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Fundamentals of air Pollution

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This cycle causes the temperature in the stratosphere to increase with altitude. ... Odd nitrogen or 'NOx' is the sum of NO and NO2. ... – PowerPoint PPT presentation

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Title: Fundamentals of air Pollution


1
Fundamentals of air Pollution Atmospheric
Photochemistry Part B
  • Yaacov Mamane
  • Visiting Scientist
  • NCR, Rome
  • Dec 2006 - May 2007
  • CNR, Monterotondo, Italy

2
Stratospheric Ozone
  • Chapman Reactions (1931)
  • O2 hn ? 2O (1)
  • O O2 M ? O3 M (2)
  • O3 hn ? O2 O (3)
  • O O3 ? 2O2 (4)
  • Reactions (1) plus (2) produce ozone.
  • O2 hn ? 2O (1)
  • 2 x ( O O2 M ? O3 M ) (2)
  • 3 O2 hn ? 2 O3 NET

3
  • While Reactions (3) plus (4) destroy ozone.
  • O3 hn ? O2 O (3)
  • O O3 ? 2O2 (4)
  • 2O3 hn? 3 O2 NET
  • Reactions (3) plus (2) add up to a null cycle,
    but they are responsible for converting solar UV
    radiation into transnational kinetic energy and
    thus heat. This cycle causes the temperature in
    the stratosphere to increase with altitude. Thus
    is the stratosphere stratified.
  • O3 hn? O2 O (3)
  • O O2 M ? O3 M (2)
  • NULL NET
  • By way of quantitative analysis, we want O3ss
    and Oss and Oxss where Ox is defined as odd
    oxygen or O O3. The rate equations are as
    follows.

4
  • (a)
  • (b)
  • (ab)
  • From the representation for O atom chemistry
  • In the middle of the stratosphere, however, R3
    gtgt2 R1 and R2 gtgt R4 thus
  • (I)
  • (R4 can be ignored in an approximation of Oss
    ).
  • The ratio of O to O3 can also be useful

5
  • (II)
  • Reactions 2 and 3 set the ratio of O to O3, while
    Reactions 1 and 4 set the absolute
    concentrations. Now we will derive the steady
    state ozone concentration for the stratosphere.
    From the assumption that Ox is in steady state it
    follows that
  • R1 R4
  • or
  • j(O2)O2 k4OO3
  • Substituting from (I), the steady state O atom
    concentration
  • or

6
  • SAMPLE CALCULATION
  • At 30 km
  • This is almost a factor of ten above the true
    concentration! What is wrong? There must be
    ozone sinks missing.

7
  • Bates and Nicolet (1950)
  • Odd hydrogen HOx is the sum of OH and HO2
    (sometimes H and H2O2 are included as well).
  • HO2 O3 ? OH 2O2 (5)
  • OH O3 ? HO2 O2 (6)
  • 2O3 ? 3O2 NET
  • The following catalytic also destroys ozone.
  • OH O3 ? HO2 O2 (6)
  • HO2 O ? OH O2 (7)
  • O O3 ? 2O2 NET

8
  • Crutzen (1970) Johnston (1971) NOx
  • Odd nitrogen or NOx is the sum of NO and NO2.
    Often NOx is used as odd nitrogen which
    includes NO3, HNO3, 2 N2O5, HONO, PAN and other
    species. This total of odd nitrogen is better
    called NOy or total reactive nitrogen. N2
    and N2O are unreactive.
  • NO O3 ? NO2 O2
  • O NO2 ? NO O2
  • O O3 ? 2O2 NET
  • This is the major means of destruction of
    stratospheric ozone. The NOx cycle accounts for
    about 70 of the ozone loss at 30 km.

9
  • Stolarski Cicerone (1974) Wofsy McElroy
    (1974) ClOx
  • Cl O3 ? ClO O2
  • ClO O ? Cl O2
  • O O3 ? 2O2 NET
  • This reaction scheme is very fast, but there is
    not much ClOx in the stratosphere yet.
  • Today ClOx accounts for about 8 of the ozone
    loss at 30 km. If all these catalytic
    destruction cycles are added together, they are
    still insufficient to explain the present
    stratosphere O3 level.

