1.3. Atmospheric composition

Earth’s atmosphere contains mainly several different gases and additionally aerosol particles. Atmospheric gases are generally classified by their amount and residence time. The residence time (or removal time or lifetime) is an average amount of time that a particle or substance spends in a particular system (as the atmosphere). The residence time can be defined as the amount of the compound in the atmosphere divided by the rate at which this compound removed from the atmosphere. Based on the quantity, major components and trace gases, while according to residence time, constant and variable (and sometimes highly variable) gases can be distinguished (Table 1.3).

The amount of atmospheric gases can be expressed by different measures. Generally used terms are the concentration (kg m–3), the volume ratio (m3 gas per m3 air) and mole fraction (mol mol–1). For trace gases, this mixing ratio are commonly given in units of parts per million volume (ppmv or simply ppm), parts per billion volume (ppbv or ppb), or parts per trillion volume (pptv or ppt); 1 ppmv = 10–6 mol mol–1, 1 ppbv = 10–9 mol mol–1 and 1 pptv = 10–12 mol mol–1.

The abundances of constant gases has remained the same over geological timescales, while residence time generally means years in case of variable gases and days in case of highly variable gases (Table 1.4).

The main constituents of the dry atmosphere are nitrogen (78.084% by volume), oxygen (20.946% by volume) and argon (0.934% by volume), but much lower concentrations other noble gases can also be found (Table 1.4). Concentrations of these gases do not vary substantially in time and space (in the lower 80 km layer of the atmosphere) and therefore they are called permanent gases.

Table 1.3: Classification of atmospheric gases

Amount

Residence time

Main components

Trace gases

Constant gases

nitrogen, oxygen and argon

(major components of the atmosphere)

other noble gases

Variable gases

carbon dioxide

other long-lived tracers

Highly variable gases

water vapour

other short-lived tracers

Table 1.4: Composition of the Earth’s atmosphere

* Concentration of water vapour is not included in dry atmosphere; (1) evaporation and transpiration; (2) oxidation of methane and non-methane hydrocarbons

Nitrogen (N2) is a relatively inert gas[3] and fundamental to all living systems. Through the nitrogen cycle nitrogen is removed from the atmosphere and becomes part of living organisms. This process is realized by nitrogen fixation[4] by soil bacteria, and by way of lighting through precipitation. Nitrogen returns to the atmosphere mainly by biomass combustion and denitrification[5].

As nitrogen, oxygen (O2) has also very important relations with life. Oxygen exchange between the atmosphere and biosphere is realized by photosynthesis and respiration[6].

Argon (Ar) in the atmosphere is the third most abundant gas. Among noble gases, argon was first detected in the atmosphere in 1894 by Lord Rayleigh and William Ramsay. Almost 100% of atmospheric argon is the radiogenic 40Ar isotope derived from decay in 40K (potassium) in the Earth's crust.

Water vapour (H2O) is a significant component of the atmosphere. Its concentration varies over a wide range both spatially and temporally. Most of water vapour concentrated in the lower atmosphere (about 90% of total atmospheric water vapour is found in the lower 5 km atmospheric layer, and more than 99% of it can be found in the troposphere[7]). The capacity of air to hold water vapour (called saturation level) is a function only of the air temperature. The higher the temperature the greater amount of water vapour can be held without condensation (Figure 1.4).

Saturated water vapour pressure in the function of air temperature.

Figure 1.4: Dependence of saturated water vapour pressure from air temperature. The higher the temperature, the greater water vapour can be held by the air.

The highest atmospheric moisture content is observable over equatorial ocean area and tropical rain forests, while the lowest water vapour concentrations can be measured over cold, polar regions, and subtropical deserts. Atmospheric water vapour has several significant direct and indirect effects on both weather and climate. It plays important roles in the radiation and the energy budgets of the atmosphere, and also in the formations of clouds and precipitations. About 70% of total absorption of the incoming shortwave solar radiation, particularly in the infrared region, and about 60% of total absorption of long-wave radiation by the Earth are realized by water vapour (Figure 1.5) thereby it is the most significant greenhouse gas[8]. Water vapour also influences heat energy transfer on the surface- atmosphere system through the latent heat flux. The latent heat flux is a component of surface energy budget. It plays an important role in the heat transfer from Earth’s surface into the atmosphere. During this process heat from evaporation and transpiration of water at the surface is transferred to the troposphere by water vapour and it is released there by condensation. Latent heat of evaporation of water at the surface is released to the atmosphere when condensation occurs. Due to the condensation, cloud, fog and precipitation can be produced.

Absorption of radiation by all atmospheric components and by water vapour.

Figure 1.5: Absorption of solar and terrestrial radiation by all atmospheric components and by water vapour. Water vapour has significant absorption bands both in shortwave (incoming solar radiation) and longwave (outgoing terrestrial radiation) spectra.

