Atmospheric chemistry

Lecture 3:Tropospheric Oxidation Chemistry

Dr. David GlowackiUniversity of Bristol,UK

david.r.glowacki@bristol.ac.uk

Yesterday…• We discussed photochemistry and kinetics• The earth’s atmosphere is a huge low temperature chemical reactor

with variable temperature, pressure, and actinic flux• All of these variables affect the rates of individual chemical reactions

Today…• Atmospheric chemistry is largely driven by free radical

chain reactions• We will discuss some of the important individual chemical

reactions that are important in the troposphere

Why is atmospheric chemistry important?

• Human activity is changing the composition of the atmosphere

• Regulatory policy requires an understanding of pollutant impact

• Atmospheric pollutants impact living organisms– Health– Vegetation (e.g., farming) &

animals– Climate change

• Atmospheric pollutants & their subsequent chemistry are responsible for:– Acid rain– Photochemical smog (e.g.,

arctic haze)– Vegetation & animals– Ozone hole

Atmospheric chemistry and Climate Change

• Atmospheric chemistry plays an important role in radiative forcing processes

Source:IPCC 4th assessment

Tropospheric Oxidation Starts with OH

• Degradation of atmospheric pollutants starts with the OH radical

• OH is often called ‘the detergent of the atmosphere’

• OH is very reactive because it has an unpaired electron:

O-H• Measuring OH is hard! There’s

not much of it, and it doesn’t live for long

• Tropospheric oxidation results in ground level O3, which is a greenhouse gas harmful to health

O3 + h O1D + O2

O1D + M O1D + MO1D + H2O 2OH

FAGE OH detection instrument in Halley Base, Antarctica

See: http://www.atmos.bham.ac.uk/chablis.htm

O3 Photolysis makes OH

O3 + hO2 + O(1D)

OH sinks

OH Sinks: oxidation of reduced speciesCO + OH CO2 + H

CH4 + OH CH3 + H2O

HCFC + OH H2O + …

Major OH sinks

GLOBAL MEAN [OH] ~ 1.0x106 molecules cm-3

Initiation

VOCOH HO2

RO2 RO

NO NO2

NONO2

High NOx

sunlight

O3

Initiation

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

VOC

Initiation

VOCOH HO2

RO2 RO

NO NO2

NONO2

O2

High NOx

Propagation

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

VOC

Ozone Formation

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

O3

O2

sunlight

VOC

Propagation

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

O3

O2

oxidation productVOC

Propagation

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

VOC

Ozone Formation

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O3sunlight

O2

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

O3

oxidation product

O3

VOC

Run Cycle

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3sunlight

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation productVOC

VOCOH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O2

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation productVOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O2

sunlight

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O2

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O3sunlight

O2

VOC

VOCOH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O2

O3

O3

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O2

sunlight

O3

O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O2

O3

O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3O3

O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O3sunlight

O2

O3

O3

VOC

VOCOH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O2

O3

O3

O3

O3

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O3

O3

O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O2

sunlight

O3 O3

O3O3

O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O2

O3

O3

O3

O3O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O3

O3

O3

O3O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O3sunlight

O2

O3

O3

O3O3

O3

VOC

OH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O3

O3

O3O3

O3

O2

VOC

Ozone Production

VOCOH HO2

RO2 RO

NO NO2

NONO2

High NOx

oxidation product

O3

O3

O3

O3O3

O3

Chemistry of ozone formation

VOCoxidation product

OH HO2

RO2 RO

NO NO2

NONO2

O2 O2

sunlight O3O2

sunlight

O2

O3

sunlight

Initiation

OH

sunlight

Low NOx

O3O3O3

Initiation

VOCOH

RO2

Low NOx

O3O3

Termination

VOCOH

RO2

Low NOx

HO2

ROOH

O3O3

General VOC oxidation schemeO3 + h O1D + O2

O1D + H2O 2OH

OH + RH (+O2) RO2 + H2O

RO2 + NO NO2 + RO

RO + O2 HO2 +R’CHO

HO2 + NO OH + NO2

NO2 + h NO + O; O + O2 O3

OVERALLNOx + VOC + sunlight ozone

The same reactions can also lead to formation of secondary organic aerosol (SOA)

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

NOx limitedPO3 [NO] & independent of [RH]

VOC limitedPO3 [NO2]-1; PO3 [RH]

Polluters:

Mobile Transportation: Generates NOx and VOC.Reductions focus on catalytic converters and fuel additives as well as congestion abatement strategies

Stationary industrial sources of VOC and NOx:Reductions involve scrubbing of pollutants from chimney stacks.

