Core Concepts
In this article, you will learn the basics of atmospheric chemistry including the chemical composition of the atmosphere, atmospheric constituents, aerosols, and a brief introduction to atmospheric cycles.
Introduction
The atmosphere is the thin layer of matter that surrounds our planet. The atmosphere actually contains gasses, liquids, and even solid particles. Colloquially, we use the terms air and atmosphere interchangeably; however, they reference different things. Air is primarily the combination of nitrogen, dioxygen, and argon. The atmosphere is the entire area of low-density fluid that surrounds the Earth. The atmosphere includes (in order of relative proximity to the Earth) the troposphere, stratosphere, and mesosphere. In the context of chemistry, the troposphere and stratosphere are the most relevant. See the image below representing the layers of the atmosphere.
Atmospheric Constituents
Atmospheric constituents are different compounds that make up the atmosphere. The most predominant atmospheric constituent is nitrogen, which accounts for about 78% of the atmosphere. There are two important concepts used to determine and represent the amount of a given substance: concentration and mixing ratio. Concentration is determined by dividing the number of moles of a substance in a given volume by that volume. Mixing ratios are determined by dividing the number of moles a specific constituent by the number of moles of air. For example, in every 100 moles of air, 78 of those moles are nitrogen. Thus, the mixing ratio of Nitrogen is about 0.78 (78%). A chart demonstrating the abundance of various constituents in the atmosphere can be seen below.
Composition of the Atmosphere
There are three types of atmospheric constituents:
- Geogenic: abiotic activities of the Earth’s crust including volcanic activity, sea sprays, and meteors.
- Biogenic: biological activities that involve chemicals including photosynthesis and respiration
- Anthropogenic: human activities that create chemicals including lead pollution and agriculture
Aerosols
One important class of atmospheric constituent is aerosols. Aerosols are liquid and solid components of the atmosphere that can measure from micrometers to millimeters. Aerosols are emitted either as condensed particle products of a complex gaseous chemical reaction (secondary aerosols) or directly into the atmosphere, like ash (primary aerosols). Aerosols have both natural and human-made sources. Natural aerosols include sea salt, biogenic aerosols (from living sources), and smoke. Aerosols are involved in many regulatory processes in the atmosphere. Depending on the composition of the aerosol, it can either absorb or reflect sunlight and heat. Soot (dark carbon particles) is the primary aerosol that absorbs light. As light is absorbed, heat is retained, and the temperature of the atmosphere rises. Soot is most frequently released from the burning of fossil fuels, wood, and other organic matter.
Measuring Atmospheric Constituents
Mixing Ratio
ri = ni / ntot – ni
Concentration
concentration= mass/volume
ni = mass of one certain constituent and ntot = mass of all other constituents
These two methodologies should be used in different situations. Concentration is dependent on pressure and temperature, whereas the mixing ratio can change based on the relative presence of water vapor (humidity). Following the principles of the ideal gas law, concentration is directly impacted by changes in pressure and temperature. When pressure is increased, the concentration is also increased. In areas of higher altitude (like mountains), the pressure decreases and so does the concentration of oxygen. This explains why it is harder to breathe in areas of higher elevations. At sea level, pressure is higher and so is oxygen concentration. In meteorology, mixing ratio is known as the mass ratio; which is the mass of water divided by the mass of dry air in a given volume. If this sample is taken from a humid atmosphere, the mixing ratio will be higher than that of a sample taken from an arid atmosphere. This is because the increased presence of water vapor in the humid sample increases the mass of water present in the sample. The arid sample is taken from an atmosphere of low water vapor, resulting in a lower mixing ratio. For this reason, concentration is primarily used for particles, and mixing ratios are used for gases. Since mixing ratios can change with humidity, it is common practice to denote the mixing ratio as either wet or dry. Most commonly, gaseous constituents are defined in terms of parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt). Since these units could be defining either volume or mass, it is best practice to define them as ppmv (parts per million volume) or ppmm (parts per million mass). Another method for determining concentration is column abundance. Column abundance is the total mass of a gas found in a column of air above a specific location. Since this unit is specific to an area, the unit is the amount per area, i.e. mol/m-2 or kg/m-2.
Atmospheric Cycles
Constant cycles are occurring in our atmosphere. Atmospheric constituents are continuously interacting and exchanging with oceans, forests, and biological organisms among many other biomes. The atmosphere is not in equilibrium, but a steady-state condition. A steady-state condition indicates an equal rate of fluctuation of gases in and out of the atmosphere. If there are more inward-flowing gases than outward, there is an accumulation of gas. In the opposite situation, there is a depletion. The buildup of carbon dioxide is an example of accumulation. An example of depletion is the current diminishing of stratospheric ozone leading to an increased risk of UV radiation for all life forms. There are many other atmospheric cycles including the carbon cycle, nitrogen cycle, and oxygen cycle.
Conclusion
The atmosphere is composed of multiple layers of matter that surround the Earth. There are many parts to this matter, both naturally occurring and as a result of human activity. Mixing ratios or concentrations can be used to measure the presence of these various atmospheric constituents. Atmospheric cycles are continuously occurring and can lead to stasis, accumulation, or depletion.