Tropospheric Ozone Chemistry

1
Tropospheric Ozone Chemistry

• David Plummer
• presented at the GCC Summer School
• Montreal, August 7-13, 2003

Outline - Solar radiation and chemistry -
Tropospheric ozone production - Methane oxidation
cycle - Nitrogen species - A look at global
tropospheric ozone - Oxidizing capacity of the
troposphere
2
Ozone in the atmosphere
Timeseries of ozone profiles over Edmonton for
2002. From World Ozone Data Centre
(www.woudc.org)

• 90 of total column O3 is found in the
  stratosphere

3
Solar radiation and chemistry

• the reaction that produces ozone in the
  atmosphere
• O O2 M ? O3 M
• difference between stratospheric and tropospheric
  ozone generation is in the source of atomic O
• for solar radiation with a wavelength of less
  than 242 nm
• O2 hv ? O O

4
Solar spectral actinic flux calculated at 50, 40,
30, 20 and 0 km above the surface. From DeMore
et al., 1997.

• little radiation with wavelengths less than 290
  nm makes it down to the troposphere

5

• photochemical production of O3 in troposphere
  tied to NOx (NO NO2)
• for wavelengths less than 424 nm
• NO2 hv ? NO O
• but NO will react with O3
• NO O3 ? NO2

• cycling has no net effect on ozone

6

• O3-NO-NO2 photochemical steady state
• consider the two reactions just seen

NO2 hv (O2) ? NO O3 J1 NO O3
? NO2 K1

• ignoring other reactions, during daylight this
  forms a fast cycle in steady-state
• dNO2/dt Prod - Loss 0
• K1NOO3 J1NO2
• NO/NO2 J1/K1O3
• partioning of NOx between NO and NO2 has
  important implications for removal of NOx from
  the atmosphere

7

• presence of peroxy radicals, from the oxidation
  of hydrocarbons, disturbs O3-NO-NO2 cycle
• NO HO2 ? NO2 OH
• NO RO2 ? NO2 RO
• leads to net production of ozone

8
The Hydroxyl Radical

• produced from ozone photolysis
• for radiation with wavelengths less than 320 nm
• O3 hv ? O(1D) O2
• followed by

O(1D) M ? O(3P) M (O2?O3) (90) O(1D)
H2O ? 2 OH (10)

• OH initiates the atmospheric oxidation of a wide
  range of compounds in the atmosphere
• referred to as detergent of the atmosphere
• typical concentrations near the surface 106 -
  107cm-3
• very reactive, effectively recycled

9
Oxidation of CO - production of ozone

• CO OH ? CO2 H
• H O2 M ? HO2 M
• NO HO2 ? NO2 OH
• NO2 hv ? NO O
• O O2 M ? O3

• CO 2 O2 hv ? CO2 O3

10
What breaks the cycle?

• cycle terminated by
• OH NO2 ? HNO3
• HO2 HO2 ? H2O2
• both HNO3 and H2O2 will photolyze or react with
  OH to, in effect, reverse these pathways
• but reactions are slow (lifetime of several days)
• both are very soluble - though H2O2 less-so
• washout by precipitation
• dry deposition
• in PBL they are effectively a loss
• situation is more complicated in the upper
  troposphere
• no dry deposition, limited wet removal

11
Methane Oxidation Cycle

• CH4 is simplest alkane species
• features of oxidation cycle common to other
  organic compounds
• long photochemical lifetime
• fairly evenly distributed throughout troposphere
• concentrations 1.8ppmv
• reactions form bedrock of the chemistry in the
  background troposphere

12

• CH4 OH ? CH3 H2O
• CH3 O2 M ? CH3O2 M
• CH3O2 NO ? CH3O NO2
• CH3O O2 ? HCHO HO2
• HO2 NO ? OH NO2
• 2NO2 hv (O2) ? NO O3

• CH4 4 O2 2 hv ? HCHO 2O3 H2O
• HCHO will also undergo further reaction

• HCHO hv ? H2 CO
• ? H HCO
• HCHO OH ? HCO H2O

HCO O2 ? HO2 CO H O2 ? HO2
13
Cycle limiting reactions

• OH NO2 ? HNO3
• HO2 HO2 ? H2O2
• but also
• HO2 CH3O2 ? CH3OOH O2
• methyl hydroperoxide (CH3OOH)
• can photolyze or react with OH with a lifetime of
  2 days
• return radicals to system
• important source of radicals in upper tropical
  troposphere
• moderately soluble and can be removed from
  atmosphere by wet or dry deposition
• loss of radicals

