CHEM 438 – Atmospheric Chemistry

Mid-Term 1 – Take-home

Each question is worth 10 points.

Due 10/19/17

1. The predominantly filled CO2 Martian atmosphere has a surface pressure of

600 Pa and surface temperature of -55 °C.

a. Given the collision cross section of CO2 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 O3 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 CO2 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:

NO2(g) + CO(g) à NO(g) + CO2(g)

Experimentally, the rate law for the reaction is second order in NO2(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

cm3 of air (80% N2 and 20% O2 by number) at 1 atm and 298 K.