© TAFE MECAT 2008 Chapter 3 Atmospheric Composition & Structure.

© TAFE MECAT 2008

Chapter 3

Atmospheric Composition & Structure

© TAFE – MECAT 2008

Introduction

This chapter will hopefully answer any question you have ever had about the atmosphere, such as ‘what is air made up from?’, and ‘where does the atmosphere end?’, and many more.

Here we describe how the atmosphere is assembled, and from what gaseous materials it is composed of.


Introduction

For example, most people are not aware that the atmosphere consists of both "permanent" and "variable" atmospheric gases, and that the atmosphere is layered like a cake!

By the end of this chapter, you will have a very good understanding of what the atmosphere is made from, and how the atmosphere is assembled


Composition of the atmosphere

So, let’s ask the question, what gases is air made up of?

Most people are aware that the air is mainly nitrogen and oxygen.

Some would be aware of carbon dioxide (due to the concern about global warming), and a few would be aware of methane gas, but what about everything else? What about water?

We need to be able to discuss the composition of the atmosphere as being either dry or moist.

Dry atmosphere values are used purely for calculations, as the atmosphere is never truly dry.


Permanent Atmospheric Gases

By ‘Permanent’ gas, we obviously mean that the gas in question is always present, and that their proportions are nearly constant near the earth's surface, as not all gases are!

Gases fall into two categories, abundant (common) and trace (rare).

The two abundant gases are nitrogen and oxygen and are the only gases that exist in the dry atmosphere that have concentrations above one percent near the earth's surface. The third gas in the dry atmosphere is Argon.


Variable Atmospheric Gases

Unlike the few "permanent" gases, the concentrations of the numerous other substances found in the earth's atmosphere are variable.

With the exception of water vapour, each one of these variable substances exists in the atmosphere in concentrations far less than one percent by volume.

Because these constituents exist in such small amounts, their concentrations are often recorded in parts per million (ppm) and parts per billion (ppb)


Water in the Atmosphere Of the variable substances in the atmosphere, water vapour

(H2O) is the most variable (and important) with concentrations ranging from 0-4% by volume.

Most water vapour enters the atmosphere via evaporation and transpiration.

Evaporation occurs when a single water molecule on a liquid water surface gains enough kinetic energy (often by solar radiation) to break the bond which holds the molecules together.

Transpiration is better explained in terms of vapour pressure. The process of water escaping from plants is referred to as transpiration. Evaporation and transpiration are collectively known as evapo-transpiration.


Constituent Symbol % by volume Molecular weight

(dry air)

Nitrogen N2 78.1 28.02

Oxygen O2 20.9 32.00

Argon Ar 0.9 39.88

Carbon Dioxide* CO2 0.036 and rising! 44.00

Neon Ne 0.0001818 20.18

Helium He 0.000524 4.00

Hydrogen H 0.00005 2.02

Nitrogen Dioxide* NO2 <0.00002 46.00

Sulfur Dioxide* SO2 <0.0001 64.00

Water Vapour* H2O >0<5 18.02


Physical Properties of the Atmosphere

Physical properties refer to the basic ‘laws of nature’ that we find all around us, all of the time.

Having a basic understanding of these properties is essential in understanding even the most fundamental of concepts in meteorology


Density

In our atmosphere, density decreases rapidly with height (i.e., number of molecules which make up the air decreases with height).

This is due to the Earth's gravitational pull. Molecules which are in the atmosphere are pulled towards the center of the earth.

Therefore, there are higher concentrations (numbers per unit volume) of molecules near the surface of the earth than there are 16 km up.


Density

In fact, over 90 percent of all molecules in the atmosphere are within the first 16 km.

So why doesn’t the atmosphere collapse into one thin layer on the surface? Well, there is a lot of mixing and vertical motion and other forces that keeps all those molecules moving around.

The dramatic decrease in density as you go up affects the air pressure, causing it to decrease at a similar rate.


Density


Pressure

The molecules in the atmosphere are in constant motion.

