4.1 Atmospheric Composition
The major gases that comprise today's atmosphere are listed in the table below. In most studies of atmospheric composition, the volume mixing ratio is used to specify the amount of a gas. The volume mixing ratio (also called the molar mixing ratio) of a gas is the number of moles of the gas divided by the number of moles of air. For example, 78 moles of every 100 moles of air is nitrogen, so nitrogen's volume mixing ratio is 0.78.
Volume Mixing Ratio
|Role in the Atmosphere|
|nitrogen (N2)||28.013||0.7808||transparent; provides heat capacity and momentum; exchanged with biomass; decomposed in combustion|
|oxygen (O2)||31.998||0.2095||transparent except in the extreme ultraviolet; provides some heat capacity and momentum; exchanged with life; source of important reactive gases like ozone|
|argon (Ar)||39.948||0.0093||no role|
|carbon dioxide (CO2)||44.010||0.000385 (385 ppmv)||transparent in visible; absorbs infrared light (i.e., contributes to global warming); exchanged with life; product of combustion|
|neon (Ne)||20.183||0.0000182||no role, but makes colorful glowing signs|
|water vapor (H2O)||18.015||2 x 10–6 to 0.05||transparent in visible; absorbs infrared light (i.e., contributes to global warming); exchanges with liquid and solid forms; exchanges with life; product of combustion|
|aerosol particles||varies||0–500 µg m–3 (note different units)||essential for cloud formation; interact with visible and infrared light; exchange with surfaces and life|
|methane (CH4)||16.04||0.00000182 (1820 ppbv)||transparent in visible; absorbs in infrared (i.e., contributes to global warming); exchange with life; source of CO2 and H2O|
|ozone (O3)||48.00||0.01–10 x 10–6 (10 ppbv to 10 ppmv)||transparent in visible; absorbs in UV and infrared; reactive and source of more reactive gases|
|particles||varies||0–100’s µg m–3 (note different units)||absorb and scatter light; act as CCN and IN (see below)|
Key features of the gases include their compressibility (i.e., ability to expand or shrink in volume), their transparency in the visible, their momentum, and their heat capacity. Key greenhouses gases—those that absorb infrared radiation and hence warm the planet—are water vapor, carbon dioxide, methane, and ozone. Water vapor has the additional important feature of exchanging with liquid and solid phases in the atmosphere and on Earth’s surface. The most important properties of small particles include their ability to dissolve in water in order to be cloud condensation nuclei (CCN) or to maintain a lattice structure similar to ice in order to be ice nuclei (IN), as well as their ability to absorb and scatter sunlight. These properties depend completely on the particle size and composition. Most atmospheric gases participate in the atmosphere's chemistry, which is initiated by sunlight, as you will soon see.
A note on the units used when quantifying atmospheric composition
The amount of a gas is typically specified in one of three different ways. You have already been introduced to the first, the volume mixing ratio, in the table above. For gases with relatively large fractions like nitrogen, oxygen, and argon, we use percent to indicate this fraction. For minor gases like carbon dioxide and ozone, we use parts per million (10–6) by volume (ppmv) or parts per billion (10–9) by volume (ppbv). The second is the mass mixing ratio, which is the mass of a chemical species divided by the total mass of air. You have already encountered this ratio with the specific humidity. The third way to specify the amount of a gas is the concentration, which is the number of molecules per unit volume.
It is straightforward to convert between volume mixing ratio and concentration. For a species X, to convert from a volume mixing ratio, notated χX, to a concentration, notated [X], use the Ideal Gas Law to find the number of total molecules in a cm3 and then multiply by χX, expressed as a fraction. For example, let p = 960 hPa, T = 296 K, and χX = 60 ppbv, then the concentration can be calculated as follows:
Here we have used the Boltzmann constant k, which is simply the universal gas constant divided by Avogadro's number.