Tropospheric Ozone Chemistry

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					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
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
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
     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
• 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
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
              d[NO2]/dt = Prod - Loss = 0
                 K1[NO][O3] = J1[NO2]
                [NO]/[NO2] = J1/K1[O3]
• partioning of NOx between NO and NO2 has
  important implications for removal of NOx from
  the atmosphere
• 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
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
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
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
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
               CH4 + OH·  CH3· + H2O
             CH3· + O2 + M  CH3O2· + M
             CH3O2· + NO  CH3O· + NO2
              CH3O· +O2  HCHO + HO2·
               HO2· + NO  OH· + NO2
             2{NO2 + 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·
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
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)
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
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
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
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
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
  – 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.
NOx Sources
            Technological                       23 - 27
            Aircraft                                 0.5
            Biomass burning                    7.0 - 8.0
            Soils                             5.0 - 12.0
            Lightning                         3.0 - 20.0
 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
An example of gridded NOx emissions
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
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
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
 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
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)
Tropospheric ozone budget
 • derived from models
    – a typical budget for present-day conditions:
           Strat-trop exchange          565 Tg/yr
           Photochemical prod.         3314 Tg/yr
           Photochemical dest.         3174 Tg/yr
           Deposition                   705 Tg/yr
           Tropospheric burden             347 Tg

 From Lelieveld and Dentener, J. Geophys. Res., 3531-3551, 105, 2000
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
             Strat-trop exchange       300 - 1100 Tg/yr
             Photochemical prod.      3000 - 5000 Tg/yr
             Photochemical dest.      2500 - 4300 Tg/yr
                Net chemistry        -500  +600 Tg/yr
             Deposition                500 - 1200 Tg/yr
             Tropospheric burden           280 - 400 Tg

Adopted from von Kuhlmann et al., J. Geophys. Res., in press, 2003.
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
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.
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
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?
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
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.
  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