Atmospheric chemistry

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Atmospheric chemistry

• Day 4
• Air pollution
• Regional ozone formation

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Regional air quality ozone formation

• Ozone is a greenhouse gas. It affects human
  health, plant growth and materials
• Ozone is a secondary pollutant and is not
  directly emitted.
• Emission of VOCs and NOx, coupled with sunlight
  leads to the formation of photochemical smog.
• Major component is ozone. Also aerosols, nitrates
  
• Need to understand chemical mechanism for
  formation in order to develop strategies and
  legislation for reduction of ozone
  concentrations.
• The European limit values are linked to these
  aims
• Is it better to control NOx or VOCs or both?

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Chemical mechanism

• Initiation OH formed from ozone photolysis at a
  rate POH ( 2k3H2OJ1O3/k2M k3H2O )
• Propagation
• OH RH (O2) ? RO2 H2O (R4)
• RO2 NO ? RO NO2 (R5)
• RO O2 ? RCHO HO2 (R6)
• HO2 NO ? OH NO2 (R7)
• Termination
• HO2 HO2 ? H2O2 (R8)
• OH NO2 M ? HNO3 M (R9)
• Ozone formation
• O3 is formed by NO2 photolysis with a rate equal
  to the sum of the rates of reactions 5 and 7 (
  v5 v7)

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NOx and VOC control of ozone formation

• Under polluted conditions, chain propagation is
  fast,
• so v4 v5 v6 v7
• PO3 v5 v7 2v7 2k7HO2NO A
• Also v4 v7 ? OH k7HO2NO/k4RH B
• Steady state for radicals rate of termination
  rate of initiation, ie POH v8 v9
• Low NOx v8 gtgt v9 POH 2k8HO22 HO2
  ?(POH/2k8)
• Sub in A PO3 2k7NO ?(POH/2k8).
• ( PO3 ? NO, independent RH NOx limited)
• 2. High NOx v8 ltlt v9 OH POH/(k9NO2M
• Sub in B HO2 POHk4RH/k7k9NONO2M
• Sub in A PO3 2k4RH/k9NO2M
• ( PO3 ? NO2-1 RH) VOC limited)

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DEPENDENCE OF OZONE PRODUCTION ON NOx AND
HYDROCARBONS
O3
HOxfamily
NO
RO2
RO
5
RH
O2
4
6
PHOx
7
O3
OH
HO2
NO
NO2
9
8
HNO3
H2O2
O3
NOx- saturated or hydrocarbon-limited regime
NOx-limited regime
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OZONE CONCENTRATIONS vs. NOx AND VOC
EMISSIONSAir pollution model calculation for a
typical urban airshed
Ridge
NOx-limited
NOx- saturated
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Can we determine the relative contributions of
different VOCs to ozone formation?Master
chemical mechanism (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, 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.
• The MCM is set up to provide input directly to
  the FACSIMILE integrator.
• It can be accessed via the web
• (http//www.chem.leeds.ac.uk/Atmospheric/MCM/mcmp
  roj.html)
• The MCM is used by Department of the Environment
  Food and Rural Affairs (DEFRA) to help develop
  its air quality strategy.

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Master chemical mechanism (MCM) A specific,
explicit implementation (http//Mcm.leeds.ac.uk/M
CM
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Navigational Features Extract
Use Mark List as primary species

• Choose output format
• HTML
• FACSIMILE
• FORTRAN
• XML
• KPP

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Navigational Features Extract Listing
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Navigational Features Source information
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• Mechanism testing using chamber experiments

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Developing and testing the MCM using chamber
experiments

• Double outdoor chambers at Valencia, Spain.
• Carry out experiments under atmospheric
  conditions, but under defined conditions.
• Heavily instrumented. Measure NOx, O3, VOCs,
  oxygenates, CO, particles, radicals (OH, HO2) vs
  time.
• Applications
• Biogenics pinenes
• aromatics

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Photo-oxidation of a-pinene / NOX gas-phase
simulation
a-pinene0 97 ppb NO0 9.7 ppb NO20
0.85 ppb Jenkin OSOA project
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Comparison of MCM3.1 to Toluene Chamber
Experiment (27/09/01)
Also possible to measure radicals OH, HO2.
Provides A sensitive test of the
mechanisms The discrepancies show that there
are significant deficiencies in the
mechanism especially related to radical
formation C. Bloss et al Atmospheric Chemistry
Physics, 2005, 5, 623 639.
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Photochemical ozone creation potentials (POCPs)

