Chapter 18 Flashcards
The atmosphere consists of the …
troposphere (up to 10 km), stratosphere (10 – 50 km), and some other “spheres”
Commercial jets fly at 10 km altitude
Combined mass of troposphere and stratosphere is…
99.9 % of the mass of atmosphere
Within the troposphere as altitude increases…
temperature drops down to -50oC at 10 km altitude
As altitude increaes, pressure…
decreases. Pressure drops to 0.26 atm at 10 km and to 0.001 torr at 100 km
Near sea level, the majority of the dry air is…
4:1 N2 and O2 (99% by volume)
Other gases (the remaining 1%) are noble gases, CO2 and CH4:
Argon 0.9% (9000 ppm)
CO2 0.04% (400 ppm)
Neon 0.002% (20 ppm)
Helium 0.0005% (5 ppm)
CH4 0.0002% (2 ppm)
Other gases present in polluted air…
CO: 1 - 50 ppm in urban areas, from incomplete combustion of fuels
NOx: 0.2 ppm in smog
O3: 0.5 ppm in photochemical smog
SO2: 0.1 – 2 ppm in urban areas
Nitrogen is largely…
unreactive due to the bond energy associated with breaking the N ≡ N.
Oxygen has a…
much lower bond enthalpy than nitrogen and is therefore more reactive.
- oxides of nonmetals are…
- oxides of (active) metals are…
- are acidic (Lewis acids)
- are basic
“Outer defense” is
Earth against radiation and high-energy particles occurs beyond the stratosphere and in the stratosphere . More than 50% of harmful radiation is removed before it reaches troposphere
Defensive reaction in the atmosphere
Earth is…
constantly bombarded by high energy radiation (mainly UV) and particles
Atmosphere removes the harmful radiation by…
using it up for photodecomposition of various gases.
Photodissociation is…
rupture of a bond due to absorption of a photon.
Usually radicals (having unpaired electrons) are formed, which are highly reactive.
The most important reaction in the upper atmosphere is…
the photodissociation of oxygen molecules to oxygen atoms. This reaction effectively “removes” from the incoming radiation UV light at wavelengths below 240 nm.
Photodissociation - Earth defense against harmful radiation
Ozone forms from…
the reaction of O2 with O atoms produced in the upper atmosphere by photodissociation of O2 .
The ozone layer occurs…
in the stratosphere, at an altitude of about 25 km.
Bond order in O3 is 1.5…
so bond energy of O3 is lower than that of O2
Ozone absorbs…
much of the radiation between 240 and 310 nm (longer wavelength than O2, protecting us from UV radiation longer than 240 nm).
O2 absorbs shorter UV light (less than 242 nm)
Absorption of radiation leads to…
photochemical decomposition of ozone: O3 + light —> O2 + O
CFCs were used for years as…
aerosol propellants and refrigerants.
The overall result of ozone formation and…
removal reactions throughout billons of years led to the so-called steady state of ozone presence in the stratosphere.
CFCs are not…
not water soluble (do not get washed out of the atmosphere by rain) and are quite unreactive (not degraded naturally), so that eventually they can diffuse to the stratosphere.
In the stratosphere, where the ozone layer exists, the C—Cl bond in CFCs is…
easily broken by light with a wavelength between 190 and 225 nm.
CFCs can reach the stratosphere, b/c at lower altitudes there is not enough of UV radiation present to decompose them.
Chlorine radicals made by photodecomposition of CFCs are..
very reactive. They react with ozone, as shown below:
Cl + O3 —> ClO + O2
ClO is a very…
unstable radical, that can decompose photochemically
Review slide 13; 18-1
Review slide 13; 18-1
The catalyzed decomposition of O3 by Cl radicals from CFCs was theoretically predicted in…
1970s. It was observed experimentally in late 1970s.
In 1987 and 1992 most nations signed the…
Montreal Protocol, in which they agreed to reduce (and eventually stop) production of CFCs. Nobel Prize was awarded to Rowland and Molina from U.C. Riverside in 1995.
As the result of the Montreal Protocol, the Ozone Hole began to…
decrease in size. However, it will take at least 50 years for ozone level in stratosphere to come back to the level from 1970s.