10
Stratospheric ozone destruction cycles Stratospheric ozone destruction cycles Stratospheric ozone destruction cycles Stratospheric ozone destruction cycles
Cycle Sources Sinks Reservoirs
HOx H2O, CH4, H2 HNO3, H2SO4nH2O H2O, H2O2
NOx N2O O(¹D) HNO3 HO2NO2, ClONO2
ClOx CH3Cl, CFC HCl HCl, HOCl
The sinks involve downward transport to the
troposphere and rainout or other local loss.
Note that some sinks are also reservoirs HCl
OH ? H2O Cl
11
The Greenhouse Effect
12
SOLAR IRRADIANCE SPECTRA
1 ?m 1000 nm 10-6 m
  • Note 1 W 1 J s-1

13
TOTAL SOLAR RADIATION RECEIVED BY EARTH
  • Solar constant for earth 1368 W m-2
  • Solar radiation received outside atmosphere
  • per unit area of sphere
  • (1370) x (? re2)/(4 ? re2) 342 W m-2

14
EFFECTIVE TEMPERATURE OF EARTH
  • Effective temperature of earth (Te)
  • Temperature detected from space
  • Albedo of surfaceatmosphere 0.3
  • 30 of incoming solar energy is reflected by
    clouds, ice, etc.
  • Energy absorbed by surfaceatmosphere 1-0.3
    0.7
  • 70 of 342 W m-2 239.4 W m-2
  • Balanced by energy emitted by surfaceatmosphere
  • Stefan-Boltzman law Energy emitted ? Te4
  • ? 5.67 x 10-8 W m-2 K-4
  • Solve ? Te4 239.4
  • Te 255 K

15
GLOBAL TEMPERATURE
  • Annual and global average temperature 15 C,
    i.e. 288 K
  • Te 255 K --gt not representative of surface
    temp. of earth
  • Te is the effective temp. of the earth
    atmosphere system
  • that would be detected by an observer in space

16
ENERGY TRANSITIONS
  • Gas molecules absorb radiation by increasing
    internal energy
  • Internal energy ? electronic, vibrational,
    rotational states
  • Energy requirements
  • Electronic transitions
  • ? UV (lt 0.4 ?m)
  • Vibrational transitions
  • ? Near-IR (lt 0.7-20 ?m)
  • Rotational transitions
  • ? Far-IR (gt 20 ?m)
  • Little absorption in visible range (0.4-0.7 ?m)
  • Gap between electronic and vibrational
    transitions
  • Greenhouse gases absorb in the range 5-50 ?m
  • Vibrational and rotational transitions

17
GREENHOUSE GASES
  • Vibrational transitions must change dipole
    moment of molecule
  • Important greenhouse gases
  • H2O, CO2, CH4, N2O, O3, CFCs
  • Non-greenhouse gases
  • N2, O2, H2, Noble gases

18
ATMOSPHERIC ABSORPTION OF RADIATION
  • 100 absorption of UV
  • Electronic transitions of
  • O2 and O3
  • Weak absorption of visible
  • Gap in electronic and
  • vibrational transition energies
  • Efficient absorption of terrestrial radiation
  • Greenhouse gas absorption
  • Important role of H2O
  • Atmospheric window between 8 and 13 ?m

19
A SIMPLE GREENHOUSE MODEL
239.4 W m-2
(1-f)? To4
f? T14
absorbed f ? To4
f? T14
? To4
  • Incoming solar radiation 70 of 342 W m-2
    239.4 W m-2
  • IR flux from surface ? To4
  • Assume atmospheric layer has an absorption
    efficiency f
  • Kirchhoffs law efficiency of abs. efficiency
    of emission
  • IR flux from atmospheric layer f ? T14 (up and
    down)