• Is there a way in which we can quantify the
  differential impact of different VOCs on ozone
  formation?
• The UK DEFRA uses POCPs to assess differences
  between VOCs and hence to develop policy.
• The method is based on the use of a photochemical
  trajectory model (PTM), in which the chemical
  evolution of an air parcel is followed as it
  travels, under anticyclonic conditions, from
  central Europe to the UK, over a period of 5
  days.
• Details
• air parcel extends from surface to top of
  boundary layer. It is 10kmx10km (horizontal
  dimensions) and has a height,h, of 300 m at
  06.00 h, rising to 1300m at 14.00h maintained at
  1300 m till early evening, then 300 m again.
• Rate equation
• dCi/dt Si Li(Ci )-viCi/h - wiCi/h
  -wv(Ci-Ci0)/h

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POCP II

• Emissions (VOCs and NOx) estimates utilise 3
  emissions inventories, UN ECE EMEP EC CORINAIR
  and UKNAEI. These give total VOC emissions, which
  are speciated into 135 organic compounds
  methane, using the UK emissions inventory.
• The master chemical mechanism is used to describe
  the chemistry and photochemistry.
• The coupled differential equations are integrated
  using the FACSIMILE integrator. Most
  concentrations are set initially to zero, except
  for NO, NO2, SO2, CO, methane, HCHO, ozone and
  hydrogen.
• The air parcel is carried on a straight line
  trajectory at 4 m s-1

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Calculation of POCP valuesPhotochemical
Trajectory Model (PTM)
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POCP III( see Derwent et al , Atmos Environment,
1996, 30, 181-199)

• The POCP is calculated by incrementing the
  emissions of each of the VOCs in turn by 4.7 kg
  km-2 across the entire domain. (corresponds to an
  increase in total VOC emissions of 4)
• The ozone formed over the 5 day trajectory is
  increased as a result and by different amounts
  for each VOC. The POCP of the ith VOC is given
  by
• POCPi 100x(ozone increment with the ith
  VOC)
• (ozone increment with C2H4)
• Examples (ethene 100)
• methane 3 ethane 14, propane 41, butane
  60
• isoprene 118
• benzene 33 toluene 77 m-xylene 109
  1,2,4 TMB 130

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MCM v3 POCP values
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Global budget for ozone (Tg O3 yr-1)

• Chemical production 3000 5000
• HO2 NO 70
• CH3O2 NO 20
• RO2 NO 10
• Transport from stratosphere 400 1100
• Chemical loss 3000 4200
• O1D H2O 40
• HO2 O3 40
• OH O3 10
• others 10
• Dry deposition 500 - 1500

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GLOBAL BUDGET OF TROPOSPHERIC OZONE recent
calculations
GEOS-CHEM model budget terms, Tg O3 yr-1
Chem prod in troposphere 4920 Chem loss in troposphere 4230
Transport from stratosphere 475 Deposition 1165
O2
hn
O3
STRATOSPHERE
8-18 km
TROPOSPHERE
hn
NO2
NO
O3
hn, H2O
OH
HO2
H2O2
Deposition
CO, VOC
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Quantifying emissions of natural VOCs using HCHO
column observations from space
GOME
Paul I. Palmer
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HCHO columns July 1996
BIOGENIC ISOPRENE IS THE MAIN SOURCE OF HCHO IN
U.S. IN SUMMER
GOME footprint 320X40 km2
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Cumulative HCHO yield per C atom from isoprene
oxidation. (O3 40 ppb, CO 100 ppb,
isoprene 1ppb. CO, NOx, O3 held constant.)

• Full MCM mechanism.
• Final yield increased from GEOS-CHEM by 16 for
  high NOx, 65 low NOx

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HCHO formation from a pineneacetone, which has
a long atmospheric lifetime, is an intermediate
in HCHO formation
Decay of a pinene
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Relating HCHO Columns to VOC Emissions (Palmer)
Master Chemical Mechanism
Ultimate Yield Y (per C) Approx. Time to Y
isoprene 0.5 2-3 hrs
? pinene 0.3 3-4 days
? pinene 0.25 3-4 days
MBO 0.4 3-4 days