Atmospheric Chemistry

1
Atmospheric Chemistry

• Lecture 4th year Environmental Science Students
• TU Bergakademie Freiberg
• Winter Term 2006/07
• Prof. Dr. Jörg Matschullat, Dr. Mark Jacob, V
  Mo
• NN Ü Fr

2
Atmospheric Chemistry

• Objective
• To understand the factors that control the
  concentrations of chemical species in the
  atmosphere
• Jacob (1999)
• Ozone hole in the stratosphere, ozone enrichments
  in the troposphere the anthropogenic greenhouse
  effect, aerosol forcing atmospheric deposition
  and acid rain major topics both of
  environmental concern and of utter relevance to
  the future of our planet. This lecture delivers
  the background for a deeper understanding and
  enables the participants to work in related
  fields.

3
Atmospheric Chemistry references

• Brasseur GP, Prinn RG, Pszenny AAP (eds)
  Atmospheric chemistry in a changing world. An
  integration and synthesis of a decade of
  tropospheric chemistry research. Springer 300
  pp.
• Brimblecombe P (1996) Air composition and
  chemistry. 2nd ed. Cambridge environmental
  chemistry series 6 253 pp.
• Jacob DJ (1999) Introduction to Atmospheric
  Chemistry. Princeton University Press, Princeton
  267 pp.
• Finlayson-Pitts BJ, Pitts JN Jr (1986)
  Atmospheric Chemistry fundamentals and
  experimental techniques. John Wiley Sons, New
  York 1098 pp.
• Graedel TE, Crutzen PJ (1994) Chemie der
  Atmosphäre. Bedeutung für Klima und Umwelt.
  Spektrum Akademischer Verlag, Heidelberg 511 S.
• Möller D (2003) Luft. Chemie, Physik, Biologie,
  Reinhaltung, Recht. Walter de Gruyter 750 S.
• Seinfeld JH, Pandis SN (1998) Atmospheric
  Chemistry and Physics. From air pollution to
  climate change. John Wiley Sons, New York 1326
  pp.

4
Atmospheric Chemistry web sites

• http//www.umweltbundesamt.net/immission/admin/Tes
  t/
• http//www.umwelt.sachsen.de/lfug/LuftOnline_neu/I
  ndex.html
• To be continued with your help and caution!

5
Atmospheric Chemistry time table

• 09.10.2006 Atmospheric composition T, P
  Thermodynamics
• 16.10.2006 Chemical kinetics (A.P.)
• 23.10.2006 Stratospheric chemistry
• 30.11.2006 Tropospheric chemistry
• 06.11.2006 Cloud chemistry, Multi-phase
  chemistry
• 13.11.2006 Tropospheric chemistry HCs, VOCs
• 20.11.2006 Aerosols
• 27.11.2006 Transport models (prepare for change)
• 04.12.2006 Vertical transport and atmospheric
  deposition
• 11.12.2006 N-species in the atmosphere
• 18.12.2006 Geochemical cycles S, C

Contents
08.01.2007 The greenhouse effect 15.01.2007
buffer day 22.01.2007 written exam 29.01.2007
evaluation
6
Atmospheric composition, pressure, T

• References
• Jacob DJ (1999) Introduction to Atmospheric
  Chemistry. 121
• Graedel TE, Crutzen PJ (1994) Chemie der
  Atmosphäre. 5978
• Seinfeld JH, Pandis SN (1998) Atmospheric
  Chemistry and Physics. 623

7
1. Atmospheric composition

• Thermosphere up to several 100 km high
  temperatures because of absorption of short
  wavelength by N2 and O2 rapid vertical mixing
  (the ionosphere with photoionization lies between
  Mesosphere and Thermosphere).
• Mesosphere from stratopause to 8090 km
  temperature decreases with altitude mesopause is
  the coldest part of the atmosphere rapid
  vertical mixing.
• Stratosphere from tropopause to 4555 km
  temperature increases with altitude slow
  vertical mixing.
• Troposphere 1015 km, depending on latitude and
  time of year decreasing temperature with height
  rapid vertical mixing.

8
Some useful numbers

• Atmospheric volume 4 x 1018 m3 (STP)
• Atmospheric mass 5 x 1021 g
• Earth surface area 5.1 x 1018 cm2
• Ocean area 3.6 x 1018 cm2
• Land area 1.5 x 1018 cm2
• Global rainfall 5 x 1020 cm3 a-1
• over oceans 4 x 1020 cm3 a-1
• over land 1 x 1020 cm3 a-1
• Global mean rainfall rate 100 cm a-1
• Scale height of the troposphere 6 km
• 1 m sec-1 2.2 miles hr-1
• 1 m sec-1 3.6 km hr-1
• 1 teragram (TG) 1012 g 106 metric tons 1 MT

9
Atmospheric composition
GWP global warming potential data after
Schönwiese (1994)
10
Atmospheric composition mixing ratio

• Mixing ratio Cx number of moles of X per mole of
  air (also mole fraction). Unit mol/mol. Also in
  units of volume of gas per volume of air (v/v),
  since the volume occupied by an ideal gas is
  proportional to the number of molecules (possible
  because atmospheric pressures are low).
  Advantage ratio remains constant with changing
  air density (after T or r changes) m.r. is a
  robust measure of atmospheric composition.
• The table lists m.r. of some major gases. The
  data are for dry air. Water vapour m.r. are
  highly variable (10-610-2 mol/mol).
• Mixing ratios of trace gases (everything but N2,
  O2, Ar, and H2O) are given in ppmv, ppbv, pptv.
  1 ppmv 10-6 mol/mol 1 ppbv 10-9 mol/mol.
  Present CO2 concentration is 370 ppmv.

