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

00

The atmosphere as an oxidizing medium

EARTHSURFACE

Emission

Reduced gasOxidized gas/aerosol

Oxidation

Uptake

Reduction

Atmospheric oxidation is critical for removal of many pollutants, e.g.• methane (major greenhouse gas)• Toxic gases such as CO, benzene, mercury…• Gases affecting the stratosphere

CO and methane account for most of reduced gas flux to atmosphere

• Methane observed from space: 1700-1900 ppb

• CO observed from space: 50-200 ppb

Production of O(1D) in troposphere takes place in narrow band [290-320 nm]

solar flux Iat sea level

ozone absorptioncross-section s

O(1D)quantumyield f

fsI

Stratospheric ozone mechanism doesn’t apply to troposphere

2

2 3

13 2

1

2

( )

( )

O h O O

O O M O M

O h O O D

O D M O M

XO 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:

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 H

H O M HO M

HO NO OH NO

NO h NO O

O 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 O

R O M RO M

RO NO RO NO

NO h NO O

RO O R CHO HO

HO NO OH NO

RH O R CHO O H O

Oxidation of VOC:

Carbonyl products can react with OH to produce additional ozone.

• CO emitted by combustion• VOCs emitted by biosphere,

combustion, industry• NOx emitted by combustion,

biosphere, lightning

Chemical mechanism for tropospheric ozone and OH

O3

HO2 H2O

NO

H2O2

CO

NO2

hn

O(1D)

hn

M OH

HNO3

NOx-limited regime: ~ [O3]1/2[H2O]1/2[NO] ~ [O3]1/2[H2O]1/2[NO]/[CO]

NOx-saturated regime: ~ [O3][H2O][CO]/[NO2] ~ [O3][H2O]/[NO2]

P(O3) [OH]

GLOBAL BUDGET OF TROPOSPHERIC OZONE

O3

O2 hn

O3

OH HO2

hn, H2O

Deposition

NO

H2O2

CO, VOC

NO2

hn

STRATOSPHERE

TROPOSPHERE

8-18 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

ATMOSPHERIC CARBON MONOXIDE

Source: incomplete combustionSink: oxidation by OH (lifetime of 2 months)

Satellite CO data at 500 hPa

ATMOSPHERIC METHANE

LIVESTOCK80-90

LANDFILLS40-70

GAS50-70

COAL30-50RICE

30-110

TERMITES20-30

WETLANDS100-230

BIOMASSBURNING10-90

2005 methane emissions

Global sources,Tg a-1 Sink: oxidation by OH

(lifetime of 10 years)

METHANE: #2 ANTHROPOGENIC GREENHOUSE GASGreenhouse radiative forcing of climate between 1750 and 2005 [IPCC, 2007]

Referenced to concentration Referenced to emission

HISTORICAL TRENDS IN METHANE

The last 1000 years

The last 30 years

Methane column observed by SCIAMACHY, 2003-2009

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

SATELLITE OBSERVATIONS OF TROPOSPHERIC NO2

SCIAMACHY data. May-Oct 2004

(R.V. Martin, Dalhousie U.)

detectionlimit

TROPOSPHERIC NO2 FROM THE OMI SATELLITE INSTRUMENT (MARCH 2006)

March 2006

NOx EMISSION TRENDS SEEN FROM SPACE

Hillboll et al. [2012]

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

OBSERVED RISE IN TROPOSPHERIC OZONEOVER 20th CENTURY

Observations at mountain sites in Europe [Marenco et al., 1994]

GLOBAL OZONE AND OH TRENDSMean mass-weighted tropospheric values computed from GEOS-Chem model

for present-day (PD), pre-industrial (PI), and last glacial maximum (LGM)

Ozone, ppb

OH, 105 molecules cm-3

PD PI LGMtwo different climatereconstructionsLee Murray, Harvard

Inferring recent OH trends from methylchloroform data

Global trendsin methylchloroform concentrations

Global trendsin OH concentrations

Montzka et al. [2011]

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 is the #1 pollutant

75 ppb (8-h average)

http://epa.gov/airtrends

4th-highest annual maximum for daily 8-h average ozone,2008-2010

The 2012 ozone season

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.)

Ozone air quality standards in the US and in the world

20082014? 1997

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., 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 VOCs

Millet et al. [2008]

1970-2003 TREND OF U.S. EMISSIONS

Focus until past decade was on VOC emission controls

OZONE TRENDS IN U.S. http://www.epa.gov/airtrends/

Boston trend

National trend

• The national trend is heavily weighted by California sites;

• other parts of the country have seen marginal improvement

DECREASE OF POWER PLANT NOx EMISSIONSOVER THE PAST DECADE

Decreasing US NOx emissions from power plants

Decrease of North American NOx emisssions, 2005-2009

as seen with annual mean NO2 columns from the OMI satellite instrument

Shailesh Kharol (Dalhousie)

20092005

Decreases in both the eastern US and eastern Canada

Ozone trends in the Northeast, 1997-2012

EVEN IN NOx-LIMITED REGIME,THE TOTAL O3 PRODUCED IS ONLY A WEAK FUNCTION OF NOx

NO NO2 HNO3hv

HO2,RO2,O3 OH, O3

P(O3) L(NOx)

3 2

2 2

( ) [ ][ ] [ ]~ ~

( ) [ ][ ] [ ]OPE =

x

P O HO NO VOC

L NO NO OH NO

Emission Deposition

Assuming NOx steady state, efficient HOx cycling, and loss of NO2 by reaction with OH:

OPE m as NOx k e strong nonlinearity; in models, decreasing NOx emissionsby 50% reduces ozone only by ~15%

Define ozone production efficiency (OPE) as the total number of O3 molecules produced per unit NOx emitted.

Median surface ozone, 2006-2010:emerging problem in the Intermountain West

Cooper et al. [2012]

Spring

Summer

Daytime surface ozone trends, 1990-2010

Spring SummerCooper et al. [2012]

8-h daily maximum ozone in the intermountain West (2006)

56.3 ppb 53.2 ppb12.3 ppb

58.1 ppb 56.4 ppb

15.0 ppb

GEOS-Chem model US pollutioncontribution

Zhang et al., 2011

North American ozone background over the US

4th highest annual North American background ozone (GEOS-Chem model)

Zhang et al. [2011]

defined as the surface ozone concentrations that would be present in the absence of North American anthropogenic emissions

Growing NOx emissions from Canadian oil sands

Oil sand recoveryIn Alberta

Satellite NO2 columns, 2004-2010

NO2 increase of 10.4 ±3.5% per year

McLinden et al. [GRL 2012]

Ozone trends in remote air at northern mid-latitudes

D.D. Parrish, NOAA

Rising surface ozone pollution in China

D.D. Parrish, NOAA

INTERCONTINENTAL OZONE POLLUTION INFLUENCES

Surface O3 enhancements from North American anthropogenic emissions

from European anthropogenic emissions

from Asian anthropogenic emissions

Lin Zhang, Harvard

GEOS-Chem model results for 2006