14
Conceptually

• photolysis of ozone most significant source of OH
• atmospheric oxidation of hydrocarbons initiated
  by OH radical
• production of peroxy radicals (HO2, RO2) which
  interact with O3-NO-NO2 cycle to photo-chemically
  produce ozone
• produce carbonyl compounds (aldehydes and
  ketones) which undergo further oxidation
• recycling of OH
• termination by formation of nitric acid (OH NO2
  ? HNO3) or peroxides (H2O2, ROOH)

15
Nitrogen species

• NOx (NO NO2) plays a critical role in the
  atmospheric oxidation of hydrocarbons
• short chemical lifetime
• from 6 hours in PBL to several days to a week
  in the upper troposphere
• large variations in concentration
• from 10s ppbv in urban areas to 10s pptv in
  remote regions (UT and remote MBL)
• gives rise to different chemical regimes

16
Regional Ozone perspective - O3 production

• More accurate to talk of NOx/VOC ratio
• VOC - volatile organic carbon
• High NOx/VOC environments
• OH reaction with NO2 dominates
• NO-NO2 cycling inefficient compared with NOx loss
• only found in urban areas
• Low NOx/VOC environments
• high peroxy radical concentrations
• peroxy radical self-reactions become important
  sink for radicals
• production of H2O2 and ROOH

17
Global perspective

• NOx concentrations almost always low enough that
  ozone production is NOx limited
• globally NOx concentrations control whether local
  chemistry creates or destroys ozone
• for NOx less than 20 pptv, chemistry results
  in net ozone destruction
• no NOx to turn-over the NO-NO2 cycle
• O3 hv ? O(1D) O2
• O(1D) H2O ? 2 OH
• also
• HO2 O3 ? OH 2 O2
• particularly important in tropical marine
  boundary layer

18
Other nitrogen species

• Peroxyacyl nitrates (PANs)
• most important being peroxyacetyl nitrate
• CH3C(O)OONO2
• formed from oxidation of acetaldehyde
• CH3CHO OH ( O2) ? CH3C(O)O2 H2O
• CH3C(O)O2 NO2 M ? CH3C(O)O2NO2 M
• decomposition is strongly temperature dependent
• from 30 minutes at 298K near the surface to
  several months under upper tropospheric
  conditions
• NOx exported from boundary layer to remote
  troposphere in the form of PAN
• observations show PAN is dominant NOy compound in
  northern hemisphere spring troposphere
• insoluble

19
Other nitrogen species

• N2O5
• formed by
• NO2 O3 ? NO3 O2
• NO2 NO3 ? N2O5
• most important is what happens to N2O5
• N2O5 H2O(s) ? 2 HNO3
• during daylight fast photolysis of NO3 limits
  production of N2O5
• NO3 hv ? NO2 O

20

• especially important NOx sink at higher latitudes
  and in winter - particularly northern hemisphere
• OH concentrations much lower

The calculated reduction in NOx and O3 amounts in
the MOZART model with the inclusion of N2O5
hydrolysis. From Tie et al. 2001.
21
NOx Sources

• Estimates of annual global NOx emissions for the
  early 1990s. Units of Tg-N/year.
• Biomass burning includes savannah burning,
  tropical deforestation, temperate wildfires and
  agricultural waste burning
• Soil emission
• enhanced by application of fertilizers
• largest uncertainty is in estimates of canopy
  transmission
• Lightning
• models use 5.0 Tg-N/yr
• scaling up from observations suggest 20 Tg-N/yr

22
An example of gridded NOx emissions
23
Impacts of NOx emission

• by mass, most NOx is emitted at the surface
• chemical impacts of NOx very non-linear
• limited impact in the continental PBL
• high OH and high NO2/NO ratio near surface result
  in a short photo-chemical lifetime
• NOx concentrations are already substantial
• per molecule, impact of NOx much greater in free
  troposphere
• venting to the free troposphere important
• emissions that occur in free troposphere
• aircraft, lightning

24
Global tropospheric ozone

• Seasonal cycle of O3 concentrations at different
  pressure levels, derived from ozonesonde data at
  eight different stations in the northern
  hemisphere. From Logan, J. Geophys. Res.,
  16115-16149, 1999.

