate for the divergence of air at the surface (Figure 4); as air sinks it heats up by compression, relative humidity goes down, leading to sunny and dry conditions. By contrast, in a low-pressure region, convergence of air near the surface causes the air to rise (Figure 4); as the air rises, it cools by expansion, its relative humidity increases, and clouds and rain may result. Thus high pressure is generally associated with fair weather, and low pressure with poor weather, in both hemispheres.

3.2.       Vertical transport

So far, we have described the circulation of the atmosphere as determined by the balance between horizontal forces. Horizontal convergence and divergence of air in the general circulation induce vertical motions but the associated vertical wind speeds are only in the range 0.001-0.01 m s-1 (compared to 1-10 m s-1 for typical horizontal wind speeds). The resulting time scale for vertical transport from the surface to the tropopause is about 3 months. Faster vertical transport can take place by locally driven buoyancy, as described in this section.

3.2.1.   Buoyancy

Consider an object of density r and volume V immersed in a fluid (gas or liquid) of density r’ (Figure 5). The fluid pressure exerted on top of the object is less than that exerted on the bottom; the resulting pressure-gradient force pushes the object upward, counteracting the downward force rVg exerted on the object by gravity. The net force exerted on the object, representing the difference between the pressure-gradient force and gravity, is called the buoyancy. Buoyancy in the atmosphere is determined by the vertical gradient of temperature. In an unstable atmosphere, buoyancy accelerates both upward and downward motions; there is no preferred direction of motion.

3.2.2.   Turbulence

Whereas buoyancy sets in vertical motions in the atmosphere, the rates of vertical transport is quantified through the process of turbulence which is the main feature of microscale circulations in the atmosphere. There are two limiting regimes for fluid flow: laminar and turbulent. Laminar flow is smooth and steady; turbulent flow is irregular and fluctuating. One finds empirically that whether a flow is laminar or turbulent depends on its Reynolds number (a dimensionless entity). The transition from laminar to turbulent flow takes place at Reynolds numbers in the range 1000-10,000. Flows in the atmosphere are generally turbulent because the values of relevant components of Reynolds numbers, viz., U (the mean speed of the flow) and L (a characteristic length defining the scale of the flow) are large.

Turbulence is important in pollution transport because it can thoroughly mix the air and its pollutants. A well-mixed layer is characterized by vertically uniform concentrations of pollutants, water vapor and potential temperature. The depth of the mixed layer is denoted as the planetary boundary layer (PBL). There are two major categories of surface-based turbulence. Mechanically-induced turbulence occurs when the prevailing horizontal wind is disrupted by a rough surface. Thermally-induced turbulence occurs when the temperature lapse rate is large, producing a relatively unstable surface layer and causing the unevenly heated surface to produce pockets of ascent and descent. Since over land both the prevailing wind speed and temperature lapse rate are typically strongest during the day, mixing is generally stronger during the day than the night. Therefore, the height of the mixed layer also varies diurnally. There is much less diurnal variation over water.

3.2.3.   Time scales for vertical transport

4. Various scale atmospheric transport processes and air pollutants

The movement of pollutants in the atmosphere is caused by transport, dispersion and deposition. Transport is movement caused by a time-averaged wind flow. Dispersion results from local turbulence, that is, motions that last less than the time used to average the transport.

It has been realized for many years that local and regional air quality are often substantially affected by human activities. Recently, there has been a growing appreciation that industrial and agricultural activities are contributing to significant modifications of the atmosphere not only in the vicinity of pollution sources, but on a global scale. The best-known example of such a global effect is the increasing concentration of carbon dioxide caused primarily by fossil fuel burning. Carbon dioxide and other trace gases that are efficient absorbers in the infrared are important contributors to the so-called greenhouse effect. The increases projected in the global concentrations of such substances will lead to an increase in the infrared opacity of the atmosphere. The resulting increase in the flux of radiation from the atmosphere to the surface is expected to have a significant effect on surface temperatures in the next century. Although carbon dioxide is chemically inert in the troposphere and stratosphere and is fairly uniformly mixed, a number of other radiatively important trace gases such as methane, ozone, and the chlorofluorocarbons undergo chemical transformations in the atmosphere that can be strongly dependent on location and time. The concentrations of such tracers thus tend to become highly variable. Unless the time scale for chemical change is very short compared to time scales characteristic of meteorological systems, their distributions are strongly affected by atmospheric motions. Thus, to understand the implications of trends in globally significant trace species it is necessary to consider their sources, sinks, and chemical transformations within the atmosphere, but also their transport by the winds. Once an air pollutant is released into the atmosphere, chemical, microphysical, and meteorological factors determine how it is distributed. The location of air pollution sources with respect to local, regional, and global air circulation patterns influences how efficiently pollutants are transported and dispersed. 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. Atmospheric stability, controlled by how temperature varies with height, determines

