Atmospheric chemistry

Lecture 3: Tropospheric Oxidation Chemistry Dr. David Glowacki University of Bristol,UK [email protected]

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 4 th 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 O 3 , which is a greenhouse gas harmful to health O O 3 1 O D + M 1 + h   D + H 2  O O 1 D + O 2 O  1 D + M 2OH FAGE OH detection instrument in Halley Base, Antarctica See: http://www.atmos.bham.ac.uk/chablis.htm

O

3

Photolysis makes OH

O 3 + h  g O 2 + O( 1 D)

OH sinks

OH Sinks: oxidation of reduced species

CO + OH

g

CO 2 + H Major OH sinks CH 4 + OH

g

CH 3 + H 2 O HCFC + OH

g

H 2 O + … GLOBAL MEAN [OH] ~ 1.0x10

6 molecules cm -3

High NO

x

Initiation

sunlight

O 3

NO 2 NO OH VOC HO 2 RO 2 RO NO NO 2

High NO

x

Initiation

VOC NO 2 NO

OH

RO 2 NO NO 2 HO 2 RO

High NO

x

Initiation

NO 2 NO OH

VOC O 2 RO 2

NO NO 2 HO 2 RO

High NO

x

Propagation

VOC NO 2 NO OH RO 2 NO

NO 2

HO 2

RO

High NO

x

Ozone Formation

VOC NO 2 NO OH HO 2 RO 2

O 3 O 2 NO

NO 2 sunlight

RO

High NO

x

Propagation

VOC NO 2 NO OH

HO 2

RO 2 O 3 NO NO 2 RO

O 2 oxidation product

High NO

x

Propagation

VOC

NO 2

NO

OH

HO 2 RO 2 O 3 NO NO 2 RO oxidation product

High NO

x

Ozone Formation

OH

sunlight

NO O 2 O 3

NO 2 HO 2 VOC RO 2 O 3 NO NO 2 RO oxidation product

High NO

x VOC OH NO 2 NO O 3 HO 2 RO 2 O 3 NO NO 2 RO oxidation product

High NO

x

Run Cycle

O 3 NO 2 NO OH HO 2 VOC RO 2 RO NO NO 2 oxidation product

High NO

x sunlight O 3 VOC OH RO 2 NO 2 NO NO NO 2 HO 2 RO oxidation product

High NO

x VOC NO 2 NO

OH

RO 2 NO NO 2 HO 2 RO oxidation product

High NO

x NO 2 NO OH

VOC O 2 RO 2

NO NO 2 HO 2 RO oxidation product

High NO

x VOC NO 2 NO OH RO 2 NO

NO 2

HO 2

RO

oxidation product

High NO

x VOC NO 2 NO OH HO 2 RO 2

O 3 O 2 NO

NO 2 sunlight

RO

oxidation product

High NO

x VOC NO 2 NO OH

HO 2

RO 2 O 3 NO NO 2 RO

O 2 oxidation product

High NO

x VOC

NO 2

NO

OH

HO 2 RO 2 O 3 NO NO 2 RO oxidation product

High NO

x VOC

OH

sunlight

NO O 2 O 3

NO 2 HO 2 RO 2 O 3 NO NO 2 RO oxidation product

High NO

x OH NO 2 NO O 3 HO 2

VOC O 2 RO 2

O 3 NO NO 2 RO oxidation product

High NO

x VOC OH NO 2 NO O 3 HO 2 RO 2 O 3 NO

NO 2 RO

oxidation product

High NO

x VOC OH NO 2 NO O 3 HO 2 RO 2

NO O 2

O 3

O 3

NO 2 sunlight

RO

oxidation product

High NO

x VOC OH NO 2 NO O 3

HO 2

RO 2 NO O 3 O 3 NO 2 RO

O 2 oxidation product

High NO

x VOC

OH NO 2

NO O 3 HO 2 RO 2 NO O 3 O 3 NO 2 RO oxidation product

High NO

x VOC

OH

sunlight

NO O 2

O 3

O 3

NO 2 HO 2 RO 2 NO O 3 O 3 NO 2 RO oxidation product

High NO

x OH NO 2 NO O 3 O 3 HO 2

VOC O 2 RO 2

NO O 3 O 3 NO 2 RO oxidation product

High NO

x VOC OH NO 2 NO O 3 O 3 HO 2 RO 2 NO O 3 O 3

NO 2 RO

oxidation product

High NO

x VOC OH NO 2 NO O 3 O 3 HO 2 RO 2

NO O 2

O 3

O 3

O 3 NO 2 sunlight

RO

oxidation product

High NO

x VOC OH NO 2 NO O 3 O 3

HO 2

RO 2 NO O 3 O 3 O 3 NO 2 RO

O 2 oxidation product

High NO

x VOC

OH NO 2

NO O 3 O 3 HO 2 RO 2 NO O 3 O 3 O 3 NO 2 RO oxidation product

High NO

x

OH

sunlight

NO O 2

O 3

O 3

O 3 NO 2 HO 2 VOC RO 2 NO O 3 O 3 O 3 NO 2 RO oxidation product

High NO

x OH NO 2 NO O 3 O 3 O 3 HO 2 VOC

O 2

RO 2 NO O 3 O 3 O 3 NO 2 RO oxidation product

High NO

x

Ozone Production

NO O 3 O 3 O 3 NO 2 OH HO 2 VOC oxidation product RO 2 RO NO NO 2 O 3 O 3 O 3

