By Kal Kaur
The greenhouse effect
Infrared gas analyser
Human-induced and natural pressures are known to affect the Earth’s climate, but the extent to which isn’t always entirely clear. Increased pressures for resources, land-use, and land-cover all collectively contribute to global change. Anthropogenic nitrogen deposition, air pollution, and industrialization are popular suggestions as possible contributing factors for destructive climate change. Consequently, this is likely to affect the ecosystems and the biodiversity that occupies planet Earth.
The Greenhouse Effect
Climate change is a complex study and even more challenging to monitor. To understand the basic principles behind climate change, and how to track the change in properties of the climate system, we must first look to the sun. The sun emits solar radiation in the visible and near-visible parts of the spectrum, which passes through the Earth’s atmosphere. Solar radiation that is at a frequency similar to visible light is responsible for warming the Earth’s surface. Radiation in the form of Gamma rays, X-rays, and ultraviolet rays (all with a wavelength frequency of less than 200 nm) is absorbed by oxygen and nitrogen, a reaction that generates heat, thus warming up the planetary surface. Ozone (O3), which occupies the stratosphere will absorb solar radiation with a wavelength between 200 to 300 nm. Carbon dioxide (CO2), water vapour, and ozone molecules are collectively responsible for absorbing solar radiation that is 700 nm or greater in wavelength.
Only half of the solar radiation penetrating the Earth’s atmosphere is absorbed. The atmosphere reflects some of the solar radiation back into space; whereas, part of the solar radiation is absorbed by greenhouse gases (GHG) that is then re-emitted as energy to the lower atmosphere (Figure1). As the Earth cools in the dark, the heat begins to escape; however, due to the abundance of GHGs in the atmosphere, this heat becomes trapped which explains why Earth has on average a temperature of approximately 59°F.
Image Credit: clinton5.nara.gov
Figure 1. Biological mechanisms to the greenhouse effect.
Gas molecules that trap heat energy in the atmosphere including CO2, methane (CH4), nitrous oxide (N20), and fluorinated gases all contribute to the greenhouse effect. All such gases are released into the atmosphere naturally and through anthropogenic processes, such as burning of fossil fuels (e.g., oil and coal) and trees, chemical reactions, agricultural practices, solid waste, and industrial processes (figure 2A).
Figure 2. (a) Global annual emissions of anthropogenic GHGs from 1970 to 2004. (b) Share of different anthropogenic GHGs in total emissions in 2004 in terms of carbon dioxide equivalents (CO2-eq). (c) Share of different sectors in total anthropogenic GHG emissions in 2004 in terms of CO2-eq. (Forestry includes deforestation.) (IPCC Fourth Assessment Report, Climate Change 2007 (AR4), Synthesis report [chapter 2: Figure 2-1])
Figure 3. Graphical interpretation of Global anthropogenic GHG emission levels from the period of 1970 to 2004. Global CO2 emissions for 1940 to 2000 and emissions ranges for categories of stabilisation scenarios from 2000 to 2100 (left-hand panel); and the corresponding relationship between the stabilisation target and the likely equilibrium global average temperature increase above pre-industrial (right hand panel). Approaching equilibrium can take several centuries, especially for scenarios with higher levels of stabilisation. Coloured shadings show stabilisation scenarios grouped according to different targets (stabilisation category I to VI). The right-hand panel shows ranges of global average temperature change above pre-industrial, using (i) ‘best estimate’ climate sensitivity of 3°C (black line in middle of shaded area), (ii) upper bound of likely range of climate sensitivity of 4.5°C (red line at top of shaded area) (iii) lower bound of likely range of climate sensitivity of 2°C (blue line at bottom of shaded area). Black dashed lines in the left panel give the emissions range of recent baseline scenarios published since the SRES (2000). Emissions ranges of the stabilisation scenarios comprise CO2-only and multi gas scenarios and correspond to the 10th to 90th percentile of the full scenario distribution. Note: CO2 emissions in most models do not include emissions from decay of above ground biomass that remains after logging and deforestation, and from peat fires and drained peat soils. (IPCC Fourth Assessment Report, Climate Change 2007 (AR4), Synthesis report [chapter 2: Figure 5-1])
Natural efforts whereby plants absorb CO2 from the atmosphere to maintain the carbon cycle in the ecosystem may to some degree help lower the CO2 levels in the atmosphere. The IPCC Fourth Assessment Report, Climate Change 2007, suggests that by stabilizing the GHG emission in the atmosphere, global temperature increase should start to slow down as demonstrated in figure 3.
Infrared Gas Analyser
An infrared gas analyzer (sensor) is typically used to measure the quantities of various gases. The basic principle to an infrared gas analyzer involves two chambers (one chamber being the reference chamber and the other chamber allowing for measurement of the type of gas and quantity). Infrared light of a particular frequency is emitted from one end of the chamber through to a series of gas chambers that contain given concentrations of different gases. As the photons from the infrared source pass through the different gas chambers, they excite symmetric and asymmetric vibrations in the gas molecules (i.e., the gas of interest will absorb some of the infrared radiation passing through the gas chamber). The detector, being the end chamber to this sensor, is responsible for converting the amount of infrared radiation absorbed by the gas into a voltage (e.g., the signal from the detector [end chamber] will change in response to varying levels of CO2 in a given sample) (see figure 4).
Figure 4: Infrared greenhouse gas analyser. Image Credit: esrl.noaa.gov
Infrared gas analysers are standard detectors for the measurement of gas in any given environment. The accuracy of the detector is maintained by generating a constant signal known that the ‘zero’ point. This is based on the understanding that gas absorbs radiation in the same proportion. However, infrared energy absorption is proportional to the number of hydrocarbons present in a gas molecule, and with this analyser being the least sensitive to molecules with single bonds (i.e., CH4, a gas known to contribute to GHG emissions); it is, therefore, limited to the types of gases that can be monitored in the environment. Modern-day GHG sensors focus on satellite technology using light detectors and ranging methods to estimate gas emission levels in the atmosphere and have the advantage of monitoring emission levels over longer periods of time.