The chemistry of the troposphere and stratosphere Prof. M. J. Pilling The University of Leeds (UK)...

The chemistry of the troposphere and stratosphere Prof. M. J. Pilling The University of Leeds (UK) helyett Turnyi Tams Eredeti Pilling eladsok: 2009. februr 23-27 A PowerPoint file-ok s a videofelvtelek letlthetk: http://garfield.chem.elte.hu/Turanyi/oktatas/Pilling.html

Structure of the atmosphere

Temperature and pressure variations in the atmosphere Heating by exothermic photochemical reactions Convective heating from surface. Absorption of IR (and some VIS-UV) radiation Barometric equation p = p 0 exp(-z/H s ) z

Atmospheric transport Random motion mixing Molecular diffusion is slow, diffusion coefficient D ~ 2x10 -5 m 2 s -1 Average distance travelled in one dimension in time t is ~ (2Dt). In the troposphere, eddy diffusion is more important: K z ~ 20 m 2 s -1. Molecular diffusion more important at v high altitudes, low p. Takes ~ month for vertical mixing (~10 km). Implications for short and long-lived species. Directed motion Advection winds, e.g. plume from power station. Occurs on Local (e.g. offshore winds) Regional (weather events) Global (Hadley circulation)

Winds due to weather patterns As air moves from high to low pressure on the surface of the rotating Earth, it is deflected by the Coriolis force.

Global circulation Hadley Cells Intertropical conversion zone (ITCZ) rapid vertical transport near the equator.

Horizontal transport timescales

Stratospheric chemistry

O 2 O( 3 P) + O( 3 P) Threshold = 242 nm O 2 O( 3 P) + O( 1 D) Threshold = 176 nm

UV absorption spectrum of O 3 at 298 K Small but significant absorption out to 350 nm (Huggins bands) Hartley bands Very strong absorption Photolysis mainly yields O( 1 D) + O 2, but as the stratosphere is very dry (H 2 O ~ 5 ppm), almost all of the O( 1 D) is collisionally relaxed to O( 3 P)

Integrated column - Dobson unit

Timescale Slow (J is small) Fast < 100 secs Fast ~ 1000 s Slow (activation barrier)

J 1 = rate of O 2 photolysis (s -1 ) J 3 = rate of O 3 photolysis (s -1 ) Graph shows the altitude dependence of the rate of photolysis of O 3 and O 2. Note how J 1 is very small until higher altitudes (1)The ratio J 1 /J 3 increases rapidly with altitude, z (2)As pressure exp (-z) then [O 2 ] 2 [M] decreases rapidly with z z This balance results in a layer of O 3 Altitude/km J1J1 J3J3 J1J1 J3J3

The Chapman mechanism overpredicts O 3 by a factor of 2. Something else must be removing O 3 (Or the production is too high, but this is very unlikely) Altitude / km HOW GOOD IS THE CHAPMAN MECHANSIM?

Catalytic ozone destruction The loss of odd oxygen can be accelerated through catalytic cycles whose net result is the same as the (slow) 4 th step in the Chapman cycle Uncatalysed: O + O 3 O 2 + O 2 k 4 Catalysed: X + O 3 XO + O 2 k 5 XO + O X + O 2 k 6 Net rxn: O + O 3 O 2 + O 2 X is a catalyst and is reformed X = OH, Cl, NO, Br (and H at higher altitudes) Reaction (4) has a significant barrier and so is slow at stratospheric temperatures Reactions (5) and (6) are fast, and hence the conversion of O and O 3 to 2 molecules of O 2 is much faster, and more ozone is destroyed.

The sources of X

CFCs are not destroyed in the troposphere. They are only removed by photolysis once they reach the stratosphere.

Data from NOAA CMDL Ozone depleting gases measured using a gas chromatograph with an electron capture detector (invented by Jim Lovelock) These are ground-based measurements. The maximum in the stratosphere is reached about 5 years later 45 years100 years Why are values in the N hemisphere slightly higher?