Biogenic Emissions:Generate VOCs, no feasible reduction strategy,Can propose urban landscapes that reduce emissions

NOx sources

Spatial distribution of

NOx emissions

NOx sinks & transport

• NOx lifetime ~1 day

• NOx sinks – primarily HNO3

• HNO3 is water soluble

• PAN allows locally produced NOx to be transported on global scales

NO3

NO2 + O3 NO3 + O2

NO3 is rapidly lost in the day by photolysis and reaction with NO ( NO2), 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.

O3

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: O3 reactions with alkenes can act as a source of OH, even at night.

Other oxidizing species

• VOC Lifetime with respect to OH:

• Atmospheric distribution depends on lifetime. The Northern Hemisphere (NH) is a major source of anthropogenic pollutants. CH4 is distributed globally with a slight NH/SH difference. Isoprene is found only close to its sources.

• The oxidising capacity of the atmosphere refers to its capacity to remove VOCs and depends on [OH] (and the concentrations of other oxidants like O3 and NO3

VOC removal by reaction with OH

€

τVOC=1

kOH +VOC [OH]

k(298K) in units of 10-12 cm3 molecule-1 s-1

OH + CH4 7.0 × 10-3

OH + CO 2.4 × 10-1

OH + isoprene 1.1 × 102

OH + ethane 2.4 × 10-1

CH4 + OH (+O2) CH3O2 + H2O

CH3O2 + NO CH3O + NO2

CH3O + O2 HO2 + HCHO

HO2 + NO OH + NO2

HCHO + OH (+O2) HO2 + CO + H2O

HCHO + h H2 + CO

HCHO + h (+2O2) 2HO2 + CO

Note:2 × (NO NO2) conversions

HCHO formation provides a route to HO2 radical formation.

CH4 Oxidation Scheme

Global budget for methane (Tg CH4 yr-1)• Sources:

Natural 160Anthropogenic 375Total 535

Natural Sources:wetlands, termites, oceans…

Anthropogenic Sources: natural gas, coal mines, enteric fermentation, rice paddies

• Sinks:– Trop. oxidation 445

by OH– Transfer to 40

stratosphere – Uptake by soils 30Total 515

Notes:1. The rate of oxidation is k5[CH4][OH], where the concentrations

are averaged over the trop.2. Concentrations of CH4 have increased from 800 to 1700 ppb since pre-industrial

times3. Methane is a greenhouse gas.

HISTORICAL TRENDS IN METHANE

Historical methane trend

Recent methane trend

1180

1200

1220

1240

1260

Jan 1995

Jan 1996

Jan 1997

Jan 1998

Jan 1999

Jan 2000

Jan 2001

Jan 2002

Jan 2003

Jan 2004

Jan 2005

Jan 2006

Jan 2007

Jan 2008

North

ern

hem

isph

ere

back

grou

nd C

H4, µ

g m

-3

Baseline

12 month mean

Recent measurements at Mace Head in W Ireland.1g 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 SH; slow transport across the intertropical conversion zone

General description of a chemical mechanism

• Constructed by University of Leeds, in collaboration with Imperial College and UK Met Office

• Explicit mechanism, based on a protocol which describes the chemistry. Includes reactions of OH, NO3 and O3 and photolysis. For development protocol see: M.E.Jenkin et al. Atmos. Env., 1997, 31, 81.

• Describes the oxidation of 123 VOCs, based on the UK emissions inventory.

• It can be accessed via the web:

http://www.chem.leeds.ac.uk/Atmospheric/MCM/mcmproj.html

• The MCM is used by the UK Department of the Energy and Climate Change (DECC) to help develop its air quality strategy.

Can we model oxidation results of other VOCs? …The MCM