Carbon dioxide (CO2) is an important greenhouse gas as it has a strong absorption capacity in the infrared and near-infrared bands. It has a natural exchange between the atmosphere and biosphere through the photosynthesis and respiration. A part of atmospheric CO2 is dissolved by the seas and oceans. Atmospheric concentration of carbon-dioxide has increased steadily worldwide by over 35% since the beginning of 1800’s. Before the Industrial Era, atmospheric carbon dioxide concentration was 280 ± 10 ppm for several thousand years. The present atmospheric CO2 concentration has not been exceeded during the past 420,000 years, and likely not during the past 20 million years. The rate of increase over the past century is unprecedented, at least during the past 20,000 years (IPCC, 2001). The increase has speeded up in the last few decades. In the beginning of the measurements of background concentration[9] of CO2 on Mauna Loa observatory in 1958, CO2 level was less than 320 ppm, while in 2010 it has reached and exceeds 390 ppm (Figure 1.6). This rapid growth is primarily due to growing anthropogenic activities, like burning of fossil fuels, deforestation, and other forms of land-use change. This man-made increase of atmospheric concentration of carbon-dioxide has definitely contributed to global warming over the last decades with the increase of greenhouse effect. Carbon-dioxide concentration shows characteristic seasonal cycle, which is related mainly to Northern Hemisphere growing season.

Monthly mean background concentration of carbon dioxide at the Mauna Loa Observatory

Figure 1.6: Monthly mean background concentration of atmospheric carbon dioxide (CO2) from 1958 to 2012, measured at the Mauna Loa Observatory. Source of data: http://www.esrl.noaa.gov/gmd/ccgg/trends/

The volume of other gases compared to the main components, are very low, and therefore they are called trace gases. Despite their low concentrations, these tracers (e.g. ozone both in troposphere and stratosphere, carbon, nitrogen, sulphur compounds etc.) can be of critical importance for several environmental issues (for example: greenhouse effect[10], atmospheric pollution etc.).

Next to the different gases, Earth’s atmosphere contains a huge number of aerosol particles. Aerosols are generally defined as suspension of solid particles or liquid droplets in a gas. Their size are very small, the particle diameters in the range of 10–9–10–4 m. Aerosol particles originate in large part from different natural sources, and in lesser extent, from anthropogenic sources. Main emission sources of primary particles – which are emitted directly to the atmosphere – are soil and mineral dusts, sea salts, volcanic eruptions, biomass burning, biological materials (e.g. pollen, plant fragments, microorganisms), incomplete combustion of fossil fuels, industrial particulates and traffic. Secondary particles are formed when gas-to-particle conversion occurs by nucleation and condensation of gaseous precursors.

The concentration, size distribution and composition of atmospheric aerosol particles vary significantly with time and space. In the lower troposphere, the aerosol mass concentration varies in a range of 1–100 µg m–3, and particle number is typically varies from 102 to 105 cm–3 (Pöschl, U., 2005). Most particles can be found over deserts and urban area, while polar atmosphere and alpine air contains fewer aerosols. Generally, aerosol concentration decreases with altitude.

Atmospheric aerosols have significant effects on different atmospheric processes, climate and human health. Aerosol particles modify the radiation balance of the Earth through scattering and absorbing incoming solar and terrestrial radiation. They are essential in the formation of clouds and precipitations, as they provide condensation nuclei. They have influence on oxidation processes, and affect the cycles of nitrogen, sulphur and atmospheric oxidants (see more information about aerosol particles in Chapter 9).



[3] Inert gas: Unreactive gas, which does not react with other chemicals to form new compounds under a set of given conditions. Inert gases are noble gases and relatively inert gas for example the nitrogen.

[4] Nitrogen fixation: a process by which atmospheric nitrogen is reduced to form ammonia.

[5] Denitrification: Reduction of nitrates to gaseous nitrogen by microorganisms in a series of biochemical reactions.

[6] Respiration: an oxidative process by which the chemical energy of organic molecules is released in a series of metabolic steps involving the consumption of oxygen and the release of carbon dioxide and water.

[7] Troposphere: a layer of the atmosphere from the surface to about 7–20 km (depending on latitude). In this layer temperature decreases with increasing altitude in the function of the distance from warming surface.

[8] Greenhouse gas (GHG): gases in the atmosphere, which cause greenhouse effect. Most abundant greenhouse gases are water vapour (H2O), carbon-dioxide (CO2), methane (NH3), nitrous-oxide (N2O), tropospheric ozone (O3) and chlorofluorocarbons (CFCs).

[9] Background concentration (or baseline concentration): a concentration of a gives species that are normally found in the atmosphere without consideration of local sources. Background concentration measurements are performed away from emission sources.

[10] Greenhouse effect: a natural process in the atmosphere, by which a part of long wave radiation from Earth’s surface is absorbed by atmospheric greenhouse gases, and is re-emitted in all directions, thereby heating the atmosphere. Due to this process, the Earth’s atmosphere is more than 33 °C warmer that it would be without it. Human activities can modify this greenhouse effect, which is the most important factor in recent climate change.