While moving, they bump into each other and other objects on the order of 10 billion times a second and each collision exerts a small force on the other object.

These collisions, in conjunction with gravity and some other forces mentioned later, all come together to create atmospheric pressure.

As the density of molecules decreases with height, so do the number of collisions and therefore, the associated pressure.


Pressure

The same concept applies the other way, and this force can be thought of as the weight of all the air above any point on the earth's surface.

Ultimately, air pressure is a measure of all the air above any point, influenced by the molecules' forces of motion and gravity.


Pressure

It is also important to understand that the pressure at any given point changes with time because the air molecules do not stay in the same location.

This changing pressure is the key to our concept of weather. Generally, as pressure decreases, the weather becomes stormier (more unstable).


Pressure


Temperature

When we hear the word "temperature" we normally think of hot and cold.

When we measure the temperature of something we are actually measuring the average kinetic energy of molecules.

Therefore, when we measure the air temperature, we are measuring the average kinetic energy of the molecules that make up the composition of the air.


Temperature

When air molecules bombard a thermometer, the kinetic energy of the air molecules is transferred to the liquid in the thermometer.

This transfer of energy causes the liquid to heat up and expand and the thermometer to "rise."

In the upper layer of the atmosphere, the thermosphere, the temperature is very hot, at 115 km it can be as hot as 65 C, but you would freeze because there are so few molecules that high up to bombard your body.


Temperature

Air temperature also changes with height.

Since the number of molecules decreases with height, it is sometimes assumed that temperature also decreases with height.

This, however, is not always the case.

Each layer in the atmosphere has its own temperature profile.

For our purposes, we will talk about temperature within the descriptions of each layer.


Volume

Density, pressure and temperature will change with altitude, but what about volume.

Obviously, the volume is what we describe it to be, it is not a physical attribute, but because the concept of volume is incorporated into so many other aspects of meteorological study, it is critical to have a good understanding of the concept.


Standard Values

Know we know what parameters make up the atmosphere, we need to know what there average or standard values are

Nothing about size, scale or amount makes any sense unless you have something to compare it to for comparison.

In order for there to be some kind of standardization for meteorological (and other) data collection, all data is related back to standard values.


Standard Values

The first standard deals with height, and is called sea level.

The reason for this is that the Earth is not even, so some measurements are taken at different heights, and so we can makes sense of the results, we must correct them to make them relative to the standard of sea level.


Standard Values

The standard air density at sea level is about 1.2 kilograms (kg) per cubic meter.

The standard air pressure at sea level (known as sea level pressure, SLP) is 1013.25 hPa (101.325 kPa).

The standard temperature for sea level is 15 °C.


Standard Values

By using these values as standard for the atmosphere, we can compare all other measured values at various locations around the world and make a valued comparison.

It means nothing to see on the news that the pressure outside is 940 hPa (94.0 kPa) if you do not know the standards.

If you do, you will realize that you are probably in the middle of a ferocious storm. However, if you saw a pressure of 1020 mb (102.0 kPa), you would know that you were probably having clear skies.


Structure of the Atmosphere

Heterosphere

On the largest scale of the atmosphere, there are two distinct layers;

a lower layer, where the composition is uniform,

and an upper layer, where there is no uniformity.

The upper layer (above about 100 km) is called the heterosphere, in which the atmosphere varies with altitude.


Structure of the Atmosphere

Homosphere

The precise altitude of the heterosphere and the layers it contains varies significantly with temperature.

Below the heteorsphere, at an altitude of about 100 km , the Earth's atmosphere has a somewhat uniform composition which creates another atmospheric layer the homosphere.


Troposphere

Excluding the Atmospheric Boundary Layer (ABL), the troposphere is the lowest layer of the atmosphere.

This is the layer where most weather takes place. Most thunderstorms don't go much above the top of the troposphere (about 10-16 km).

In this layer, pressure and density rapidly decrease with height, and temperature generally decreases with height at a constant rate.


Troposphere

The other main characteristic of the troposphere is that it is well-mixed.

