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.