7.1: Atmospheric Composition
- Page ID
- 33887
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)We know quite a lot about the atmosphere which is a thin layer of gases that protects life on Earth. It has taken decades, if not centuries, of careful observation and insightful theory that is based on solid physical and chemical laws.
The atmosphere consists mostly of dry air composed mostly of nitrogen (78%), oxygen (21%), and argon (0.9%), and highly variable amounts of water vapor (from parts per million in air to a few percent). Now we will consider gases and particles in the atmosphere at trace levels. The most abundant of the trace gases in the global atmosphere is carbon dioxide (~400 parts per million, or 400 x 10-6), but there are thousands of trace gases with fractions much less than a few parts per million. These trace gases and particles are as important to atmospheric structure and weather as are nitrogen, oxygen, and water vapor and they also play a huge role in human and ecological health and global climate.
The major gases that comprise today's atmosphere are in Table \(\PageIndex{1}\).
| Constituent | Percent of Total Gas in Atmosphere | Role in the Atmosphere |
|---|---|---|
| nitrogen (N2) | 78% | transparent; provides heat capacity and momentum; exchanged with biomass; decomposed in combustion |
| oxygen (O2) | 21% | transparent except for in extreme ultraviolet; provides some heat capacity and momentum; exchanged with life; source of important reactive gases like ozone |
| argon (Ar) | 0.9% | no role |
| carbon dioxide (CO2) | <1% | transparent in visible; absorbs infrared light (i.e., contributes to global warming); exchanged with life; product of combustion |
| neon (Ne) | <1% | no role, but makes colorful glowing signs |
| water vapor (H2O) | varies <1 to 5% | gas transparent in visible; absorbs infrared light (i.e., contributes to global warming); exists as vapor, liquid, and solid; exchanged with life; product of combustion |
| aerosol particles | <1% | essential for cloud formation; interact with visible and infrared light; exchanged with surfaces and life |
| methane (CH4) | <1% | transparent in visible; absorbs in infrared (i.e. contributes to global warming); exchanged with life; source of CO2 and H2O |
| ozone (O3) | <1% | transparent in visible; absorbs in UV and infrared; reactive and source of more reactive gases |
| particles | <1% | 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), 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 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.