Fundamentals of Atmospheric Science

4.1 Atmospheric Composition

The major gases that comprise today's atmosphere are in the table below. The mixing ratio of a gas X is defined as the fraction of total moles that are the moles of gas X. For instance, 78 moles of every 100 total moles of air is nitrogen, so nitrogen's mixing ratio is 0.78. Note that in atmospheric composition, the mixing ratio is the moles of the gas divided by the total moles of air. In contrast, the water vapor mixing ratio is the mass of water vapor divided by the mass of dry air.

Major Constituents in Earth’s Present Atmosphere
Constituent Molecular Mass (g/mol)

Mixing Ratio (mol mol-1)

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 for in 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; exchanged with life; product of combustion
neon (Ne) 20.183 0.0000182 no role, but makes colorful glowing signs
water vapor (H2O) 18.015 2x10-6 to 0.05 gas transparent in visible; absorbs infrared light; exists as vapor, liquid, and solid; exchanged with life; product of combustion
aerosol particles varies 0-500 ug m-3 (note different units) essential for cloud formation; interact with visible and infrared light; exchanged with surfaces and life
methane (CH4) 16.04 0.00000182 (1820 ppbv) transparent invisible; absorbs in infrared; exchanged with life; source of CO2 and H2O
ozone (O3) 48.00 0.01 – 10 ppm transparent invisible; absorbs in UV and infrared; reactive and source of more reactive gases
particles varies 0-100’s µg m-3 of air absorbs and scatters light; acts 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. Water vapor has the additional important feature of existing in the vapor, 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

Three different units are typically used when specifying the amounts of gases. One 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 with the specific humidity of water vapor. A second is the volume mixing ratio, which is just the number of molecules of a chemical species in a unit volume divided by the total number of all molecules in a unit volume. 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) ppmv or parts per billion (10-9) ppbv by volume (meaning by number not mass). Lastly we need to use the concentration, or number per unit volume, to calculate reaction rates and lifetimes.

To convert between volume mixing ratios and concentrations, use the following procedure. For a species X, to convert from a 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. Suppose p = 960 hPa (or mb) and T = 296K, and X = 60 ppbv, then

[X]= p kT χ X = 96000Pa ( 1.38x 10 23 J K 1 molecul e 1 )(296K) ( 1 m 3 10 6 c m 3 )60x 10 9 =1.4x 10 12 moleculesc m 3