atmospheric chemistry lecture 3: tropospheric oxidation chemistry dr. david glowacki university of...
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Atmospheric chemistry
Lecture 3:
Tropospheric Oxidation Chemistry
Dr. David GlowackiUniversity of Bristol,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 + M
O1D + 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 species
CO + 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
O3
O3O3
Initiation
VOCOH
RO2
Low NOx
O3O3
Termination
VOCOH
RO2
Low NOx
HO2
ROOH
O3O3
General VOC oxidation scheme
O3 + 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
OVERALL
NOx + 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 160
Anthropogenic 375
Total 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 30
Total 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
No
rth
ern
hem
isp
her
e b
ackg
rou
nd
CH
4, 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