Atmospheric Composition

The beginning of Earth’s atmosphere can be traced back to early history of the planet and it is thought to be formed by the release of the trapped volatile compounds from the planet itself. The early atmosphere is believed to be a mixture of carbon dioxide, nitrogen and water vapour, with trace amounts of hydrogen. The majority of the out gassed water vapour formed oceans, the outgassed carbon dioxide formed sedimentary carbonate rocks after dissolution in the oceans, while nitrogen became the most abundant component as being inert, insoluble and non-condensible.

 

Present-Day Earth’s Atmosphere

The sunlight that reaches the Earth constitutes the basis of life. However, the atmosphere helps to maintain an accurate balance between the inflow and outflow of solar energy, which determines the temperature of the Earth’s surface. Atmosphere warms the surface through net infrared radiation trapping (greenhouse effect) and reduces the extremes of diurnal temperature variation, absorbs the biologically harmful ultraviolet solar radiation.

Atmosphere is a gaseous mass which envelopes the Earth and it is held in place by Earth’s gravity. The present-day Earth’s atmosphere is composed of the nitrogen (78 %), oxygen (21 %) and argon (1 %), whose concentrations are controlled by the biosphere, crustal uptake and release, degassing of the interior. They are called the ‘constant gases’, as their abundances has remained the same over geological timescales.  Water vapour, with its highly variable concentrations (reaching 3 %), is also a common component of the Earth’s atmosphere. Its abundance is controlled by precipitation and evaporation processes. The remaining atmospheric constituents, called ‘trace gases’ or ‘variable gases’ (it is among others: carbon dioxide, methane, ozone) (Table 1), constitute less than 1 % of the atmosphere. Even though they represent a small fraction of the atmosphere as a whole, their impact on our environment is of a great significance. They play a critical role in the radiative balance of Earth and in the chemical properties of the atmosphere. The abundances of trace gases have changed significantly over the past centuries.

Gas name	Chemical formula	Percent volume
Nitrogen	N2	78.08 %
Oxygen	O2	20.95 %
Water*	H2O	0 to 3 %
Argon	Ar	0.93 %
Carbon dioxide*	CO2	0.0360 %
Neon	Ne	0.0018 %
Helium	He	0.0005 %
Methane*	CH4	0.00017 %
Hydrogen	H2	0.00005 %
Nitrous Oxide*	N2O	0.00003 %
Ozone*	O3	0.000004 %

^* variable gases

Table 1. Average composition of the atmosphere up to an altitude 25 km. 

The atmosphere is not a homogeneous body, but it has a layered structure defined by vertical temperature changes. The atmosphere is divided into lower and upper regions; lower atmosphere extends to the top of the stratosphere (~ 50 km). The regions of the atmosphere are (Figure 1):

Troposphere – extends from the surface up to the tropopause (10-15 km depending on latitude and season). Despite it constitutes a small fraction of atmosphere’s total height, it contains 80% of its total mass, with almost all of the atmosphere’s water vapour.  The temperature here decreases with height.

Stratosphere – extends from tropopause to the stratopause (~ 45-55 km). Temperature increases with altitude, reaching 271 K at the top of the stratosphere (not much lower than average Earth’s surface temperature, 288 K). The thermal inversion is caused by absorption of ultraviolet solar radiation by ozone.

Mesosphere – extends from stratopause to the mesopause (~ 80-90 km). Temperature decreases with increasing altitude. The coldest place in the atmosphere is mesopause.

Thermosphere – region spanning above mesopause. The absorption of short-wave radiation by nitrogen and oxygen is causing high temperatures existing in this region. In the upper mesosphere and lower thermosphere there is an ionosphere, where ions are produced by photoionization.

Exosphere – the outermost atmospheric layer (> 500 km), where gas molecules can escape the Earth’s gravitation if they carry sufficient energy.

Figure 1. Layers of the atmosphere (from Seinfeld and Pandis, 2006)

Changes of atmosphere’s composition

Analysis of air trapped in ice cores of Greenland and Antarctica combined with a present-day measurements indicate a striking, global increase of gases such as, carbon dioxide, methane (Figure 2), nitrous oxide and halogen-containing compounds over recent 200 years.

Figure 2. Methane concentrations over the last 1000 years based on analysis of ice cores from Antarctice and Greenland (IPCC, 1005) (based on Seinfeld and Pandis, 2006)

The changes in atmospheric composition are triggered by both, natural (e.g. volcanic activity, weathering of land, internal feedbacks between climate and carbon cycle), and human factor (e.g. direct emissions of greenhouse gases, emissions of precursors of greenhouse gases, changes the surface of lands).

The recent dramatic changes in the concentrations of atmospheric constituents are attributed to human activities, and, like never observed before, are exceedingly rapid in speed and magnitude, and, in case of accumulation of carbon dioxide, practically irreversible.

 

Aviation emissions and impact on atmospheric composition

Aircraft emissions are a specific part of human activity, as it is the only anthropogenic source injected directly into the relatively clean parts of the atmosphere, which is the upper troposphere and lower stratosphere (UTLS) region.

Aviation affects the atmosphere through a wide range of components:

• emissions of carbon dioxide,
• emissions of nitrogen oxides (produces ozone and destroys ambient methane),
• emissions of water vapour,
• formation of persistent linear contrails,
• aviation induced cloudiness (ACI),
• emissions of sulphate particles,
• emissions of soot particles.

Through these emissions and clouds effect the upper atmosphere is modified in term of its chemical and physical properties. The level of scientific understanding (LOSU) is ‘high’ for carbon dioxide only, the LOSU for the rest of the components varies from medium-low (nitrogen dioxides) to very low (ACI).

Impact of aircraft NO_x emissions on atmosphere is investigated in CATE using a Global Chemistry-Transport Model (MOZART-3.5 CTM). The coupled NO_x-O₃-CH₄ system, affected by emissions of NO_x, results in a short-term positive O₃ perturbation and a long-term negative CH₄ response (Figure 3). Depletion of CH₄ has a feedback effect on O₃ and thus causes a long-term and small negative O₃ perturbation. These processes act on a different spatial (ozone: continental to hemispheric, methane: global) and temporal (ozone: days, methane: years) scales, which is why, defining unanimous result describing the impact of aviation NO_x on climate remains challenging and controversial.

 

Figure 3. The monthly mean perturbations of ozone (upper) and methane (lower) in July 2006 at 227 hPa in a response to emissions of aircraft nitrogen oxides based on experiments done by MOZART-3.5 CTM.