Introduction
The atmosphere surrounding the Earth is a thin layer of gases retained by gravity (Fig). Table lists the most abundant atmospheric gases. Concentrations are expressed as mole fractions, commonly called mixing ratios. The principal constituents are molecular nitrogen (N2), molecular oxygen (O2), and argon (Ar). Their mixing ratios are controlled by interactions with geochemical reservoirs below the Earth’s surface on very long timescales. Water vapor is present at highly variable mixing ratios (10–6–10–2 mol mol–1), determined by evaporation from the Earth’s surface and precipitation. In addition to these major constituents, the atmosphere contains a very large number of trace gases with mixing ratios lower than 10–3 mol mol–1, including carbon dioxide (CO2), methane (CH4), ozone (O3), and many others. It also contains solid and liquid aerosol particles, typically 0.01–10 μm in size and present at concentrations of 101–104 particles cm–3. These trace gases and aerosol particles do not contribute significantly to atmospheric mass, but are of central interest for environmental issues and for atmospheric reactivity.
Table: Mixing ratios of gases in dry air
| Gas | Mixing ratio (mol mol–1) |
|---|---|
| Nitrogen (N2) | 0.78 |
| Oxygen (O2) | 0.21 |
| Argon (Ar) | 0.0093 |
| Carbon dioxide (CO2) | 400 × 10–6 |
| Neon (Ne) | 18 × 10–6 |
| Ozone (O3) | 0.01–10 × 10–6 |
| Helium (He) | 5.2 × 10–6 |
| Methane (CH4) | 1.8 × 10–6 |
| Krypton (Kr) | 1.1 × 10–6 |
| Hydrogen (H2) | 500 × 10–9 |
| Nitrous oxide (N2O) | 330 × 10–9 |
The mean atmospheric pressure at the Earth’s surface is 984 hPa, which combined with the Earth’s radius of 6378 km yields a total mass for the atmosphere of 5.14 × 1018 kg. As we will see, atmospheric pressure decreases quasi-exponentially with height: 50% of total atmospheric mass is found below 5.6 km altitude and 90% below 16 km. Atmospheric pressures are sufficiently low for the ideal gas law to be obeyed within 1% under all conditions. The global mean surface air temperature is 288 K, and the corresponding air density is 1.2 kg m–3 or 2.5 × 1019 molecules cm–3; air density also decreases quasi-exponentially with height.