Chemistry of ozone formation

sunlight OH sunlight NO

O 2

O 3 NO 2 HO 2 VOC

O 2

RO 2 O 3

O 2

NO NO 2 sunlight RO

O 2

oxidation product

Low NO

x O 3 O 3

O 3

sunlight

Initiation

OH

Low NO

x O 3 O 3

Initiation

VOC

OH

RO 2

Low NO

x

Termination

O 3 O 3 OH VOC HO 2 RO 2

ROOH

General VOC oxidation scheme O 3 + h   O 1 D + H 2 O O 1 D + O 2  2OH OH + RH (+O 2 )  RO 2 + NO  RO NO 2 2 + H + RO 2 O RO + O HO 2 + 2  NO HO 2  +R’CHO 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)

OZONE CONCENTRATIONS vs. NO x AND VOC EMISSIONS Air pollution model calculation for a typical urban airshed P O3 

NOx limited

[NO] & independent of [RH] P O3 

VOC limited

[NO 2 ] -1 ; P O3  [RH]

Polluters: Mobile Transportation: Generates NO x and VOC.

Reductions focus on catalytic converters and fuel additives as well as congestion abatement strategies Stationary industrial sources of VOC and NO x : Reductions involve scrubbing of pollutants from chimney stacks.

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

NO x sources

Spatial distribution of NOx emissions

NO x sinks & transport • NO x lifetime ~1 day • NO x sinks HNO 3 – primarily • HNO 3 is water soluble • PAN allows locally produced NO x transported on global scales to be

Other oxidizing species NO 3 NO 2 + O 3  NO 3 + O 2 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 can act as a source of OH, even at night. reactions with alkenes

VOC removal by reaction with OH • VOC Lifetime with respect to OH: 

VOC

 1

k OH



VOC

[

OH

] 10 k(298K) in units of -12 cm 3 molecule -1 s -1 OH + CH 4 OH + CO  OH + isoprene OH + ethane 7.0 × 2.4 × 1.1 × 2.4 × 10 -3 10 -1 10 2 10 -1 • Atmospheric distribution depends on lifetime. The Northern Hemisphere (NH) is a major source of anthropogenic pollutants. CH 4 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 O 3 and NO 3

CH 4 Oxidation Scheme CH 4 CH 3 + O 2 OH (+O 2 )  + NO  CH 3 O + O 2  CH HO CH 2 3 3 O O 2 + H + NO 2 + HCHO 2 O HO 2 + NO  OH + NO 2 HCHO + OH (+O2)  HO 2 + CO + H 2 O HCHO + h HCHO + h    (+2O 2 )  H 2 + CO 2HO 2 + CO Note: 2 × (NO  NO 2 ) conversions HCHO formation provides a route to HO 2 radical

formation

.

Global budget for methane (Tg CH 4 yr -1 ) •

Sources:

Natural Anthropogenic 160 375

Total 535

Natural Sources: wetlands, termites, oceans… Anthropogenic Sources: natural gas, coal mines, enteric fermentation, rice paddies •

Sinks:

– Trop. oxidation by OH – Transfer to stratosphere – Uptake by soils

Total

445 40 30

515

Notes: 1. The rate of oxidation is k 5 [CH 4 ][OH], where the concentrations are averaged over the trop.

2. Concentrations of CH times 4 have increased from 800 to 1700 ppb since pre-industrial 3. Methane is a greenhouse gas.

HISTORICAL TRENDS IN METHANE 1260 1240 1220 1200 1180

Historical methane trend Recent methane trend

Recent measurements at Mace Head in W Ireland.

1 m g m -3 = 0.65 ppb Baseline 12 month mean 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

Can we model oxidation results of other VOCs? …The MCM • 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, NO 1997,

31

, 81.

emissions inventory.

• It can be accessed via the web: 3 and O 3 and photolysis. For development protocol see: M.E.Jenkin et al. Atmos. Env., • Describes the oxidation of 123 VOCs, based on the UK 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.