Do nothing cycles O x is not destroyed Reduces efficiency of O 3 destruction Removal of the catalyst X. Reservoir is unreactive and relatively stable to photolysis. X can be regenerated from the reservoir, but only slowly. [X] is reduced by these cycles. For Cl atom, destroys 100,000 molecules of O 3 before being removed to form HCl

Interactions between different catalytic cycles Reservoir species limit the destruction of ozone ClONO 2 stores two catalytic agents ClO and NO 2

Effects of catalytic cycles are not additive due to coupling MechanismOzone Column (Dobson units) Chapman only (C)644 C + NO x 332 C + HO x 392 C + ClO x 300 C + NO x + HO x + ClO x 376 Coupling to NO leads to null cycles for HO x and ClO x cycles Increase of Cl and NO concentrations in the atmosphere has less effect than if Cl or NO concentrations were increased separately (because ClOx and NOx cycles couple, hence lowering [X])

Bromine cycle Br + O 3 BrO + O 2 Cl + O 3 ClO + O 2 BrO + ClO Br + ClOO ClOO Cl + O 2 Net 2O 3 3 O 2 Br and Cl are regenerated, and cycle does not require O atoms, so can occur at lower altitude Source of bromine : CH 3 Br (natural emissions from soil and used as a soil fumigant) Halons (fire retardants) Catalytic cycles are more efficient as HBr and BrONO 2 (reservoirs for active Br) are more easily photolysed than HCl or ClONO 2 But, there is less bromine than chlorine Bromine is very important for O 3 destruction in the Antarctic stratosphere where [O] is low

Total Ozone Mapping Spectrometer (TOMS) Monthly October averages for ozone, 1979, 1982, 1984, 1989, 1997, 2001 Dobson units (total O 3 column)

October 2000 For the Second time in less than a week dangerous levels of UV rays bombard Chile and Argentina, The public should avoid going outside during the peak hours of 11:00 a.m. and 3:00 p.m. to avoid exposure to the UV rays Ushaia, Argentina The most southerly city in the world

At 15 km, all the ozone disappears in less than 2 months This cannot be explained using gas- phase chemistry alone US Base in Antarctica

Steps leading to ozone depletion within the Antarctic vortex ClO+BrO Cl+Br+O 2

Simultaneous measurements of ClO and O 3 on the ER-2 Late August 1987September 16 th 1987 The smoking gun experiment proved the theory was OK Still dark over AntarcticaDaylight returns

Ozone loss does appear in the Arctic, but not as dramatic Above Spitzbergen Some years see significant depletion, some years not, and always much less than over Antarctica

Tropospheric chemistry

Global tropospheric chemistry Questions to be addressed: 1.Many organic compounds emitted to the atmosphere are oxidised, eventually forming CO 2 and H 2 O. What determines the oxidising capacity of the atmosphere? 2.Methane is a greenhouse gas, whose atmospheric concentration has more than doubled since the industrial revolution. What governs it concentration? 3.Tropospheric oxidation is strongly influenced by NOx, whose lifetime is ~ 1 day. How is NOx transported to regions with no NOx emissions? 4.Ozone is a secondary pollutant. In the boundary layer it affects human health, growth of vegetation and materials. It is also a greenhouse gas. What governs its concentration?

Methane oxidation CH 4 + OH (+O 2 ) CH 3 O 2 + H 2 O CH 3 O 2 + NO CH 3 O + NO 2 CH 3 O + O 2 HO 2 + HCHO HO 2 + NO OH + NO 2 HCHO + OH (+O2) HO 2 + CO + H 2 O HCHO + h H 2 + CO HCHO + h (+2O 2 ) 2HO 2 + CO Note: 2 x(NO NO 2 ) conversions HCHO formation provides a route to radical formation.

