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Air Pollution and Atmospheric Chemistry

Sasha Madronich Atmospheric Chemistry DivisionNational Center for Atmospheric Research Boulder, Colorado USA 27 July 2004, Boulder

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Components of Air Quality Models

Spatial and Temporal Grids• Horizontal domain (local; regional; global)• Vertical extent (PBL; troposphere; trop+strat+mesosphere)• Time span (day or week episode; interannual; climatologic)

Chemical Inputs• Natural emissions• Anthropogenic emissions• Inflow from model boundaries• Initial conditions

Chemical Transformations• Gas phase• Condensed phase (aerosols, clouds)

Transport• Horizontal advection• Vertical diffusion and convection• Update environment (T, P, H2O, h

Deposition• Wet (rain, snow)• Dry (gas & aerosol on surfaces)

Solution forward in time• Coupled non-linear stiff differential equations

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Earth’s Atmosphere

Composition• 78% nitrogen• 21% oxygen• 1-2% water (gas, liquid, ice)• trace amounts (<< 1%) of many other species, some natural

and some “pollutants”

Reactivity dominated by • oxygen chemistry• solar photons

To understand fate of pollutants, must first understand oxygen photochemistry

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Energetics of Oxygen in the Atmosphere

Hf (298K) kcal mol-1

Excited atoms O*(1D) 104.9

Ground state atoms O (3P) 59.6

Ozone O3 34.1

“Normal” molecules O2 0

Increasingstability

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Atmospheric OxygenThermodynamic vs. Actual

1E-110

1E-100

1E-90

1E-80

1E-70

1E-60

1E-50

1E-40

1E-30

1E-20

1E-10

1

200 220 240 260 280 300

Temperature, K

Co

nc

en

tra

tio

n, a

tm. O2 (=0.21)

thermodyn. O3

thermodyn. O

thermodyn. O*

observed O3

inferred O

inferred O*

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Photochemistry

Thermodynamics alone cannot explain atmospheric amounts of O3, O, O*

Need – energy input, e.g.

O2 + h O + O ( < 250 nm)

– chemical reactions, e.g. O + O2 (+ M) O3 (+ M)

= Photochemistry

WMO, 2002

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Stratospheric Odd Oxygen (Ox = O + O3)

Chapman, 1930’s: Pure oxygen photochemistry

O3 production:

O2 + h ( < 240 nm) 2 O

O + O2 + M O3 + M

O3 destruction:O3 + h ( < 800 nm) O + O2

O + O3 2 O2

Correctly predicts vertical profile shape, but too much O3.

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Stratospheric Odd Hydrogen (HOx = OH + HO2)

Bates and Nicolet, 1950’s: Hydrogen-containing “contaminants”

Formation of excited oxygen atoms:

O3 + h (<330 nm) O2 + O*

Formation of HOx radicals from H2O and CH4:

H2O + O* OH + OHCH4 + O* OH + CH3

Catalytic destruction of O3 by HOx:

O3 + OH O2 + HO2

O + HO2 O2 + OHO3 + HO2 2 O2 + OH

Better, but still too much O3

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Stratospheric Odd Nitrogen (NOx = NO + NO2)

Crutzen, 1970: Nitrogen containing “contaminants”

Formation of excited oxygen atoms:

O3 + h (<330 nm) O2 + O*

Formation of NOx radicals from N2O:

N2O + O* NO + NO

Catalytic destruction of O3 by NOx:

O3 + NO O2 + NO2

O + NO2 O2 + NOworks for natural stratosphere

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Stratospheric Halogens (Cl, Br, I, …)

Rowland and Molina, 1974: Chlorofluorocarbons (CFCs) can make it to stratosphere because they are not destroyed in troposphere:

Formation of chlorine atoms from photolysis of chlorofluorocarbons:

CH3Cl + h CH3 + Cl

CF2Cl2 + h CF2Cl + Cl

Catalytic destruction of O3 by Clx:

O3 + Cl O2 + ClOO + ClO O2 + Cl

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Stratospheric Reservoirs

Formation of less-reactive reservoirs:

Cl + CH4 HCl + CH3

ClO + NO2 + M ClONO2 + M

OH + NO2 + M HNO3 + M

Reservoirs can either be removed by diffusion to troposphere, or can be transformed back to reactive species.

Strong reactivation of halogens occurs on surfaces of polar stratospheric clouds.

SOLAR SPECTRUM

UNEP, 2002

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Detrimental Effects of UV Radiation

Human and animal health– Skin cancer, skin ageing, sunburns– Ocular damage– Immune system suppression

Reduced Growth in Plants– Terrestrial (agriculture, forests)– Marine (less phytoplankton)

Air Quality– More UV means more urban ozone, secondary aerosols

Materials– Degradation of plastics (PVC, PC)

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Global UV Changes (1990’s/1980’s)

Clear sky(ozone change only)

All conditions(ozone and cloud changes)

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Atmospheric Halogens are Decreasing or Stabilizing

WMO, 2002

The

Future

Avoided

WMO, 2002

WMO, 2002

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Tropospheric Ozone Formation – how?

Urban ozone (O3) is generated when air containing hydrocarbons and nitrogen oxides (NOx = NO + NO2) is exposed to UV radiation (Haagen-Smit, 1950’s).

Laboratory studies show that O3 is made almost exclusively by the reaction:

O2 + O + M O3 + M

But troposphere lacks short-wavelength photons (<250 nm) needed to break O2 directly.

So: what is the source of tropospheric O atoms??

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Tropospheric O3 - From NO2?

NO2 photolysis is a source of O atoms:

NO2 + h ( < 420 nm) NO + O

O + O2 + M O3 + M

Two problems:Reversal by NO + O3 NO2 + O2

Usually O3 >> NO2

Makes some O3, but not enough!