whether vertical transport will be slow or rapid. After emission, pollutants may undergo chemical transformation, be subjected to depletion processes such as particle scavenging and dry or wet deposition, or mix into the atmosphere to become a component of the background concentration. This module provides a general description of the atmosphere and a synopsis of air circulation and weather patterns that influence the distribution of air pollutants. The major atmospheric transport processes are atmospheric advection and turbulent diffusion. Atmospheric transport processes are conveniently divided between those processes that involve bulk motions of the atmosphere, referred to as “advection” by meteorologists, and those processes that may be characterized as turbulent, or diffusive in nature. In the case of point sources, such as power plant plumes, the distinction is quite clear; advection moves the center of mass of the plume along the direction of the average wind, while turbulent diffusion disperses the plume in the plane orthogonal to the average wind. On a global scale distinction between advective and diffusive processes is not always clear. Since the atmosphere is characterized by spatially and temporally varying motions with a wide range of scales, there is no obvious physical separation between “mean” and “turbulent” motions. In practice, those transport processes that are explicitly resolved by the particular observational network or transport model being utilized are often regarded as the advective motions, while the remaining unresolved motion.

4.1. Global circulation

The global scale general circulation of winds can be considered the multiyear seasonal average of the daily winds. Global circulation plays very important role in producing long-range transport as depicted in the following points:

• Winds in the middle-latitude troposphere are mostly from the west (zonal flow), causing most intercontinental transport to be from west to east.The north-south (meridional) component of the wind in the middle and upper troposphere is usually much weaker than the zonal component. The zonal and meridional components can have similar magnitudes near the surface.
• Wind speeds are generally stronger during winter than summer, causing more rapid transport during the winter months.
• Areas of rising air tend to be smaller and shorter lived than areas of subsidence, which generally cover larger areas and persist longer.

4.2.  Synoptic systems

Synoptic circulation features have sizes of about one- to two- thousand kilometers and lifetimes of several days to a week. Transient middle-latitude cyclones (lows) and anticyclones (highs) are prime examples of these circulations. Anticyclones generally are regions of tranquil weather with sinking air that leads to relatively cloud-free skies and stable conditions that suppress mixing and tend to trap pollutants. Their light winds also reduce horizontal transport. Anticyclones with little forward motion allow these stagnating conditions to persist over days or even weeks.

It is noteworthy that air masses also can be transported long distances without being lifted (i.e., the air and its pollutants remain in the lower troposphere). This generally occurs in the absence of transient synoptic systems that would contain mechanisms for ascent. Arctic haze has been attributed to this low-level transport. Similar phenomena have been observed downwind of North America over the North Atlantic Ocean and during the winter monsoon over the North Pacific Ocean and the Indian Ocean.

4.3. Mesoscale systems

Mesoscale weather systems have typical sizes of a few hundred kilometers and lifetimes ranging from a few hours to a day. Important examples associated with pollutant transport are thunderstorms, land and sea circulations, and mountain and valley breezes. These circulations either can be superimposed on the larger scale transient systems or they can occur alone.

Thunderstorms occur frequently over many parts of the world, ranging from isolated cells to organized clusters called mesoscale convective systems (MCSs). The bases of thunderstorms typically are about 1.5 km above the surface, while the tops of non severe isolated cells or disorganized clusters extend to near the local tropopause. Updrafts and downdrafts are generally less than 10 m s−1. 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. Conversely, the downdrafts that occur during the mature and dissipating stages of a storm transport upper tropospheric air to the surface. Non severe storms can be associated with cyclones and frontal systems or be embedded within homogeneous synoptic air masses.