• Remote northern stations
• spring-time maximum
• nearer to industrial emissions
• broader maximum stretching through summer

25
O3 at the surface

• Seasonal cycle of O3 concentrations at the
  surface for different rural locations in the
  United States.
• From Logan, J. Geophys. Res., 16115-16149, 1999.

• Surface sites in industrialized regions show an
  even more pronounced summer-time peak

26
Global distribution

• Spatial distribution of climatological O3
  concentrations at 1000hPa.
• From Logan, J. Geophys. Res., 16115-16149, 1999.

• constructed from surface observations,
  ozonesondes and a bit of intuition
• note very low concentrations over tropical
  Pacific ocean

27
Measurements from satellite

• Data from asd-www.larc.nasa.gov/TOR/data.html
• See Fishman et al., Atmos. Chem. Phys., 3,
  893-907, 2003.
• Tropospheric residual method
• total column (from TOMS) - stratospheric column
  (SBUV)

28
Tropospheric ozone budget

• derived from models
• a typical budget for present-day conditions

From Lelieveld and Dentener, J. Geophys. Res.,
3531-3551, 105, 2000
29
Range of model predictions

• all global models compared to available
  measurements
• comparisons becoming more sophisticated
• all show believable ozone
• budgets show large spread in individual terms

Adopted from von Kuhlmann et al., J. Geophys.
Res., in press, 2003.
30
Future concerns

• How much have emissions of precursors perturbed
  ozone already?
• Ozone is reactive
• no ice-core records
• some re-constructed records
• Montsouris measurements suggested surface O3 was
  10 ppbv
• other information from model simulations
• emissions, particularly biomass burning, hard to
  quantify
• suggest tropospheric ozone burden has increased
  between 25 and 60 since pre-industrial

31
The more recent past

• Statistically significant negative trends of 1-2
  per year found at several stations in Canada for
  1980-1993 (Tarasick et al., Geophys. Res. Lett.,
  409-412, 22, 1995)
• trends at most other stations in NH ambiguous

• Monthly averaged O3 concentration between 630 and
  400 hPa from 9 ozonesonde stations located
  between 36 and 59N. From Logan et al. J.
  Geophys. Res., 104, 26373-26399, 1999.

32
IPCC OxComp simulations for 2100

• Emissions for year 2100 were a bit of a worst
  case scenario
• CH4 4.3 ppmv NOx 110 Tg-N/yr (32.5)
• CO 2500 Tg/yr (1050) VOC 350 Tg/yr (150)
• mid-latitude O3 increases by 20-30 ppbv at the
  surface
• puts background O3 in 60-70 ppbv range
• these models did not include impacts of global
  warming
• increased H2O vapour
• temperature effects on reaction rates
• increasingly coupled models
• inclusion of biosphere-atmosphere interactions
• lightning

33
Stability of global OH

• OH originates with O3
• very reactive and very short-lived
• recycling critically important
• OH is responsible for initiating atmospheric
  oxidation of hydrocarbons
• CH4 lifetime of 10 years
• are changes in chemical composition of the
  troposphere affecting average OH?

34
Information from methyl chloroform

• CH3CCl3 used as solvent by industry
• atmospheric lifetime of 5-6 years
• main loss by reaction with OH
• some entered stratosphere and enhanced Cl levels
• banned under Montreal protocol
• use was to stop in 1996 in developed countries
• assuming one knows the sources of MCF, it is
  possible to calculate an average global OH by
  fitting to observed decay

35

• Observed MCF concentrations at Barbados.
  Vertical bars represent the monthly standard
  deviations. Different colour symbols represent
  measurements made as part of different networks.
  See Prinn et al., J. Geophys. Res., 105,
  17751-17792, 2000.

36

• Global average OH determined from fitting to
  observed MCF concentrations over 3 and 5 year
  periods and as a second-order polynomial. From
  Krol and Lelieveld, J. Geophys. Res., in press,
  2002.

• Minor changes in the time profile of emissions
  can give constant OH
• banking of MCF in early 1990s
• release in late 1990s
• aircraft observations of plumes of MCF in 2000
  over Europe