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

Fundamentals of air Fundamentals of air Pollution – Atmospheric Pollution – Atmospheric Photochemistry – Part BPhotochemistry – Part B

Yaacov MamaneYaacov Mamane

Visiting ScientistVisiting ScientistNCR, RomeNCR, Rome

Dec 2006 - May 2007Dec 2006 - May 2007CNR, Monterotondo, ItalyCNR, Monterotondo, Italy

Stratospheric OzoneStratospheric Ozone

Chapman Reactions (1931)

O₂ + h → 2O (1)

O + O₂ + M → O₃ + M (2)

O₃ + h → O₂ + O (3)

O + O₃ → 2O₂ (4)

Reactions (1) plus (2) produce ozone.

O₂ + h → 2O (1)

2 x ( O + O₂ + M → O₃ + M ) (2)

3 O₂ + h→ 2 O₃ NET

While Reactions (3) plus (4) destroy ozone.

O₃ + h → O₂ + O (3)

O + O₃ → 2O₂ (4)

2O₃ + h→ 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 + h→ 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.

(a)

(b)

(a+b)

From the representation for O atom chemistry:

In the middle of the stratosphere, however, R₃ >>2 R₁ and R₂ >> R₄ thus:

(I)

(R₄ can be ignored in an approximation of [O]ss ).

The ratio of [O] to [O₃] can also be useful:

413

4321

4323

22/][/][

2/][

/][

RRdtOxddtOOd

RRRRdtOd

RRRdtOd

][]][[

])[(2])[(][

3422

2233

OkMOk

OOjOOjO SS

]][[

])[(][

22

33

MOk

OOjO SS

(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:

or

]][[

)(

][

][

22

3

3 MOk

Oj

O

O

SS

SS

]][[

])[(])[(

22

2334

22 MOk

OOjkOOj

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.

)(

][])[(][

34

22

223 Ojk

MkOOjO SS

ppmSS

O

scmk

scmk

sOj

sOj

30]3

[

101

105.4

101)(

106)(

13154

16342

133

1112

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

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

CycleSourcesSinksReservoirs

HOxH₂O, CH₄, H₂HNO₃, H₂SO₄nH₂O

H₂O, H₂O₂

NOxN₂O + O(¹D)HNO₃HO₂NO₂, ClONO₂

ClOxCH₃Cl, CFCHClHCl, 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 EffectThe Greenhouse Effect

SOLAR IRRADIANCE SPECTRASOLAR IRRADIANCE SPECTRA

1 m = 1000 nm = 10-6 m

• Note: 1 W = 1 J s-1

• Solar radiation received outside atmosphere per unit area of sphere = (1370) x ( re

2)/(4 re2) = 342 W m-2

TOTAL SOLAR RADIATION RECEIVED BY EARTHTOTAL SOLAR RADIATION RECEIVED BY EARTH

• Solar constant for earth: 1368 W m-2

EFFECTIVE TEMPERATURE OF EARTHEFFECTIVE 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 TEMPERATUREGLOBAL 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 TRANSITIONSENERGY 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 GASESGREENHOUSE 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 RADIATIONATMOSPHERIC 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 MODELA SIMPLE GREENHOUSE MODEL

• 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)

239.4 W m-2

absorbed= f To

4 To

4

(1-f) To4

f T14

f T14

RADIATION BALANCE EQUATIONSRADIATION BALANCE EQUATIONS

239.4 W m-2

absorbed= f To

4 To

4

(1-f) To4

f T14

f T14

• Balance at top of atmosphere

f T14 + (1-f) To

4 = 239.4

• Balance for atmospheric layer

f T14 + f T1

4 = f To4

THE GREENHOUSE EFFECTTHE GREENHOUSE EFFECT

239.4 W m-2

absorbed= f To

4 To

4

(1-f) To4

f T14

f T14

• 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 ASSESSMENTTHE IPCC THIRD ASSESSMENT

CONCEPT OF RADIATIVE FORCINGCONCEPT OF RADIATIVE FORCING

239.4 W m-2

absorbed= f To

4 To

4

(1-f) To4

f T14

f T14

• 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 CHANGERADIATIVE FORCING AND TEMPERATURE CHANGE

239.4 W m-2

absorbed= f To

4 To

4

(1-f) To4

f T14

f T14

• 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 FORCINGRADIATIVE 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 POTENTIALGLOBAL WARMING POTENTIAL

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

FORCINGS AND SURFACE TEMPERATUREFORCINGS AND SURFACE TEMPERATURE

• Climate sensitvity parameter (): To = F

• Global climate models = 0.3-1.4 K m2 W-1

THE TEMPERATURE RECORDTHE TEMPERATURE RECORD

• 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

RECENT CHANGES IN SURFACE TEMPERATURERECENT CHANGES IN SURFACE TEMPERATURE

POTENTIAL CAUSES OF TEMPERATURE CHANGESPOTENTIAL CAUSES OF TEMPERATURE CHANGES

• 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)

239.4 W m-2

absorbed= f To

4

SOLAR VARIABILITYSOLAR VARIABILITY

• Changes in sunspots and surface conditions

CHANGES IN CLOUD COVERCHANGES 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 PASTMODEL SIMULATIONS OF RECENT PAST

CLIMATE PROJECTIONSCLIMATE PROJECTIONS

POTENTIAL IMPACTSPOTENTIAL IMPACTS

JULY HEAT INDEX FOR South-East U.S.JULY HEAT INDEX FOR South-East U.S.