Watch SI units! Use e.g., µmol mol-1 instead of
ppmv
11
Atmospheric composition number density

• Number density nX number of molecules of X per
  unit volume of air. Commonly expressed in numbers
  of molecules of X per cm-3 air. Critical for
  gas-phase reaction rates.
• Bimolecular gas-phase reaction X Y P Q
  The loss of X frequency of collisions between
  molecules X and Y, multiplied by the probability
  that a collision will result in a chemical
  reaction. The collision frequency is proportional
  to the product of number densities nXnY.

Concentrations must be expressed as number
densities when this reaction rate equation with k
as a rate constant is being used. Absorption or
scattering calculation of optically active gases
is another application (see figure at right).
12
Atmospheric composition partial pressure

• Partial pressure of a gas X in a mixture of gases
  under a total pressure P the pressure exerted
  by X if all other gases were removed. Daltons
  law states that PX relates to P through the
  mixing ratio CX
• PX CX P
• P total atmospheric pressure. The ideal gas law
  relates PX to nX
• PX nX/Av RT
• nX number density Av Avogadros number 6.022
  x 1023 molecules mol-1

13
Atmospheric composition partial pressure
Example PH2O to illustrate cloud formation, when
PH2O ³ PH2O, sat. Phase rule n independent
determining variables of c chemical components
between a number p of phases.Water vapour 1
component, 2 phases T dependence (see figure
below).
Relative humidity (RH), cloud formation occurs at
RH ³ 100
Dew point (Td), T at which air parcel is
saturated with liquid water
14
Atmospheric composition questions

• Calculate the number densities of air and CO2 at
  sea level for P 1013 hPa, T 0C
• In surface air over tropical oceans, the mixing
  ratio of water vapour can be as high as 0.03 mol
  mol-1. What is the molecular weight of this moist
  air?
• How many independent variables determine the
  liquid-vapour equilibrium of the H2O-NaCl system?
  What do you conclude regarding the ability of sea
  salt aerosol particles in the atmosphere to take
  up water?

15
2. Atmospheric pressure

• The atmospheric pressure is the weight exerted by
  the overhead atmosphere on a unit area of
  surface. p F/a, with p pressure, F force and
  a area
• Customary pressure units are the atmosphere (1
  atm 1.013 105 Pa), the bar (1 bar 105 Pa),
  the millibar (1 mb 100 Pa), and the torr (1
  torr 1 mm Hg 134 Pa). The SI unit is
  hectoPascals (hPa).
• Atmospheric mean pressure at sea level 1.013
  105 Pa 1013 hPa 1013 mb 1 atm 760 torr
• Global mean pressure at Earths surface PS 984
  hPa
• Total mass of atmosphere ma 4pR2PS/g 5.2 1018
  kg, with R 6400 km (Earths radius). Total
  number of moles of air in the atmosphere Na
  ma/Ma 1.8 1020 moles.

16
Air pressure and height
Up low pressure, low air density small gradient
within a given scale height interval. Down high
pressure, high air density, large pressure
decrease within a given scale height interval
17
Air pressure and temperature
Standard atmosphere _ _ _ in warm air ........ in
cold air
height (km)
Influence of air temperature on the pressure
gradient Warm air lower pressure Cold air
higher pressure
C
W
air pressure (mb)
18
Barometric law
dz thickness of atmosphere slab A area, z
altitude Pressure-gradient force the net force
of up- and downward pressures. Barometric law
P(z) P(0) exp(-Mag/RT z) or P(z) P(0)e-z/H
explains the exponential dependence of P on z.
Hydrostatic equilibrium calm air slab
hydrostatic pressure P within this slab amount
of air above this slab
19
Barometric law specials
Scale height H thickness of the theoretical
atmosphere consisting of an ideal gas and being a
homogenous atmosphere (density constant with
height) Isothermal atmosphere temperature
constant with height, resulting in exponential
pressure decrease with height (good approximation
for small atmospheric slabs) Non-isothermal
atmosphere most simple form of the barometric
law. Inverted hypsometric formula calculate a
height from T and P measurements Polytrope
atmosphere temperature decrease constant with
height (almost true for troposphere)
20
Sea-breeze circulation
The circulating cell usually extends ca. 10 km
horizontally across the coastline and ca. 1 km
vertically. At night, a reversed circulation can
be observed, the land breeze, since land cools
faster than water.
21
Atmospheric pressure questions

• Atmospheric CO2 concentrations have increased
  from 280 ppmv in pre-industrial times to 370 ppmv
  today. What is the corresponding increase in the
  mass of atmospheric carbon? Assume CO2 to be well
  mixed in the atmosphere.
• What fraction of total atmospheric mass at 30N
  is in the troposphere? In the stratosphere? Use
  data from figure _at_ right
• The cruising altitude of subsonic and supersonic
  aircraft are 12 and 20 km respectively. What is
  the relative difference in air density between
  these two altitudes?