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

Stratospheric Ozone Chapman Reactions (1931) O ₂ + h → 2O(1) O + O ₂ + M → O ₃ + M(2) O ₃ + h → O ₂ + O(3) O + O ₃ → 2O ₂ (4) Reactions (1) plus (2) produce ozone. O ₂ + h → 2O(1) 2 x ( O + O ₂ + M → O ₃ + M )(2) 3 O ₂ + h  → 2 O ₃ NET

While Reactions (3) plus (4) destroy ozone. O ₃ + h → O ₂ + O (3) O + O ₃ → 2O ₂ (4) 2O ₃ + h → 3 O ₂ 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. O ₃ + h → O ₂ + O (3) O + O ₂ + M → O ₃ + M*(2) NULLNET By way of quantitative analysis, we want [O ₃ ]ss and [O]ss and [Ox]ss where “Ox” is defined as odd oxygen or O + O ₃. The rate equations are as follows.

(a) (b) (a+b) From the representation for O atom chemistry: In the middle of the stratosphere, however, R ₃ >>2 R ₁ and R ₂ >> R ₄ thus: (I) (R ₄ can be ignored in an approximation of [O]ss ). The ratio of [O] to [O ₃ ] can also be useful:

(II) Reactions 2 and 3 set the ratio of O to O ₃, 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: R ₁ = R ₄ or j(O ₂ )[O ₂ ] = k ₄ [O][O ₃ ] Substituting from (I), the steady state O atom concentration: or

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.

Bates and Nicolet (1950) Odd hydrogen “HOx” is the sum of OH and HO ₂ (sometimes H and H ₂ O ₂ are included as well). HO ₂ + O ₃ → OH + 2O ₂ (5) OH + O ₃ → HO ₂ + O ₂ (6) 2O ₃ → 3O ₂ NET The following catalytic also destroys ozone. OH + O ₃ → HO ₂ + O ₂ (6) HO ₂ + O → OH + O ₂ (7) O + O ₃ → 2O ₂ NET

Crutzen (1970); Johnston (1971) “NOx” Odd nitrogen or “NOx” is the sum of NO and NO ₂. Often “NOx” is used as “odd nitrogen” which includes NO ₃, HNO ₃, 2 N ₂ O ₅, HONO, PAN and other species. This total of “odd nitrogen” is better called “NOy” or “total reactive nitrogen.” N ₂ and N ₂ O are unreactive. NO + O ₃ → NO ₂ + O ₂ O + NO ₂ → NO + O ₂ O + O ₃ → 2O ₂ 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 + O ₃ → ClO + O ₂ ClO + O → Cl + O ₂ O + O ₃ → 2O ₂ 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 O ₃ level.

Stratospheric ozone destruction cycles CycleSourcesSinksReservoirs HOx H ₂ O, CH ₄, H ₂ HNO ₃, H ₂ SO ₄ nH ₂ O H ₂ O, H ₂ O ₂ NOx N ₂ O + O(¹D)HNO ₃ HO ₂ NO ₂, ClONO ₂ ClOx CH ₃ Cl, CFC HClHCl, HOCl The sinks involve downward transport to the troposphere and rainout or other local loss. Note that some sinks are also reservoirs: HCl + OH → H ₂ O + Cl

The Greenhouse Effect

SOLAR IRRADIANCE SPECTRA 1  m = 1000 nm = m Note: 1 W = 1 J s -1

Solar radiation received outside atmosphere per unit area of sphere = (1370) x (  r e 2 )/(4  r e 2 ) = 342 W m -2 TOTAL SOLAR RADIATION RECEIVED BY EARTH Solar constant for earth: 1368 W m -2

EFFECTIVE TEMPERATURE OF EARTH Effective temperature of earth (T e ) Temperature detected from space Albedo of surface+atmosphere ~ % of incoming solar energy is reflected by clouds, ice, etc. Energy absorbed by surface+atmosphere = = % of 342 W m -2 = W m -2 Balanced by energy emitted by surface+atmosphere Stefan-Boltzman law: Energy emitted =  T e 4  = 5.67 x W m -2 K -4 Solve  T e 4 = T e = 255 K

GLOBAL TEMPERATURE Annual and global average temperature ~ 15 C, i.e. 288 K T e = 255 K --> not representative of surface temp. of earth T e 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 (<  m) Rotational transitions  Far-IR (> 20  m) Little absorption in visible range (  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 H 2 O, CO 2, CH 4, N 2 O, O 3, CFCs Non-greenhouse gases N 2, O 2, H 2, Noble gases

ATMOSPHERIC ABSORPTION OF RADIATION ~100% absorption of UV Electronic transitions of O 2 and O 3 Weak absorption of visible Gap in electronic and vibrational transition energies Efficient absorption of terrestrial radiation Greenhouse gas absorption Important role of H 2 O Atmospheric window between 8 and 13  m

A SIMPLE GREENHOUSE MODEL Incoming solar radiation = 70% of 342 W m -2 = W m -2 IR flux from surface =  T o 4 Assume atmospheric layer has an absorption efficiency = f Kirchhoff’s law: efficiency of abs. = efficiency of emission IR flux from atmospheric layer = f  T 1 4 (up and down) W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4

RADIATION BALANCE EQUATIONS W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 Balance at top of atmosphere f  T (1-f)  T o 4 = Balance for atmospheric layer f  T f  T 1 4 = f  T o 4

THE GREENHOUSE EFFECT W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 T o = 288 K f = 0.77; T 1 = 241 K Greenhouse gases  gases that affect f As f increases, T o and T 1 increase

THE IPCC THIRD ASSESSMENT

CONCEPT OF RADIATIVE FORCING W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 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 W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 Response to imbalance T o and T 1 increase  may cause other greenhouse gases to change  f  (positive feedback) or  (negative feedback)  T o and T 1 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 ( ):  T o =  F Global climate models  = K m 2 W -1

THE 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 C RECENT CHANGES IN SURFACE TEMPERATURE

POTENTIAL 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) W m -2 absorbed = f  T o 4

SOLAR VARIABILITY Changes in sunspots and surface conditions

CHANGES IN CLOUD COVER Incoming solar radiation = 0.7 x 342 W m -2 = W m -2 Consider albedo change of 2.5% Albedo = 0.3 x = Incoming solar radiation = x 342 W m -2 = W m -2 Radiative forcing = – = 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.