L2 - Impact of Human Activities on the Earth’s Atmosphere Flashcards
List and briefly state the characteristics of Earth’s four, thermal (atmospheric) layers.
Troposphere: 80% mass of atmosphere. Strong infrared heating. Temperature decrease ~6.5ºC km-1.
Stratosphere: Very dry. Heating via ultraviolet absorption by ozone defines stratosphere.
Mesosphere: Cool region. Temperature decreases with height.
Thermosphere: temperature increases with height due to UV/X-ray photodissociation and photoionization of N2 and O2.
Explain what Earth’s primordial atmosphere was like and why it changed radically.
Earth’s earliest atmosphere rich in CO2 and H2O. CO2 fractional concentration 103 times present value, i.e. ~38%. Minor amounts of N2 and sulphates.
Most atmospheric H2O condensed to form primordial oceans. Most atmospheric CO2 dissolved in oceans. Some dissolved CO2 combined with metal (Ca, Mg) ions to form carbonate minerals such as CaCO3. Inert atmospheric N2 became enriched over geological time periods. O2 came from plants (photosynthesis) and cyanobacteria.
Explain the underlying physics of Earth’s greenhouse effect and the role of anthropogenic gases (CO2, CH4, etc.).
A garden greenhouse transmits sunlight, which heats the ground. Heated ground radiates IR radiation, some of which is reflected or re-radiated (as thermal radiation) by the glass and H2O, CO2, CH4. Result = net heating effect. Earth’s atmosphere is quite transparent to incoming, visible radiation and opaque to outgoing (IR) radiation on account of absorbent, greenhouse gases (H2O, CO2, CH4, N2O, halocarbons, etc.), cloud droplets and aerosols.
Explain why some atmospheric gases are greenhouse gases while others are not.
Molecules oscillate via their bonds. Diatomic molecules, e.g. CO, possess a dipole moment if their electrons are unevenly distributed. The size of the dipole moment depends on the size of the partial charges (δ+, δ-), the atoms, and on the distance between them. Generally, heteronuclear, diatomic molecules (like CO) are IR active while homonuclear diatomic atoms (like N2) are not.
In the case of polyatomic molecules such as H2O, the two bond dipoles cancel each other out. BUT the bending and stretching modes result in different dipole modes. Hence, the overall H2O molecule has an electric dipole moment and is IR active. Being IR active means that molecules – such as the greenhouse gases – can absorb or emit photons in the infrared part of the spectrum. Tropospheric H2O, for example, thermal (IR) radiation from the Sun heated surface of the Earth. This gives the H2O more thermal energy (they become warmer).
Give one example of paleo-climatic (ancient climate) data, which shows the dramatic increase in anthropogenic gases in Earth’s atmosphere.
Paleoclimatic data from various sources, e.g. ice cores in Greenland and Antarctica down to 3 km. Ice contains ancient air bubbles with trace gases (CO2, CH4, etc.). Analyses of bubbles shows atmospheric concentration of CH4 was constant until 1700s but has increased dramatically in the last century.
Explain the origins and negative effects of acid rain and photochemical smog.
Causes: Reactions of industrial emission gases, NOx and SO2. Reactions of NOx and hydrocarbons from biomass burning. NOx and SO2converted to HNO3 and H2SO4 via a chain of reactions, e.g. involving OH (hydroxyl radical).
Fresh water ecosystems and biodiversity:
• Low pH values toxic to fish eggs especially, which impacts food-chain, e.g. birds.
• Low pH leaches soil of nutrients/minerals (e.g. Mg, K, Ca) while concentrating toxic Al.
• Some microbes in soils cannot survive acidic conditions.
• Toxic Al finds its way into local lakes and rivers.
Forests:
• Soils leached of nutrients and enriched in toxic Al.
• Acids strip waxy coating of leaves, inhibiting photosynthesis.
• Above effects make trees vulnerable to disease and exposure.
Other effects:
• Acidic damage to ornamental stones and carvings, e.g. CaCO3 (limestone, marble).
Explain the underlying physics and chemistry relating to the ozone hole over Antarctica and why ozone destruction is so intensive during (Antarctic) spring.
Over Antarctica, there’s a polar vortex much of the year, i.e. a cyclone system isolating frigid air.
During the sunless winter at 15-25 km (<80 °C), rare polar stratospheric clouds (PSCs) form
PSCs composed of tiny crystals of H2O, NOx, HCl, ClNO3, etc.
Crystal surfaces are the locations where CFC residues react catalytically to produce more active forms Cl, which accumulates during the winter.
Antarctic Spring brings sunlight (UV photons), which photodissociate molecular Cl generating a large reservoir of Cl atoms.
Reactive Cl now available to complete the twostep, catalytic reaction:
- Cl + O3 → ClO + O2
- ClO + O → Cl + O2
