Transcript Slide 1
Fundamentals of air
Pollution – Atmospheric
Photochemistry – Part B
Yaacov Mamane
Visiting Scientist
NCR, Rome
Dec 2006 - May 2007
CNR, Monterotondo, Italy
Stratospheric Ozone
Chapman Reactions (1931)
O₂ + hn → 2O
O + O₂ + M → O₃ + M
O₃ + hn → O₂ + O
O + O₃ → 2O₂
Reactions (1) plus (2) produce ozone.
O₂ + hn → 2O
2 x ( O + O₂ + M → O₃ + M )
3 O₂ + hn → 2 O₃ NET
(1)
(2)
(3)
(4)
(1)
(2)
While Reactions (3) plus (4) destroy ozone.
O₃ + hn → O₂ + O
(3)
O + O₃ → 2O₂
2O₃ + hn→ 3 O₂
(4)
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 [O₃]ss and [O]ss and [Ox]ss
where “Ox” is defined as odd oxygen or O + O₃. The rate equations
are as follows.
d [O3 ] / dt R2 R3 R4
d [O] / dt 2 R1 R2 R3 R4
d [O3 O] / dt d [Ox] / dt 2 R1 2 R4
(a)
(b)
(a+b)
From the representation for O atom chemistry:
[O]SS
j (O3 )[O3 ] 2 j (O2 )[O2 ]
k2 [O2 ][M ] k4 [O3 ]
In the middle of the stratosphere, however, R₃ >>2 R₁ and R₂ >> R₄ thus:
[O]SS
j (O3 )[O3 ]
k 2 [O2 ][M ]
(R₄ can be ignored in an approximation of [O]ss ).
The ratio of [O] to [O₃] can also be useful:
(I)
[O]SS
j (O3 )
[O3 ]SS k2 [O2 ][M ]
(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] = k₄[O][O₃]
Substituting from (I), the steady state O atom concentration:
k4 j (O3 )[O3 ]2
j (O2 )[O2 ]
k2 [O2 ][M ]
or
[O3 ]SS
j (O2 )[ O2 ]2 k 2 [ M ]
k 4 j (O3 )
SAMPLE CALCULATION
At 30 km
j (O2 ) 6 1011 s 1
j (O3 ) 1 103 s 1
k 2 4.5 1034 cm6 s 1
k 4 1 1015 cm3 s 1
[O ]
30 ppm
3 SS
This is almost a factor of ten above the true concentration! What is
wrong? There must be ozone sinks missing.
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
OH + O₃ → HO₂ + O₂
2O₃ → 3O₂
The following catalytic also destroys ozone.
OH + O₃ → HO₂ + O₂
HO₂ + O → OH + O₂
O + O₃ → 2O₂
(5)
(6)
NET
(6)
(7)
NET
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.
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.
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
The Greenhouse Effect
SOLAR IRRADIANCE SPECTRA
1 m = 1000 nm = 10-6 m
• Note: 1 W = 1 J s-1
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
EFFECTIVE TEMPERATURE OF EARTH
• Effective temperature of earth (Te)
Temperature detected from space
• Albedo of surface+atmosphere ~ 0.3
30% of incoming solar energy is reflected by clouds, ice, etc.
• Energy absorbed by surface+atmosphere = 1-0.3 = 0.7
70% of 342 W m-2 = 239.4 W m-2
• Balanced by energy emitted by surface+atmosphere
Stefan-Boltzman law: Energy emitted = Te4
= 5.67 x 10-8 W m-2 K-4
• Solve Te4 = 239.4
Te = 255 K
GLOBAL TEMPERATURE
• Annual and global average temperature ~ 15 C, i.e. 288 K
• Te = 255 K --> 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
ENERGY TRANSITIONS
• Gas molecules absorb radiation by increasing internal energy
Internal energy electronic, vibrational, & rotational states
• Energy requirements
Electronic transitions
UV (< 0.4 m)
Vibrational transitions
Near-IR (< 0.7-20 m)
Rotational transitions
Far-IR (> 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
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
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
A SIMPLE GREENHOUSE MODEL
239.4 W m-2
f T14
(1-f) To4
f T14
To4
absorbed
= f 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
• Kirchhoff’s law: efficiency of abs. = efficiency of emission
• IR flux from atmospheric layer = f T14 (up and down)
RADIATION BALANCE EQUATIONS
239.4 W m-2
f T14
(1-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
absorbed
= f To4
THE GREENHOUSE EFFECT
239.4 W m-2
f T14
(1-f) To4
f T14
To4
absorbed
= f To4
• To = 288 K
f = 0.77; T1 = 241 K
• Greenhouse gases gases that affect f
As f increases, To and T1 increase
THE IPCC THIRD ASSESSMENT
CONCEPT OF RADIATIVE FORCING
239.4 W m-2
f T14
(1-f) To4
f T14
To4
absorbed
= f 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
RADIATIVE FORCING AND TEMPERATURE CHANGE
239.4 W m-2
f T14
(1-f) To4
f T14
To4
absorbed
= f 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
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)
GLOBAL WARMING POTENTIAL
• Index used to quant.
compare radiative forcings
of various gases
• Takes into account lifetimes,
saturation of absorption
FORCINGS AND SURFACE TEMPERATURE
• Climate sensitvity parameter (): To = F
• Global climate models = 0.3-1.4 K m2 W-1
THE TEMPERATURE RECORD
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
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)
SOLAR VARIABILITY
• Changes in sunspots and surface conditions
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
MODEL SIMULATIONS OF RECENT PAST
CLIMATE PROJECTIONS
POTENTIAL IMPACTS
JULY HEAT INDEX FOR South-East U.S.