Many of the concepts developed in this chapter can be applied to understanding the atmosphere. Because atmospheric air is comprised primarily of N₂ (roughly 78% by volume), approximate the atmosphere as consisting only of N₂ in answering the following questions:

a. What is the single-particle collisional frequency at sea level, with T = 298 K and P = 1.0 atm? The corresponding single-particle collisional frequency is reported as 10¹⁰ s^–1 in the CRC Handbook of Chemistry and Physics (62nd ed., p. F-171).

b. At the tropopause (11 km in altitude), the collisional frequency decreases to 3.16 × 10⁹ s^–1, primarily due to a reduction in temperature and barometric pressure (i.e., fewer particles). The temperature at the tropopause is ~220 K. What is the pressure of N₂ at this altitude?

c. At the tropopause, what is the mean free path for N₂? 

The collisional cross section of N₂ is σ = 4.3 × 10^–19 m² (see Table 33.1) and M = 0.0280 kg mol^–1.