20
RADIATION BALANCE EQUATIONS
239.4 W m-2
(1-f)? To4
f? T14
absorbed f ? To4
f? T14
? To4
  • Balance at top of atmosphere
  • f ? T14 (1-f) ? To4 239.4
  • Balance for atmospheric layer
  • f ? T14 f ? T14 f ? To4

21
THE GREENHOUSE EFFECT
239.4 W m-2
(1-f)? To4
f? T14
f? T14
absorbed f ? To4
? To4
  • To 288 K
  • f 0.77 T1 241 K
  • Greenhouse gases ? gases that affect f
  • As f increases, To and T1 increase

22
THE IPCC THIRD ASSESSMENT
23
CONCEPT OF RADIATIVE FORCING
239.4 W m-2
(1-f)? To4
f? T14
absorbed f ? To4
f? T14
? To4
  • Consider increase in concentration of a
    greenhouse gases
  • If nothing else changes
  • ? f increases ?outgoing terrestrial radiation
    decreases
  • Change in outgoing terrestrial radiation
    radiative forcing

24
RADIATIVE FORCING AND TEMPERATURE CHANGE
239.4 W m-2
(1-f)? To4
f? T14
absorbed f ? To4
f? T14
? To4
  • Response to imbalance
  • To and T1 increase ? may cause other
    greenhouse gases to
  • change ? f ? (positive feedback) or ?
    (negative feedback)
  • ?To and T1 may ? or ? ? ?f ? ?T ? ? Rad.
    balance
  • Radiative forcing is measure of initial change
    in outgoing flux

25
RADIATIVE FORCING
  • Permits assessment of potential climate effects
    of
  • different gases
  • Radiative forcing of a gas depends not only on
    change in
  • concentration, but also what wavelengths it
    absorbs
  • Aerosols can exert a negative radiative effect
    (i.e. have a
  • cooling effect) by reflecting radiation (direct
    effect) and
  • by increasing reflectivity of clouds (indirect
    effect)

26
GLOBAL WARMING POTENTIAL
  • Index used to quant.
  • compare radiative forcings
  • of various gases
  • Takes into account lifetimes,
  • saturation of absorption

27
FORCINGS AND SURFACE TEMPERATURE
  • Climate sensitvity parameter (?) ?To ? ?F
  • Global climate models ? ? 0.3-1.4 K m2 W-1

28
THE TEMPERATURE RECORD
29
RECENT CHANGES IN SURFACE TEMPERATURE
  • Trend differences due to
  • differences in spatial av.,
  • diff. in sea-surface temps.,
  • and handling of urbanization
  • Same basic trend over last
  • 100 years
  • Increase in T by 0.6-0.7 C

30
POTENTIAL CAUSES OF TEMPERATURE CHANGES
239.4 W m-2
absorbed f ? To4
  • Variations in solar radiation at top of
    atmosphere
  • Changes in albedo (e.g. due to changes in cloud
    cover)
  • Changes in greenhouse gas forcing (i.e., change
    in f)

31
SOLAR VARIABILITY
  • Changes in sunspots and surface conditions

32
CHANGES IN CLOUD COVER
  • Incoming solar radiation 0.7 x 342 W m-2
    239.4 W m-2
  • Consider albedo change of 2.5
  • Albedo 0.3 x 1.025 0.3075
  • Incoming solar radiation 0.6925 x 342 W m-2
    236.8 W m-2
  • Radiative forcing 236.8 239.4 - 2.6 W m-2
  • ? Comparable but opposite to greenhouse gas
    forcing
  • Clouds are also efficient absorbers of
    terrestrial radiation
  • ? Positive forcing
  • Cloud effects are larege source of uncertainty
    in climate
  • projections

33
MODEL SIMULATIONS OF RECENT PAST
34
CLIMATE PROJECTIONS
35
POTENTIAL IMPACTS
36
JULY HEAT INDEX FOR South-East U.S.
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