General oxidation scheme for VOCs O 3 + h O 1 D + O 2 O 1 D + H 2 O 2OH OH + RH (+O 2 ) RO 2 + H 2 O RO 2 + NO NO 2 + RO RO HO 2 (+RCHO) HO 2 + NO OH + NO 2 NO 2 + h NO + O; O + O 2 O 3 OVERALL NO x + VOC + sunlight ozone The same reactions can also lead to formation of secondary organic aerosol (SOA)

THE OH RADICAL: MAIN TROPOSPHERIC OXIDANT O 3 + h O 2 + O( 1 D) (1) O( 1 D) + M O + M (2) O( 1 D) + H 2 O 2OH (3) Primary source: Sink: oxidation of reduced species CO + OH CO 2 + H CH 4 + OH CH 3 + H 2 O HCFC + OH H 2 O + Major OH sinks GLOBAL MEAN [OH] ~ 1.0x10 6 molecules cm -3

Other oxidising species NO 3 NO 2 + O 3 NO 3 + O 2 NO 2 + NO 3 + M N 2 O 5 + M NO 3 is rapidly lost in the day by photolysis and reaction with NO ( NO 2 ), so that its daytime concentration is low. It is an important night time oxidant. It adds to alkenes to form nitroalkyl radicals which form peroxy radicals in the usual way. O 3 Ozone reacts with alkenes to form a carbonyl + an energised Criegee biradical. The latter can be stabilised or decompose. One important reaction product is OH: O 3 reactions with alkenes can act as a source of OH, even at night.

Global budget for methane (Tg CH 4 yr -1 ) Sources: Natural160 Anthropogenic375 Total535 Natural Sources: wetlands, termites, oceans Anthropogenic Sources: natural gas, coal mines, enteric fermentation, rice paddies, Sinks: Trop. oxidation445 by OH Transfer to 40 stratosphere Uptake by soils 30 Total515 Notes: 1.The rate of oxidation is k 5 [CH 4 ][OH], where the concentrations are averaged over the troposphere 2. Concentrations of CH 4 have increased from 800 to 1700 ppb since pre- industrial times 3. Methane is a greenhouse gas.

HISTORICAL TRENDS IN METHANE Historical methane trend Recent methane trend Recent measurements at Mace Head in W Ireland. 1 g m -3 = 0.65 ppb NB seasonal variation higher in winter

GLOBAL DISTRIBUTION OF METHANE NOAA/CMDL surface air measurements Seasonal dependence higher in winter than summer (maximum in NH correlates with minimum in SH). NH concentrations > SH main sources are in NH; slow transport across ITCZ.

GLOBAL BUDGET OF CO

GLOBAL DISTRIBUTION OF CO NOAA/CMDL surface air measurements Compare CH 4. What are the differences and why? ( Rate coefficients at 298 K/10 -12 cm 3 molecule -1 s -1 : CH 4 : 7x10 -3; CO: 0.24)

Global VOC emissions (Tg yr -1 ) Anthropogenic: fuel production and distribution 17; fuel consumption 49; road transport 36; chemical industry 2; solvents 20; waste burning 8, other 10. Total 142 Tg yr -1 Biogenic: isoprene 503; monoterpenes 127; other reactive VOCs 260, unreactive VOCs 260; Total 1150 Tg yr -1 Typical atmospheric lifetimes (for [OH] = 1x10 6 molecule cm -3 ) = 1/k[OH] CH 4 6 yrisoprene2.7 h CO48 daysethane46 days benzene6 daysethene30 h

Global budget for NOx Global sources (Tg N yr -1 ): Fossil fuel combustion 21;Biomass burning: 12 Soils6Lightning 3 Ammonia oxidation3Aircraft 0.5 Transport from strat0.1 Coupling (rapid - ~ 1 minute in the day) NO + O 3 NO 2 + O 2 NO 2 + Light NO + O; O + O 2 + M O 3 + M Also HO 2 + NO NO 2 + OH Loss OH + NO 2 + M HNO 3 + M Rainout of HNO 3 Lifetime of NO x is about 1 day. NO x is a key component in ozone formation. Can it be transported to regions where it is not strongly emitted?