The name troposphere is derived from the Greek tropein, which means to turn or change.

The troposphere is bounded above by the tropopause, a boundary marked as the point where the temperature stops decreasing with height and becomes constant with height.


Stratosphere

The stratosphere is the layer above the troposphere, characterized primarily as a stable, stratified layer (hence, stratosphere) with a large temperature inversion throughout which prevents large storms from extending much beyond the tropopause.


Stratosphere

The other main impact important to life deals with ozone.

Ozone (O3) is a form of oxygen that absorbs ultraviolet (UVc) light and prevents it from reaching the earth's surface at dangerous levels.

The maximum concentrations of ozone are at about 25 km above the surface, or near the middle of the stratosphere.

The interaction between UV light, ozone, and the atmosphere at that level releases heat, which warms the atmosphere and helping to create the temperature inversion in this layer.


Mesosphere

The mesosphere is the middle layer in the atmosphere (hence, mesosphere). There are two key points about the mesosphere. First, temperature in the mesosphere decreases with height. At the top of the mesosphere, air temperature reaches its coldest value, around -90 degrees Celsius (or -130 degrees Fahrenheit). The second point is that the air is extremely thin at this level; over 99.9 percent of the atmosphere's mass lies below the mesosphere. However, the proportion of nitrogen and oxygen at these levels is about the same as at sea level.

The mesosphere is bounded above by the mesopause. The average height of the mesopause is about 85 km. Above the mesosphere is the thermosphere.


Atmospheric Boundary Layer

At some point in the atmosphere, there is a zone where friction goes from significant to insignificant.

The lower layer of air which is subjected to frictional processes is known as the atmospheric boundary layer (ABL).

The remaining air in the troposphere is known as the free atmosphere


Planetary Boundary Layer

It turns out that there is a well defined, but quite variable height, where the influence of friction stops.

Below this point, the earth's surface has direct influences on motion in the atmosphere.

These influences include frictional drag, evapotranspiration, terrain induced flow modification, heat and energy transfer, and pollution emission.

The ABL is that layer which is influenced directly by the earth's surface.


Planetary Boundary Layer


Processes in the Boundary Layer

The planetary boundary layer is the area of the atmosphere in which we live, and nearly all of our activities take place there.

So, what are some of the processes that occur in the PBL?


Weather processes

The PBL is where nearly all of our weather is produced.

Temperature and pressure gradients caused by differential heating force the winds that drive air masses together producing warm, cold and occluded fronts.

The lifting mechanisms produce the upward motion which causes the cooling necessary for cloud development to occur and precipitation to form.

Important in the role they play in the transport, dispersion, and removal of pollution.



Pollutant removal mechanisms Pollutants are released in various forms and from various

sources (sulphur dioxide from factory stacks, carbon monoxide from automobile exhausts, etc).

These pollutants are mixed into the air and transported down wind.

Many of these pollutants do not remain in the atmosphere but are removed by natural processes that occur within the planetary boundary layer.

We call these processes removal mechanisms, and the duration a pollutant resides or is suspended in the atmosphere is referred to as its residence time.



There are basically three removal mechanisms that act on airborne pollutants;

Wet deposition involves the absorption of pollutants, both particles and gases, into liquid droplets or ice crystals.

Dry deposition, as the name suggests, refers to the removal of pollutants that are not absorbed into liquid or ice, but rather, are removed as either dry particles or gases.

Chemical reactions that transform different substances are also considered a removal mechanism.



Evapo-transpiration

Amount of water vapour in the air varies from 0 to 4 percent, and this water vapour enters the atmosphere through evapo-transpiration.

Evapo-transpiration is the combined process of evaporation and transpiration.

It is used to describe the exchange of water vapour from the surface to the air via water reservoirs, soils, and plant life.

Evapo-transpiration is, therefore, an important process within the atmospheric boundary layer.


© TAFE MECAT 2008 Chapter 3 Atmospheric Composition & Structure.

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Transcript of © TAFE MECAT 2008 Chapter 3 Atmospheric Composition & Structure.