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Trace gases are those gases in the atmosphere other than nitrogen (78.1%), oxygen (20.9%), and argon (0.934%) which, in combination, make up 99.934% of the gases in the atmosphere (not including water vapor).

Contents

Abundance, sources and sinksEdit

The abundance of a trace gas can range from a few parts per trillion (ppt) by volume to several hundred parts per million by volume (ppmv).[1] When a trace gas is added into the atmosphere, that process is called a source. There are two possible types of sources - natural or anthropogenic. Natural sources are caused by processes that occur in nature. In contrast, anthropogenic sources are caused by human activity. Some of the sources of a trace gas are biogenic, solid Earth (outgassing), the ocean, industrial activities, or in situ formation.[1] A few examples of biogenic sources include photosynthesis, animal excrements, termites, rice paddies, and wetlands. Volcanoes are the main source for trace gases from solid earth. The global ocean is also a source of several trace gases, in particular sulfur-containing gases. In situ trace gas formation occurs through chemical reactions in the gas-phase.[1] Anthropogenic sources are caused by human related activities such as fossil fuel combustion (e.g. in transportation), fossil fuel mining, biomass burning, and industrial activity.

In contrast, a sink is when a trace gas is removed from the atmosphere. Some of the sinks of trace gases are chemical reactions in the atmosphere, mainly with the OH radical, gas-to-particle conversion forming aerosols, wet deposition and dry deposition.[1] Other sinks include microbiological activity in soils.

Below is a chart of several trace gases including their abundances, atmospheric lifetimes, sources, and sinks.  

Trace gases – taken at pressure 1 atm[1]

Gas Chemical Formula Fraction of Volume of Air by the Species Residence Time or Lifetime Major Sources Major Sinks
Carbon Dioxide CO2 405 ppmv (in 2018) 3 – 4 years Biological, oceanic, combustion, anthropogenic photosynthesis
Neon Ne 18.18 ppmv _________ Volcanic ________
Helium He 5.24 ppmv _________ Radiogenic ________
Methane CH4 1.8 ppmv 9 years Biological, anthropogenic OH
Hydrogen H2 0.56 ppmv ~ 2 years Biological, HCHO photolysis soil uptake
Nitrous Oxide N2O 0.33 ppmv 150 years Biological, anthropogenic O(1D) in stratosphere
Carbon Monoxide CO 40 – 200 ppbv ~ 60 days Photochemical, combustion, anthropogenic OH
Ozone O3 10 – 200 ppbv (troposphere) Days - Months Photochemical photolysis
Formaldehyde HCHO 0.1 – 10 ppbv ~ 1.5 hours Photochemical OH, photolysis
Nitrogen Species NOx 10 pptv – 1 ppmv variable Soils, anthropogenic, lightning OH
Ammonia NH3 10 pptv - 1 ppbv 2 – 10 days Biological gas-to-particle conversion
Sulfur Dioxide SO2 10 pptv – 1 ppbv Days Photochemical, volcanic, anthropogenic OH, water-based oxidation
Dimethyl sulfide (CH3)2S several pptv – several ppbv Days Biological, ocean OH

Greenhouse gasesEdit

A few examples of the major greenhouse gases are water, carbon dioxide, methane, nitrous oxide, ozone, and CFCs. These gases can absorb infrared radiation from the Earth's surface as it passes through the atmosphere. The most important greenhouse gas is water vapor because it can trap about 80 percent of outgoing IR radiation.[2] The second most important greenhouse gas, and the most important one affected by man-made sources into the atmosphere, is carbon dioxide.[2] The reason for why greenhouse gases can absorb infrared radiation is their molecular structure. For example, carbon dioxide has two basic modes of vibration that create a strong dipole-moment, which causes its strong absorption of infrared radiation.[2] Below is a table of some of the major greenhouse gases with man-made sources and their contribution to the enhanced greenhouse effect.

Key Greenhouse Gases and Sources[2]

Gas Chemical Formula Major Human Sources Contribution to Increase (Estimated)
Carbon Dioxide CO2 fossil fuel combustion, deforestation 55%
Methane CH4 rice fields, cattle and dairy cows, landfills, oil and gas production 15%
Nitrous Oxide N2O fertilizers, deforestation 6%

In contrast, the most abundant gases in the atmosphere are not greenhouse gases. The main reasons is because they cannot absorb infrared radiation as they do not have vibrations with a dipole moment. [2] For instance, the triple bonds of atmospheric dinitrogen make for a highly symmetric molecule that is very inert in the atmosphere.

MixingEdit

The residence time of a trace gas depends on the abundance and rate of removal. The Junge (empirical) relationship describes the relationship between concentration fluctuations and residence time of a gas in the atmosphere. It can expressed as fc = br, where fc is the coefficient of variation, τr is the residence time in years, and b is an empirical constant, which Junge originally gave as 0.14 years.[3] As residence time increases, the concentration variability decreases. This implies that the most reactive gases have the most concentration variability because of their shorter lifetimes. In contrast, more inert gases are non-variable and have longer lifetimes. When measured far from their sources and sinks, the relationship can be used to estimate tropospheric residence-times of gases.[3]

ReferencesEdit

  1. ^ a b c d e Wallace, John; Hobbs, Peter (2006). Atmospheric Science: An Introductory Survey. Amsterdam, Boston: Elsevier Academic Press. ISBN 9780127329512.
  2. ^ a b c d e Trogler, William C. (1995). "The Environmental Chemistry of Trace Atmospheric Gases". Journal of Chemical Education. 72 (11): 973. doi:10.1021/ed072p973.
  3. ^ a b Slinn, W. G. N. (1988). "A Simple Model for Junge's Relationship between Concentration Fluctuations and Residence Times for Tropospheric Trace Gases". Tellus B: Chemical and Physical Meteorology. 40 (3): 229–232. doi:10.3402/tellusb.v40i3.15909.

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