PEROXYACETYLNITRATE (PAN) AS RESERVOIR FOR LONG-RANGE TRANSPORT OF NO x

NO 2 as an air pollutant. UK NOx emissions, 1970 - 2000 Recent road transport data for the UK

Spatial distribution of NOx emissions

Maps of annual mean background NO 2 concentrations UK 2001 UK 2010 Key AQ objective is annual mean of 40 g m -3 to be achieved by 2010 (EU Directive)

Annual mean NO 2 concentrations, London London 2010 670 road links out of 1888 exceed 40 g m -3 London 1999 1407 road links out of 1888 exceed 40 g m -3

Hungarian air quality network http://www.kvvm.hu/olm/index.php

NO 2 in Budapest and Hungary in 2005

MAPPING OF TROPOSPHERIC NO 2 FROM THE GOME SATELLITE INSTRUMENT (July 1996) Martin et al. [2002]

Global budget for ozone (Tg O 3 yr -1 ) Ozone is a secondary pollutant and is not directly omitted. Chemical production3000 4600 HO 2 + NO70% CH 3 O 2 + NO20% RO 2 + NO10% Transport from stratosphere400 1100 Chemical loss3000 4200 O 1 D + H 2 O40% HO 2 + O 3 40% OH + O 3 10% others10% Dry deposition500 - 1500 Ozone is a greenhouse gas. It affects human health, plant growth and materials

Regional ozone formation

Regional air quality ozone formation Ozone is a greenhouse gas. It affects human health, plant growth and materials Ozone is a secondary pollutant and is not directly emitted. Emission of VOCs and NOx, coupled with sunlight leads to the formation of photochemical smog. Major component is ozone. Also aerosols, nitrates Need to understand chemical mechanism for formation in order to develop strategies and legislation for reduction of ozone concentrations. The European limit values are linked to these aims Is it better to control NOx or VOCs or both?

Chemical mechanism Initiation: OH formed from ozone photolysis at a rate P OH (= 2k 3 [H 2 O]J 1 [O 3 ]/{k 2 [M] + k 3 [H 2 O]} ) Propagation OH + RH (+O 2 ) RO 2 + H 2 O(R4) RO 2 + NO RO + NO 2 (R5) RO + O 2 RCHO + HO 2 (R6) HO 2 + NO OH + NO 2 (R7) Termination HO 2 + HO 2 H 2 O 2 (R8) OH + NO 2 + M HNO 3 + M(R9) Ozone formation O 3 is formed by NO 2 photolysis with a rate equal to the sum of the rates of reactions 5 and 7 (= v 5 + v 7 )

DEPENDENCE OF OZONE PRODUCTION ON NO x AND HYDROCARBONS HO x family OH RO 2 RO HO 2 HNO 3 H2O2H2O2 O3O3 O3O3 O3O3 P HOx 4 5 6 7 8 9 NO x - saturated or hydrocarbon-limited regime NO x -limited regime RH NO O2O2 NO 2

OZONE CONCENTRATIONS vs. NO x AND VOC EMISSIONS Air pollution model calculation for a typical urban airshed NO x - saturated NO x -limited Ridge

Air quality and climate change

Impact of air pollution UK Air Quality Strategy, 2007 Air pollution is currently estimated to reduce the life expectancy of every person in the UK by an average of 7- 8 months. The measures outlined in the strategy could help to reduce the impact on average life expectancy to five months by 2020, and provide a significant step forward in protecting our environment. Defra estimate the health impact of air pollution in 2005 cost 9.121.4 billion pa.

Timescales of ozone chemistry 1.Global chemistry. Dominated by NO x + CH 4 + sunlight. Timescales are long as are transport distances. 2.Regional chemistry. Many VOCs are emitted, e.g. over Europe. Each has its own lifetime governed by its rate constant for reaction with OH. The timescales of ozone production takes from hours to days. The transport distance for a wind speed of 5 m s -1 and a lifetime of 1 day is ~500 km. 3.Urban chemistry: high concentrations of NO from transport sources. Ozone is depressed by the reaction: NO + O 3 NO 2 + O 2

Radiative Forcing Radiative forcing: the change in the net radiation balance at the tropopause caused by a particular external factor in the absence of any climate feedbacks. These forcing mechanisms can be caused by: change in the atmospheric constituents such as the increase in greenhouse gases (GHGs) aerosols due to anthropogenic activity, changes in other components of the Earth/atmosphere system such as changes in the surface albedo (the fraction of incoming radiation that is reflected). Albedo changes are caused, e.g., by changesin vegetation (e.g. burn scars or agriculture).

