2017-18 EXAMINATIONS

PART 2 (Second Year)

LANCASTER ENVIRONMENT CENTRE Summer
LEC.279 Atmospheric Science (2 hours)


Answer two questions: one question from Section A and one question from Section B.

Use of a non-programmable calculator is permitted.

Each answer must be in a separate book.

Note that a formula sheet is provided at the end of this paper.



Page 2 of 4

Section A

1.
i) Calculate the volume of 500 g of dry air at the top of a mountain at 700 mb where
it is 5 °C. How does this change if the air is brought down to sea level at 1000 mb
where the temperature is 20 °C, and why? [20%]

ii) Explain what is meant by the term Potential Temperature. At what pressure in the
atmosphere does air with a potential temperature of 300 K have an actual
temperature of 0°C? [20%]

iii) What is the Inter-Tropical Convergence Zone (ITCZ), and what meteorological
phenomenon is dependent on its annual latitudinal migration? [20%]

iv) Calculate the change in temperature of an air mass that is forced to rise 50 mb to
pass over a mountain. You may assume that it follows an isentropic trajectory and
that the density of the air is 1.1 kg m-3. [20%]

v) Describe the conditions required for turbulence to occur in the atmosphere and list
three causes of turbulence in the planetary boundary layer. [20%]


2. Bromoform (CHBr3) is a naturally-emitted trace gas. It is destroyed in the troposphere
by reaction with OH radicals, Cl radicals and by photolysis in elementary reactions 1-3
below.
Reaction 1: CHBr3 + OH → CBr3 + H2O k1 = 2.7×10-13 cm3 molecule-1 s-1
Reaction 2: CHBr3 + Cl → CBr3 + HCl k2 = 2.8×10-13 cm3 molecule-1 s-1
Reaction 3: CHBr3 + hv → Br2 + Br + products j3= 8×10-7 s-1
k1 and k2 denote the second order rate constants at 298 K for Reactions 1 and 2,
respectively, and j3 is the CHBr3 photolysis frequency. Assume tropospheric [OH] =
1×106 molecules cm-3 and tropospheric [Cl] = 1.2×103 atoms cm-3.
i) Reactions 1 and 2 can be treated as pseudo-first order reactions. For Reaction 1, explain
why this is the case, and outline a route by which OH is produced in the troposphere.
[20%]

ii) Calculate the half-life (t1/2) of CHBr3, in days, with respect to photolysis. [20%]

iii) Calculate the CHBr3 lifetime (τ) with respect to each of the three loss processes and
the total CHBr3 lifetime (τtot). Express your answers in days and comment on the relative
importance of each CHBr3 sink. [40%]

iv) Write an expression describing the rate of H2O production from Reaction 1. Your
expression should take the form d[H2O]/dt =… [20%]


PLEASE TURN OVER
Page 3 of 4

Section B

3. Describe the structure and life-cycle of a mid-latitude depression of the type that affects
the UK. Your description should focus on:
i) The structure of a depression and its associated fronts and airstreams. [40%]

ii) The formation, development and decay of the system. [30%]

iii) The observational evidence available to confirm the structure that you have described.
[30%]

4. Answer all parts of this question, which are equally weighted.
i) Give three examples of trace species that are components of the Earth’s
troposphere: one that is very long-lived, one that is short-lived, and one that is very
short-lived. In each case, state the approximate atmospheric lifetime of the species
and its typical concentration (or range of concentrations) in the troposphere.

ii) Describe the main sources and sinks of each of the three trace species you have
named in part (ii), and explain briefly how these have been affected by human
activity.


iii) Describe the atmospheric distribution of each of the three trace species you have
named, and explain how this is affected by dynamical processes in the Earth’s
atmosphere. You should clearly distinguish between the effects of latitudinal,
longitudinal and vertical transport.












END OF PAPER
Page 4 of 4

Useful relations:
p RTρ=
g
z
p ρ−=
∂
∂
z
Hp p eo
−
=
dpdTCdQ p ρ
1
−=
pCR
p
pT
/
0






=θ
= 2 sinf φΩ
1
g
pU
f nρ
∂
=
∂

2
2 2
g
NzRi
u u
z z
θ
θ
∂ 
 ∂ = =
∂ ∂   
   ∂ ∂   

( )fQ
z
ζ θ
ρ
+ ∂
=
∂


Definition of symbols:
Cp = specific heat capacity of dry air,
1004 J K-1 kg-1
dp = change in pressure of air parcel, Pa
dQ = heat input/output to an air parcel,
J kg-1
dT = change in temperature of air parcel,
K
f = Coriolis parameter, also known as
planetary vorticity, s-1
g = gravitational acceleration, 9.81 m s-2
n = distance in direction at right-angles
to the isobars, m
N = Brunt-Väisälä frequency, s-1
p = pressure of air parcel, Pa or mbar
p0 = reference pressure to define
potential temperature (1000 mbar)
Q = Potential vorticity, K m2 kg-1 s-1, or,
in thermodynamics, heat content
(see the definition of dQ, above)
R = specific gas constant for dry air,
287 J K-1 kg-1
Ri = Richardson number
T = temperature of air parcel, K
T0 = 273.15 K
u = mean wind speed, m s-1
u* = friction velocity, m s-1
Ug = geostrophic wind, m s-1
z = height, m
z0 = roughness length, m
κ = von Karman’s constant = 0.4
ϕ = latitude, degrees North
θ = potential temperature, K
ρ = density of air, kg m-3
ζ = relative vorticity, s-1
Ω = Earth's angular velocity,
7.292x10−5 s−1






=
0
* ln
z
zuu
κ