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Tropospheric O3 Formation – Need h, HCs, NOx

Initiation by UV radiation (Levy, 1970):

O3 + h ( < 330 nm) O*(1D) + O2

O*(1D) + H2O OH + OH

Hydrocarbon consumption (oxygen entry point):

OH + RH R + H2O

R + O2 + M ROO + M

Single-bonded oxygen transferred to NOx:

ROO + NO RO + NO2

NOx gives up oxygen atoms (as before):

NO2 + h ( < 420 nm) NO + O

O + O2 + M O3 + M

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Tropospheric O3 Formation – Secondary Reactions

PropagationRO + O2 R’CO + HO2

HO2 + NO OH + NO2

more O3, OH

TerminationOH + NO2 + M HNO3 + M

HO2 + HO2 + M H2O2 + M

HO2 + O3 OH + 2 O2

slows the chemistry

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Tropospheric Chemical Mechanisms

This talk: 15 reactions

Typical 3D model used for air quality: 100 - 200 reactions

Typical 0D (box) models used for sensitivity studies:5,000 - 10,000 reactions

Fully explicit (computer-generated) mechanisms:106 - 107 reactions

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Hydrocarbon Chemistry is Complex!

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

2 3 4 5 6 7 8 9

Number of carbons

n-alkanes

i-alkanes

1-alkenes

isoprene

Reactions

Species

Aumont and Madronich, 2003

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Consequences of tropospheric O3 chemistry - 1

Surface O3 pollution

Urban: 100-500 ppb

Regional: 50-100 ppb

Global background increase

10-20 ppb 35-45 ppb in NH

10-20 ppb 25-35 ppb in SH

Damage to health and vegetation Greenhouse role of O3

Changes in global oxidation capacity

California EPA, 2004

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Consequences of tropospheric O3 chemistry - 2

Formation of peroxides and acids:

HO2 + HO2 H2O2 + O2

OH + NO2 + M HNO3 + M

OH + SO2 … H2SO4

H2O2(aq) + SO2(aq) … H2SO4(aq)

Damage to vegetation and structures (acid precipitation)

Sulfate aerosol formation (visibility, climate)

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Consequences of tropospheric O3 chemistry - 3

Products of hydrocarbon oxidation

CO2 (minor compared to direct emissions)

CO (~ 1/2 of total global emissions)

Oxygenated organics: aldehydes, ketones, alcohols, organic acids, nitrates, peroxides

Damage to health, vegetation Secondary organic aerosol formation (health,

visibility, climate) Changes in global oxidation capacity

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Global Oxidation (self-cleaning) Capacity

Solar UV radiation

Oxidation, e.g.:

CH4 + OH … CO2 + H2O

Insoluble Soluble

EmissionsCH4 CmHn

SO2

NO

CO

NO2

HalocarbonsDeposition(dry, wet)

HNO3, NO3-

H2SO4, SO4=

HCl, Cl-

Carboxylic acids

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Consequences of tropospheric O3 chemistry - 4

OH increase because of increasing emissions of NOx?

OH increase because of increasing UV radiation?OR

OH decrease because of increasing emissions of CO, CmHn, SO2, and other reduced compounds?

Decreased OH (oxidizing capacity) implies generally higher amounts of most pollutants including:• Higher amounts of greenhouse gases• Higher amounts of substances that deplete the

ozone layer• More global spread

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How Climate Change Can Affect Pollution - 1

Changes in Anthropogenic and Biogenic Emissions:

• Anthropogenic emissions of ozone precursor compounds (CO, NOx, SOx, NMHC) and aerosols are expected to increase over the next 50 years.

• Biogenic emissions of NMHCs and CO are expected to be affected significantly by future changes in temperature, relative humidity and photosynthetically available radiation (PAR).

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How Climate Change Can Affect Pollution - 2

Changes in Transport:

• Modification of inter-continental transport of pollutants.

• Modification of moist convective activity, with associated changes in wet removal processes and vertical redistribution of pollutants.

• Modification of the boundary-layer height and ventilation rates.

• Modification of stratosphere-troposphere exchange, with consequently different inputs of ozone to the troposphere.

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How Climate Change Can Affect Pollution - 3

Changes in Chemically Relevant Environmental Variables:

• Increased temperatures lead to faster kinetics of O3 production.

• Changes in H2O, affecting both the gas phase chemistry, e.g. OH production via O(1D) + H2O, and the growth of aerosols near the deliquescence point.

• Changes in cloud distributions, with associated changes in aqueous chemical processes (e.g. sulfate formation), NOx production by lightning, wet removal, and photochemistry.

• Increased aerosol loading, with associated enhancements of heterogeneous chemistry, and – depending on aerosol type – either increased or decreased photochemistry.

• Changes in stratospheric ozone, with associated changes in photochemistry.

INTERACTIONS:

Climate change

&

Stratospheric

ozone

WMO, 2002

INTERACTIONS: Climate, Clouds, and UVR:

2130 – Present, SH Summer

Madronich, Tie, Rasch, unpubl.

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INTERACTIONS: Climate & Air Pollutants

IPCC, 2001

IPCC, 2001

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INTERACTIONS: Heat, Air Pollution & Health

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INTERACTIONS: Carbon cycle & Tropospheric O3

Loya et al., Nature, 425, 705, 2003

Stratospheric Ozone Depletion

Air Quality Climate Change

+ halocarbons+ H2O

+ UV+CFC replacement

+ CH4, + O3, + soot, + sulfate, ± clouds

+ T, + H2O, ± emissions, ± rain, ± winds, ± clouds

- T± H2O

+ OH+ IR cooling

+ CFC replacement

Good?Bad?Unclear?

(a very incomplete picture)