Download Lecture #7: Composition of the Present Day Atmosphere

Lecture #7: Composition of the Present Day Atmosphere
Fixed and Variable Gases:
The most abundant gases in the homosphere are nitrogen (N2) and oxygen (O2).
Nitrogen and oxygen make up about 99% of the air in the homomsphere. Next in line are
argon (Ar), neon (Ne), helium (He), and water (H2O). All of the above, minus water, are
considered “fixed” gases, i.e. their concentration does not change significantly over time,
and their mixing ratios remain constant throughout the atmosphere (homosphere).
“Fixed” gases
N2 (nitrogen)
O2 (oxygen)
Ar (argon)
Ne (neon)
He (helium)
Kr (krypton)
Xe (xeon)
Mixing Ratio
78.08%
20.95%
0.93%
18.2 ppmv
5.2 ppmv
1.14 ppmv
0.09 ppmv
Variable gases are gases whose mixing ratios change considerably with time and
space. Many of these gases degrade (react chemically) in the lower parts of the
atmosphere so that very little reaches the upper parts of the atmosphere (i.e. stratosphere
and beyond).
from Turco “Earth Under Siege”
“Variable” gases
H2O (water vapor)
CO2 (carbon dioxide)
CH4 (methane)
H2 (hydrogen)
N2O (nitrous oxide)
CO (carbon monoxide)
O3 (ozone)
CF2Cl2 (fluorocarbon 12)
Mixing ratio
4% (maximum in the tropics, minimum at
the South Pole)
~370 ppmv (increasing by ~0.4% per year
due to fossil fuel burning)
~2.0 ppmv
~0.6 ppmv
~ 0.3 ppmv
~0.09 ppmv
~0.4 ppmv
~0.0005 ppmv = ~0.5 pptv
Although the variable gases are present in only “trace” amounts in the atmosphere, their
presence has profound impacts on Earth’s climate and human (and plant) health. The
importance and impact of some of these variable gases are discussed below.
Some Selected Variable Gases:
Water vapor (H2O):
Water vapor is the most important (and abundant) variable gas. Since it absorbs
IR radiation, it’s a greenhouse gas (it has a warming effect on Earth’s climate). The
cycling of water through the atmosphere, land and oceans represents the hydrological
cycle, and allows life on Earth to exist. The main source of water vapor in the
atmosphere is evaporation from the oceans. The atmosphere has a maximum amount of
water vapor that it can hold (4 – 5% of total air) before it condenses as a liquid (and
subsequently rains out of the atmosphere).
Carbon dioxide (CO2), carbon monoxide (CO), and methane (CH4):
Carbon dioxide is important because it is the 2nd most abundant greenhouse gas in
the atmosphere (after water vapor). Carbon dioxide is not toxic, so it does not directly
impact human health. However, since it is an important component of the climate
system, it can affect human societies and ecosystems through climate change. Because of
the profound impacts climate change can have on humans and ecosystems, and the fact
that humans are adding large amounts of carbon dioxide into the atmosphere through
fossil fuel burning, some (rather unsuccessful) attempts at international regulation have
been made (Kyoto Protocol). This will be discussed further in week 10. Carbon dioxide
is produced through many of the biological processes discussed in Ch. 2, fossil fuel
combustion (over the past ~100 years), and chemical oxidation from the reduced carbon
compounds, carbon monoxide and methane:
CO + OH + O2 Æ CO2 + HO2
CH4 + OH + 3O2 Æ Æ Æ CO2 + 5H2O
CO2 is lost from the atmosphere through chemical loss (photolysis) in the stratosphere,
oxygen producing photosynthesis, dissolution into ocean water, and chemical weathering.
Fossil fuel burning has increased the composition of CO2 in the atmosphere to levels
greater than any time in the last half a million years or more, and an unprecedented rate.
Carbon monoxide is directly harmful to human health as it binds to hemoglobin
(Hb) in the blood replacing oxygen. The conversion of O2Hb(aq) Æ COHb(aq) causes
headaches at exposures of 300 ppmv for one hour, and death at exposures of 700 ppmv
for one hour. CO also leads to the formation of tropospheric ozone. Ozone in the
troposphere is harmful to human health and ecosystems. The main sources of CO are
fossil fuel combustion and biomass burning, and the main sink is oxidation to CO2 (see
chemical reaction above).
Methane (CH4), also known as natural gas, is a greenhouse gas. It absorbs IR
radiation more efficiently than CO2 (25 times more efficiently), but it is much less
abundant than CO2. The largest natural source of methane is production from
methanogenic bacteria in wetlands, although today human emissions of methane from
rice paddies, natural gas leaks, biomass burning, and fossil fuel combustion dominate
atmospheric sources of methane. The main sink (loss process) of methane in the
atmosphere is oxidation chemistry (see abbreviated chemical reaction above). Methane is
not toxic and has no direct effects on human health. It does however have indirect
impacts on human health and well being through its impacts on climate (greenhouse gas),
and because it contributes to the formation of tropospheric ozone (we’ll talk about this in
weeks 3 and 4).
Ozone (O3):
Ozone in the stratosphere protects life on Earth by absorbing harmful UV
radiation. Most (90%) of the ozone in the atmosphere resides in the stratosphere.
However, ozone is toxic, so at the surface it is harmful to human health and ecosystems.
Ozone has been shown to decrease lung function, and may accelerate the aging of lung
tissue. (We will cover the health effects of various pollutants in more detail in week 7.)
Ozone also interferes with the growth of plants, causing them stress and making them
more susceptible to disease. Ozone is not emitted directly but forms through chemical
reactions in the atmosphere:
O + O2 Æ O3
This chemical reaction starts with the emission of pollutants such as nitrogen oxides and
carbon monoxide. Ozone formation will be discussed further in weeks 3 and 4. Ozone is
lost from the atmosphere through photolysis by sunlight, and chemical reactions with
other trace species.
Sulfur dioxide (SO2):
Sulfur dioxide is important because it forms aerosols, or particles, in the
atmosphere. An aerosol is defined as a solid or liquid particle suspended in the
atmosphere. Small particles in the atmosphere can be inhaled and subsequently absorbed
in the respiratory tract, harming lung function. Particles also contribute to the formation
of clouds, thus affecting the hydrological cycle and climate. Particulate sulfur forms
through oxidation of sulfur dioxide such as:
SO2(g) + OH(g) + H2O(g) Æ Æ H2SO4(aq)
Sulfur dioxide also contributes to the formation of acid rain, which has harmful effects on
ecosystems and structures. Acid rain will be covered in more detail in week 5. Today,
the main source of SO2 to the atmosphere is fossil fuel burning, primarily from coal due
to its relatively high sulfur content. The largest natural sources of sulfur to the
atmosphere are from marine phytoplankton, and volcanoes. Sulfur is lost from the
atmosphere by deposition (transfer) to the surface of the Earth.
Lead (Pb):
Lead is a solid metal. Small particles of lead can become suspended in the
atmosphere and inhaled. Lead has very small natural sources, originating from the
Earth’s crust. Human sources are relatively large, mainly from the combustion of leaded
fuel. Lead’s harmful health effects are well known and documented, which has lead to its
essential elimination from gasoline in many countries. Severe effects of lead poisoning
include mental retardation and neurological disorders. Other human sources of lead to
the atmosphere are lead ore smelting and lead-acid battery production. Lead is lost from
the atmosphere through deposition (transfer to the surface of the Earth).