Mechanisms of the radiative forcing due to greenhouse gases and of the direct radiative forcings due to aerosols

Global-average radiative forcing (RF) estimates and ranges in 2005 (relative to 1750) for anthropogenic GHGs and other important agents and mechanisms

Carbon dioxide and methane mixing ratios versus time (NOAA Climate Monitoring and Diagnostics Laboratory http://www.cmdl.noaa.gov/ccgg/insitu.html)

Climate System

Evolution of models

SRES (IPCC Special Report on Emission Scenarios) scenarios The A1 storyline is for a future world with very rapid economic growth, global population that peaks in mid-century and declines thereafter, the rapid introduction of new and more efficient technologies and with a substantial reduction in regional differences in per capita income. Within this family are three sub-scenarios with different technological emphasis: A1FI A1, fossil fuel intensive A1T A1, with non-fossil energy source emphasis A1B A1, with a balance across energy sources. The A2 storyline is a more pessimistic scenario, describing a very heterogeneous world based on self-reliance, regional differences in economic and technological development and continuous increase in global population. The B1 storyline describes a convergent world like A1, with global population peaking in mid-century, but with rapid changes in economic structures, introduction of clean and resource-efficient technologies, emphasis on global solutions to social and environmental sustainability. The B2 storyline describes a world with emphasis on local solutions to social and environmental sustainability, less rapid and more diverse than in B1 and A1, with continuously increasing global population, but at a lower rate than A2.

Royal Society Report on ozone over next 100 years Level of automobile emission limits in Asian countries, compared with the EuropeanUnion. Source: Clean Air Initiative for Asian cities

Impact of improved technologies in Asian countries on assessment of NOx emissions

New estimates of CO emissions

New estimates of CH 4 emissions

Predicted lobal temperature rise for different scenarios

Future summer temperatures Using a climate model simulation with greenhouse gas emissions that follow an IPCC SRES A2 emissions scenario, Hadley Centre predict that more than half of all European summers are likely to be warmer than that of 2003 by the 2040s, and by the 2060s a 2003- type summer would be unusually cool Stott et al. Nature, December 2004 2003: hottest on record (1860) Probably hottest since 1500. 15 000 excess deaths in Europe

Impact of climate change on air quality - ozone

O 3 from climate change Warmer temperatures &higher humidities increase O 3 destruction over the oceans But also a role from increases in isoprene emissions from vegetation &changes in lightning NO x 2020s CLEcc- 2020s CLE O 3 + h O 1 D + O 2 O 1 D + H 2 O 2OH O 1 D + N 2, O 2 O 3 P OH+RH(+O 2 ) RO 2 + H 2 O RO 2 + NO RO + NO 2 NO 2 + h (+O 2 ) NO+O 3

PAN peroxy acetyl nitrate PAN is a reservoir compound for nitrogen oxides and provides a mechanism for their transport, especially in the upper troposphere. It provides a means of carrying nitrogen oxides from polluted to less polluted regions. It is a major player in the intercontinental transport of pollutants PAN is formed from reactions of the acetyl peroxy radical and NO2: e.g. CH3CHO + OH (+O2) CH3COO2 + H2O CH3COO2 + NO2 CH3COO2NO2 (PAN)

Heat wave in Europe, August 2003 Monitoring stations in Europe reporting high band concentrations of ozone >15 000 excess deaths in France; 2000 in UK, ~30% from air pollution. Temperatures exceeded 35 0 C in SE England. What about Hungary? How frequent will such summers be in the future?

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The chemistry of the troposphere and stratosphere Prof. M. J. Pilling The University of Leeds (UK)...

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