TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY

Troposphere

Stratosphere:90% of total

The many faces of atmospheric ozone:

In stratosphere: UV shield

In middle/upper troposphere: greenhouse gas

In lower/middle troposphere: precursor of OH, main atmospheric oxidant

In surface air: toxic to humans and vegetation

THE ATMOSPHERE: OXIDIZING MEDIUM IN GLOBAL BIOGEOCHEMICAL CYCLES

EARTHSURFACE

Emission

Reduced gas Oxidized gas/aerosol

Oxidation

Uptake

Reduction

Atmospheric oxidation is responsible for removal of many pollutants, e.g.• methane (major greenhouse gas)• CO (toxic pollutant)• HCFCs (ClOx sources in stratosphere)

TROPOSPHERE WAS VIEWED AS CHEMICALLY INERT UNTIL 1970

• “The chemistry of the troposphere is mainly that of of a large number of atmospheric constituents and of their reactions with molecular oxygen…Methane and CO are chemically quite inert in the troposphere” [Cadle and Allen, Atmospheric Photochemistry, Science, 1970]

• Lifetime of CO estimated at 2.7 years (removal by soil) leads to concern about global CO pollution from increasing car emissions [Robbins and Robbins, Sources, Abundance, and Fate of Gaseous Atmospheric Pollutants, SRI report, 1967]

FIRST BREAKTHROUGH:

• Measurements of cosmogenic 14CO place a constraint of ~ 0.1 yr on the tropospheric lifetime of CO [Weinstock, Science, 1969]

SECOND BREAKTHROUGH:• Tropospheric OH ~1x106 cm-3 predicted from O(1D)+H2O, results in

tropospheric lifetimes of ~0.1 yr for CO and ~2 yr for CH4 [Levy, Science, 1971, J. Geophys. Res. 1973]

THIRD BREAKTHROUGH:• Methylchlroform observations provide indirect evidence for OH at levels

of 2-5x105 cm-3 [Singh, Geophys. Res. Lett. 1977]

WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT?Production of O(1D) in troposphere takes place in narrow band [290-320 nm]

solar flux I

ozone absorptioncross-section s

O(1D)quantumyield f

fsI

~tropopause

10 ppmv

40 ppbv

TYPICAL OZONE PROFILE: ~10% OF OZONE COLUMN GLOBALLY IS IN THE TROPOSPHERE

Stratospheric ozone mechanism doesn’t apply to troposphere

2

2 3

13 2

1

2

( )

( )

O h O OO O M O M

O h O O D

O D M O MXO O X O

O2+hvO3+hv

By contrast, in troposphere:

• no photons < 240 nm no oxygen photolysis;

• neglible O atom conc.gno XO + O loss

In stratosphere:

• Estimate ozone flux FO3 across tropopause (strat-trop exchange)– Total O3 col = 5x1013 moles– 10% of that is in troposphere– Res. time of air in strat = 1.4 yr

• Estimate CH4 source SCH4:– Mean concentration = 1.7 ppmv– Lifetime = 9 years

• Estimate CO source SCO:– Mean concentration = 100 ppbv– Lifetime = 2 months

UNTIL ~1990, PREVAILING VIEW WAS THAT TROPOSPHERIC OZONE ORIGINATED MAINLY FROM STRATOSPHERE…but that cannot work.

FO3 = 3x1013 moles yr-1

SCH4 = 3x1013 moles yr-1

SCO = 9.7x1013moles yr-1

SCO+ SCH4 > 2FO3 e OH would be titrated!

OZONE PRODUCTION IN TROPOSPHEREPhotochemical oxidation of CO and volatile organic compounds (VOCs)

catalyzed by HOx and NOx

HOx ≡ H + OH + HO2 + RO + RO2

NOx ≡ NO + NO2

Oxidation of CO:

2

2 2

2 2

2

2 3

2 2 3Net: 2

CO OH CO HH O M HO MHO NO OH NONO h NO OO O M O M

CO O CO O

2

2

2 2

2 2

2 3

2 2

2 2

2 3 2

'

Net: 4 ' 2

O

RH OH R H OR O M RO MRO NO RO NO

NO h NO ORO O R CHO HOHO NO OH NO

RH O R CHO O H O

Oxidation of VOC:

RO can also decompose or isomerize; range of carbonyl products

Carbonyl products can react with OH to produce additional ozone, or photolyze to generate more HOx radicals (branching reaction)

OH can also add to double bonds of unsaturated VOCs

GLOBAL BUDGET OF TROPOSPHERIC OZONE (MODEL)

O3

O2 h

O3

OH HO2

h, H2O

Deposition

NO

H2O2

CO, VOC

NO2

h

STRATOSPHERE

TROPOSPHERE8-18 km

Chem prod in troposphere,Tg y-1

43001600

Chem loss in troposphere,Tg y-1

40001600

Transport from stratosphere,Tg y-1

400400

Deposition,Tg y-1

700400

Burden, Tg 360230

Lifetime, days 2842

Present-day Preindustrial

OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSBox model calculation

NOx-saturatedregime

NOx-limited regime Ridge

SATELLITE OBSERVATIONS OF TROPOSPHERIC NO2

SCIAMACHY data. May-Oct 2004(R.V. Martin, Dalhousie U.)

