## CHEM 438 – Atmospheric Chemistry

25 Oct

CHEM 438 – Atmospheric Chemistry
Mid-Term 1 – Take-home
Each question is worth 10 points.
Due 10/19/17
1. The predominantly filled CO
2 Martian atmosphere has a surface pressure of
600 Pa and surface temperature of -55 °C.
a. Given the collision cross section of CO
2 is 0.318 nm2 calculate the
collision frequency and mean free path of gas molecules at zero
altitude.
b. Given the gravitational constant for Mars of 3.8 m s
-2, calculate the
scale height of Mars
c. Given that at an altitude of 50 km the atmosphere has a temperature
of 160 K, calculate the number density of the atmosphere.
2. Odin is a Earth orbiting satellite mission jointly run by Sweden, Canada,
Finland and France. On board is OSIRIS (Optical Spectrograph and InfraRed
Imager System). The figure below is data returned from OSIRIS that
measures ozone abundances in the atmosphere.
a. Calculate the mixing ratio of O
3 at the peak of the O3 layer. Use lecture
notes to estimate typical pressure and temperature conditions at this
altitude.
b. Using the figure below,
estimate the total ozone column number
density.
c. How many Dobson units of ozone is in this column of air?

3. a) Using the barometric law, calculate at what altitude the Earth’s pressure is
1% of the surface air pressure.
b) If Earth’s atmosphere was primarily made of CO
2 instead of N2/O2, at what
altitude would it reach 1% of the surface air pressure?
4. In the troposphere, carbon monoxide and nitrogen dioxide undergo the
following reaction:
NO
2(g) + CO(g) à NO(g) + CO2(g)
Experimentally, the rate law for the reaction is second order in NO
2(g), zeroorder in CO(g), and NO3(g) has been identified as an intermediate in the
reaction. The rate of the reaction increases with temperature. Construct a
reaction mechanism that is consistent with these experimental observations.
Explain why your mechanism fits with the observations.

 5. Consider two parallel reactions: Aà B k = 0.005 s-1 Aà C k = 0.022 s-1

Determine:
a) The relative amounts of B and C at equilibrium
b) The half life of A
c) If the initial concentration of A is 0.78 M, what are the concentrations of A,
B and C after 100 seconds?
6. Calculate the total number of molecular collisions that occur per second in 1
cm
3 of air (80% N2 and 20% O2 by number) at 1 atm and 298 K.