Sea and land breezes, mountain and valley circulations are important sources of mesoscale transport in all three dimensions. They are examples of diurnally varying mesoscale thermal circulations. The land breeze is usually much weaker than the sea breeze. Sea and land breezes can transport coastal emissions offshore during the day and onshore during the night. In mountain-valley situations, the mountains act as elevated heat source during the day, causing air and its pollutants to rise up along the slope. At night, the horizontal temperature gradient reverses and this aided by the force of gravity results in down slope flow of pollutants and its accumulation in the nearby valley.

5. Distribution of air pollutants into the atmosphere

Once an air pollutant is released into the atmosphere, chemical, microphysical, and meteorological factors determine how it is distributed. The location of air pollution sources with respect to local, regional, and global air circulation patterns influences how efficiently pollutants are transported and dispersed. 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. Atmospheric stability, controlled by how temperature varies with height, determines whether vertical transport will be slow or rapid. After emission, pollutants may undergo chemical transformation, be subjected to depletion processes such as particle scavenging and dry or wet deposition, or mix into the atmosphere to become a component of the background concentration.

Role of vertical layers of atmosphere and Jet Streams:

Pollutant transport occurs in the lowest two layers of the atmosphere—the troposphere and stratosphere. Most weather phenomena that affect pollutant transport occur in the troposphere, which extends from the surface to about 18 km in the tropics and about 8 km near the poles (Figure 7). The height of the tropopause does not uniformly decrease in the poleward direction. Instead, there are two climatologically occurring breaks in each hemisphere, one containing the subtropical jet stream (near 30° N), and the other containing the polar jet stream (near 45° N). These jet streams are not stagnant in location, but shift with season, moving closer to the equator in winter and more poleward during summer.

The jet streams are important distributors of air pollutants for two reasons. They create major areas of air exchange between the troposphere and stratosphere. And the strong winds of the jet stream can rapidly transport pollutants. For example, if one assumes an average wind speed of 35 m s–1 (~ 70 kt) at 40° latitude, an eastward-moving air parcel will circumnavigate the globe in only 10 days. The stratosphere, which extends to ~ 50 km, has regions of strong winds, but virtually no turbulent mixing except for occasional overshooting thunderstorms, certain types of lightning, and occasional thin clouds.

The atmosphere’s vertical temperature profile actually determines how quickly an air pollutant will be dispersed upward from its point of emission. Conditions like those near the surface on a sunny day are associated with atmospheric instability and turbulence. On the contrary, situations like cold, windless night denote stability that suppresses vertical motion. Layers containing temperature inversions are very stable, greatly inhibiting vertical transport and promoting the accumulation of pollutants. The stratosphere is a permanently stable region, with a near zero lapse rate between 11 and 20 km and increasingly negative (stable) rates above. As a result, 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). Exceptions are regions of cloud cover where radiative processes and water vapor phase changes can be major sources or sinks of heat. In addition, air parcels in the surface boundary layer undergo temperature changes due to exchanges of radiationwith Earth’s surface. Isentropic surfaces slope upward toward the north, with isentropic values increasing vertically. As a result poleward moving air conserving its potential temperature tends to ascend, while equatorward-moving air tends to sink. This concept has applications for pollution transport into the Arctic. Specifically, pollution-laden parcels beginning at low altitudes and heading north that conserve their potential temperature will ascend to the middle troposphere. Conversely, for low-level parcels to remain near the surface during northward excursions, they either must be very cold initially or undergo considerable loss of heat due to passing over ice-covered surfaces, especially during the long polar winter seasons. Northern Eurasia is sufficiently cold that its pollutants can be transported quasi-horizontally to the Arctic, making it a major source of Arctic pollution during winter.

6. Summary

• Many meteorological phenomena, on a variety of spatial and temporal scales, transport surface pollutants out of the boundary layer and into the free troposphere. Examples include thunderstorms, turbulence, sea breezes, and the warm conveyor belts (WCBs) of cyclones.
• Advection is the most important mechanism for transporting tracer to the free troposphere; and the addition of upright convection and turbulent mixing increases the amount significantly. Convection and turbulent mixing are not linearly additive processes, therefore, it is important to represent all such processes in meteorological modeling studies.
• Generally, the long-range intercontinental transport of pollutants can be considered a 2-step process. Firstly, 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. The strength of the winds determines how rapidly the transport will occur, and there can be considerable mixing with stratospheric air above the troposphere.