Transcript 3280 – Atmospheric chemistry

Atmospheric chemistry
Day 3
Tropospheric chemistry
Global tropospheric chemistry
Questions to be addressed:
1. Many organic compounds emitted to the atmosphere
are oxidised, eventually forming CO2 and H2O. What
determines the oxidising capacity of the atmosphere?
2. Methane is a greenhouse gas, whose atmospheric
concentration has more than doubled since the
industrial revolution. What governs it concentration?
3. Tropospheric oxidation is strongly influenced by NOx,
whose lifetime is ~ 1 day. How is NOx transported to
regions with no NOx emissions?
4. Ozone is a secondary pollutant. In the boundary layer it
affects human health, growth of vegetation and
materials. It is also a greenhouse gas. What governs its
concentration?
Methane oxidation
CH4 + OH
CH3O2 +
CH3O +
HO2 +
(+O2)  CH3O2 + H2O
NO  CH3O + NO2
O2  HO2 + HCHO
NO  OH + NO2
HCHO + OH (+O2)  HO2 + CO + H2O
HCHO + hn  H2 + CO
HCHO + hn (+2O2)  2HO2 + CO
Note:
2 x(NO  NO2) conversions
HCHO formation provides a route to radical formation.
General oxidation scheme for VOCs
O3 + hn  O1D + O2
O1D + H2O  2OH
OH + RH (+O2)  RO2 + H2O
RO2 + NO  NO2 + RO
RO  HO2 (+R’CHO)
HO2 + NO  OH + NO2
NO2 + hn  NO + O; O + O2  O3
OVERALL
NOx + VOC + sunlight  ozone
The same reactions can also lead to formation of
secondary organic aerosol (SOA)
THE OH RADICAL: MAIN TROPOSPHERIC OXIDANT
Primary source:
O3 + hn g O2 + O(1D)
(1)
O(1D) + M g O + M
(2)
O(1D) + H2O g 2OH
(3)
Sink: oxidation of reduced species
CO + OH g
CO2 + H
CH4 + OH g CH3 + H2O
HCFC + OH g H2O + …
GLOBAL MEAN [OH] ~ 1.0x106 molecules cm-3
Major
OH sinks
Other oxidising species
NO3
NO2 + O3  NO3 + O2
NO2 + NO3 + M N2O5
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.
Removal of organic compounds from the atmosphere by reaction
with OH
• Lifetime = 1/{k[OH]} = 1/{1x106k} s
Rate coefficients at 298 K/10-12 cm3 molecule-1 s-1
CH4: 7x10-3; CO: 0.24; isoprene: 110; ethane: 0.25
• Atmospheric distribution depends on lifetime. NH is
major source of anthropogenic pollutants. CH4 is
distributed globally with only a slight NH/SH difference.
Isoprene is found only close to its (very extensive)
sources.
• The oxidising capacity of the atmosphere refers to its
capacity to remove VOCs and depends on [OH] (and the
concentrations of other oxidants – O3, NO3
Global budget for methane (Tg CH4 yr-1)
• Sources:
• Sinks:
Natural
160
– Trop. oxidation
Anthropogenic
375
by OH
Total
535
– Transfer to
Natural Sources:
stratosphere
wetlands, termites, oceans…
– Uptake by soils
Anthropogenic Sources:
Total
natural gas, coal mines, enteric
fermentation, rice paddies,
445
40
30
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 preindustrial times
3. Methane is a greenhouse gas.
HISTORICAL TRENDS IN METHANE
Historical methane trend
Northern hemisphere background CH4, µg m -3
Recent methane trend
1260
1240
Baseline
1220
12 month mean
1200
1180
Recent measurements at Mace
Head in W Ireland.
1mg m-3 = 0.65 ppb
NB – seasonal variation –
higher in winter
Jan 2008
Jan 2007
Jan 2006
Jan 2005
Jan 2004
Jan 2003
Jan 2002
Jan 2001
Jan 2000
Jan 1999
Jan 1998
Jan 1997
Jan 1996
Jan 1995
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 ITCZ.
GLOBAL BUDGET OF CO
GLOBAL DISTRIBUTION OF CO
NOAA/CMDL surface air measurements
• Compare CH4. What are the differences and why?