detectionlimit

NOx EMISSIONS (Tg N a-1) TO TROPOSPHERE

FOSSIL FUEL 23.1

AIRCRAFT 0.5

BIOFUEL 2.2

BIOMASSBURNING 5.2

SOILS 5.1

LIGHTNING 5.8

STRATOSPHERE 0.2

LIGHTNING FLASHES SEEN FROM SPACE (2000)

DJF

JJA

GLOBAL DISTRIBUTION OF TROPOSPHERIC OZONE

Zhang et al. [2010]

TES thermal IR satellite observations for 2006, seasonal means at 500 hPa

• Maximum values at northern mid-latitudes in spring-summer due to anthropogenic pollution;

• High values in tropical regions affected by seasonal biomass burning;

• Minimum values over tropical oceans due to chemical loss

LONDON FOGAerosols a.k.a.particulate matter (PM) from domestic+industrial coal combustion

“Killer fog” of December 1952 resulted in 10,000 excess deaths

Coal combustionTemperature

Altitude

inversion

sulfateorganic carbonblack carbon

particles< 1km

LOS ANGELES SMOGRespiratory problems, vegetation damage due to high surface ozone

troposphere

stratosphere8-18 km

temperature

inversionozone

altitude

Nitrogen oxides (NOx ≡ NO + NO2) Volatile organic compounds (VOCs)

UV radiation Ozone (O3)

vehicles, industry, vegetation

produced by photolysisof oxygen (O2)

AIR POLLUTION IN THE US TODAY:Ozone and fine particulate matter (PM2.5) are the two main pollutants

75 ppb (8-h average)15 mg m-3 (1-y av.)

http://epa.gov/airtrends/2010/

OzonePM2.5

2008 REVISION TO OZONE STANDARDFROM 84 to 75 PPB CAUSED MORE U.S. AREAS TO BE OUT OF COMPLIANCE

0 20 40 60 80 100 120 ppb

Europe AQS(seasonal)

U.S. AQS(8-h avg.)

U.S. AQS(1-h avg.)

Preindustrialozone

background

Present-day ozone background at

northern mid-latitudes

Europe AQS (8-h avg.)

Canadian AQS (8-h avg.)

Mexican AQS(1-h avg.)

…AND INCREASED THE IMPORTANCE OF THE OZONE BACKGROUND

20082014?

Currently proposed 60-70 ppb standard would have extensive non-compliance

OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSAir pollution model calculation for a typical urban airshed

NOx-saturated

NOx-limited Ridge

LARGE SUPPLY OF BIOGENIC VOCs – unrecognized until the 1990s

Isoprene (biogenic VOC)Anthropogenic VOCs

Jacob et al., J. Geophys. Res. [1993]

Switches polluted areas in U.S. from NOx-saturated to NOx-limited regime!recognized in Revised Clean Air Act of 1999

MAPPING OF VOC EMISSIONS FROM SPACEusing satellite measurements of formaldehyde

confirms dominance of biogenic over anthropogenic VOCsMillet et al. [2008]

1970-2003 TREND OF U.S. EMISSIONS

Focus until past decade was on VOC emission controls

DECREASE OF POWER PLANT NOx EMISSIONSOVER THE PAST DECADE

Decreasing US NOx emissions from power plants

Growth Trends

0%

100%

200%

300%

1980 1990 2000 2010 2020

Norm

aliz

ed to

198

0

Population

Vehicle Miles

Traveled

Gross State

Product

Emission Trends

0%

50%

100%

150%

1980 1990 2000 2010 2020

Norm

aliz

ed to

198

0 CO2

NOx

SOx

ROG

CO

PM10 Trends

0

100

200

300

Max

24-

hr P

M10

(µg/

m3 )

South Coast

State Standard

San Joaquin Valley

0.0

0.1

0.2

0.3

0.4

0.5

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

Max

1-h

r O

zone

(pp

m)

State Standard

South Coast

San Joaquin Valley

Ozone Trends

Historical Ozone Levels

0

50

100

150

200

250

300

1965 1970 1975 1980 1985 1990 1995 2000 Year

Above Califor niaOzone Standar d

Stage I E pisodes

Stage II Episodes

Num

ber o

f Day

s

Stage II > 350 ppb O3Stage I > 200 ppb O3

SOUTH COAST O3 HISTORY

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000Years

Max

O3 (

ppm

)

1952

– P

rof.

Haa

gen-

Smit

disc

over

s sm

og fo

rmat

ion

1963

– C

lean

Air

Act

(CA

A)

1970

– E

PA c

reat

ed, C

AA

Am

men

d.

1976

– V

olvo

intr

oduc

es fi

rst c

ar to

us

e 3-

way

cat

alys

t

1984

– S

mog

Che

ck P

rogr

am

1990

– C

AA

A

1994

– S

mog

Che

ck II

1971

– C

AR

B a

dopt

s 1s

t aut

o N

Ox

stan

dard

sEP

A pr

omul

gate

s N

AA

QS

NAAQS

Improvement Over Past 20 Years

-80%

-60%

-40%

-20%

0%NitrogenDioxide

SulfurDioxide

CarbonMonoxide

Ozone PM10 Air Toxics(CancerRisk)

Perc

ent

Chan

ge

Approaching StandardsAttained

Standards

T H A N K S ! !