(Rate coefficients at 298 K/10-12 cm3 molecule-1 s-1:
CH4: 7x10-3;
CO: 0.24)
Global VOC emissions (Tg yr-1)
Anthropogenic:
fuel production and distribution 17; fuel consumption 49; road
transport 36; chemical industry 2; solvents 20; waste burning 8,
other 10. Total 142 Tg yr-1
Biogenic:
isoprene 503; monoterpenes 127; other reactive VOCs 260,
unreactive VOCs 260; Total 1150 Tg yr-1
Typical atmospheric lifetimes (for [OH] = 1x106 molecule cm-3)
t = 1/k[OH]
CH4
6 yr
isoprene
2.7 h
CO
48 days
ethane
46 days
benzene
6 days
ethene
30 h
Global budget for NOx
• Global sources (Tg N yr-1):
Fossil fuel combustion
21;
Biomass burning:
12
Soils
6
Lightning
3
Ammonia oxidation
3
Aircraft
0.5
Transport from strat
0.1
• Coupling (rapid - ~ 1 minute in the day
NO + O3 → NO2 + O2
NO2 + Light → NO + O; O + O2 + M → O3 + M
Also HO2 + NO → NO2 + OH
• Loss
OH + NO2 + M → HNO3 + M
Rainout of HNO3
• Lifetime of NOx is about 1 day. NOx is a key component in ozone
formation. Can it be transported to regions where it is not strongly
emitted?
PEROXYACETYLNITRATE (PAN) AS RESERVOIR FOR
LONG-RANGE TRANSPORT OF NOx
3000
2500
2000
Other transport &
machinery
Road transport
1500
Domestic
Industry
1000
Public power
500
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1985
1980
1975
0
1970
NOx emission (ktonnes)
Other
Recent road transport
data for the UK
NO2 as an air pollutant.
UK NOx emissions,
1970 - 2000
Spatial distribution
of NOx emissions
EU Air quality objectives – limit values
legally binding
NO2
1 hour mean
200 mg m-3 (105 ppb); (not to be exceeded >18 times per year)
annual mean
40 mg m-3 (21 ppb)
To be achieved by 2010. Possible to apply for derogation
to 2015
Maps of annual mean background NO2 concentrations
UK 2001
UK 2010
Key AQ objective is annual mean of
40 mg m-3 to be achieved by
2010 (EU Directive)
Annual mean NO2 concentrations, London
London 1999
1407 road links out of 1888
exceed 40 mg m-3
London 2010
670 road links out of 1888
exceed 40 mg m-3
Hungarian air quality network
http://www.kvvm.hu/olm/index.php
NO2 in Budapest
and Hungary in
2005
MAPPING OF TROPOSPHERIC NO2
FROM THE GOME SATELLITE INSTRUMENT (July 1996)
Martin et al. [2002]
Global budget for ozone (Tg O3 yr-1)
• Ozone is a secondary pollutant and is not directly omitted.
• Chemical production
3000 – 4600
HO2 + NO
CH3O2 + NO
RO2 + NO
• Transport from stratosphere 400 – 1100
• Chemical loss
O1 D + H 2 O
HO2 + O3
OH + O3
others
• Dry deposition
70%
20%
10%
3000 – 4200
40%
40%
10%
10%
500 - 1500
• Ozone is a greenhouse gas. It affects human health, plant growth
and materials
GLOBAL BUDGET OF TROPOSPHERIC OZONE
GEOS-CHEM model budget terms, Tg O3 yr-1
O2
hn
O3
STRATOSPHERE
8-18 km
Chem prod in
troposphere
4920
Chem loss in
troposphere
4230
Transport
from
stratosphere
475
Deposition
1165
TROPOSPHERE
hn
O3
Deposition
NO2
NO
OH
HO2
hn, H2O
CO, VOC
H2O2
01/12/2006
01/11/2006
01/10/2006
01/09/2006
01/08/2006
01/07/2006
01/06/2006
01/05/2006
01/04/2006
01/03/2006
01/02/2006
01/01/2006
O3, ug/m3
Sources of ozone in W Ireland
120
100
80
Europe-regional
North America
60
Asia
Europe-intercontinental
40
Extra-continental
Stratosphere
20
0