sensitivity of ozone formation to photons
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Sensitivity of Ozone Formation To Photons. Sasha Madronich National Center for Atmospheric Research Boulder Colorado USA Mexico City, 14 August 2009. Tropospheric Ozone Formation:. - PowerPoint PPT PresentationTRANSCRIPT
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Sensitivity of Ozone FormationTo Photons
Sasha MadronichNational Center for Atmospheric ResearchBoulder Colorado USA
Mexico City, 14 August 2009
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Tropospheric Ozone Formation:
Urban ozone is generated when air containing hydrocarbons and nitrogen oxides is exposed to ultraviolet radiation
- Haagen-Smit (1950s)
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3
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Mexico City’s O3 Production is VOC-limited, NOx-inhibited
WRF-Chem --- sensitivity studies ● observations CAMx --- sensitivity studies
Tie et al., 2007 Lei et al., 2007
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NOx-VOC Regimes NOx-limited
Very low NOx:O3 ~ J0.5 [NOx]
VOC-limited
NOx-inhibited
Very high NOx:O3 ~ J [VOC] / [NOx]
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Sensitivity (%/%) of O3 in Mexico City
NCAR Master Mechanism box model
Madronich, unpubl..
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O3 production is always PHOTON-LIMITED
Radiative transfer modeling ok for ideal conditions: cloud-free, pollution-free
Large uncertainties for realistic conditions, not well modeled
Affected by long term trends in aerosols, absorbing gases, clouds
Few comprehensive studies on photon-limitation
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Quantifying Photolysis Processes
Photolysis reaction: AB + hn A + B
Photolysis frequency (s-1) J = l F(l) s(l) f(l) dl
(other names: photo-dissociation rate coefficient, J-value)
Photolysis rates:
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CALCULATION OF PHOTOLYSIS COEFFICIENTS
J (s-1) = l F(l) s(l) f(l) dl
F(l) = spectral actinic flux, quanta cm-2 s-1 nm-1
probability of photon near molecule.
s(l) = absorption cross section, cm2 molec-1
probability that photon is absorbed.
f(l) = photodissociation quantum yield, molec quanta-1
probability that absorbed photon causes dissociation.
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Solar Spectrum
UNEP, 2002
O2 and O3 absorball UV-C (l<280 nm)before it reaches the troposphere
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Spectral Region ForTropospheric Photochemistry
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
280 300 320 340 360 380 400 420
Wavelength, nm
dJ/d
l (r
el) O3->O2+O1D
NO2->NO+OH2O2->2OHHONO->HO+NOCH2O->H+HCO
surface, overhead sunMadronich, unpubl..
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Typical Vertical Optical Depths, t
0.01
0.1
1
10
100
280 300 320 340 360 380 400 420
Wavelength, nm
Opt
ical
dep
th O3 (300 DU)RayleighAerosol (25 km)Cloud (32)
Direct transmission = exp(-t)Diffuse transmission can be much larger
Madronich, unpubl..
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Effect of Pollutants on UV Irradiance
-30
-20
-10
0300 320 340 360
Wavelength, nmU
V re
duct
ion,
% SO2 30 ppbhaze 25kmNO2 50 ppbO3 120ppb
Model calculations for 21 June, 35 N, noon, pollutants distributed over a 1 km boundary layer
Madronich, unpubl..
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UV Actinic Flux Reduction Slower Photochemistry
300 320 340 360 380 4000
100000000000000
200000000000000
300000000000000
obs
tuv-clean
tuv-polluted
Wavelength, nm
Qua
nta
cm-2
s-1
nm
-118 March 16:55 LTsolar zenith angle = 65o
Madronich, Shetter, Halls, Lefer, AGU’07
Mexico City (T1)
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JNO2 Observed/Model_cleanMarch 2006 T1 supersite
thin curves = individual daysthick blue curve = average
Madronich, unpubl..
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Aerosol Impacts on Photochemistry
Outside Mexico City (Tres Marias) 15 April 94
6 9 12 15 18
Local time, hrs.
Mexico City 11 Feb 94
0.E+00
2.E-03
4.E-03
6.E-03
8.E-03
1.E-02
6 9 12 15 18
Local time, hrs
J NO
2, s-1
JNO2_expcleanwo=0.95wo=0.80
Castro et al. 2001
NO2 + hn NO + O (at surface)
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O3 Suppression from Aerosol (Mexico City)
Castro et al. 2001
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Vertical Structure of Aerosol EffectsNO2 Photolysis Frequency
19N, April, noon, AOD = 1 at 380 nm
0
0.5
1
1.5
2
5.0E-03 1.0E-02 1.5E-02
JNO2, s-1
z, k
m
clean
purelyscattering
moderately absorbing(wo=0.8)
Castro et al. 2001
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Aerosol Single Scattering AlbedoMexico City
UV-MFRSR (T1)AERONET (T1)Barnard et al. (CENICA)
Corr et al., 2009
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DIURNAL CYCLE OF AEROSOL OPTICS550 nm
Paredes-Miranda et al., 2008
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Clouds
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UNIFORM CLOUD LAYER
• Above cloud: - high radiation because of reflection
• Below cloud: - lower radiation because of attenuation by cloud
• Inside cloud: - complicated behavior– Top half: very high values (for high sun)– Bottom half: lower values
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EFFECT OF UNIFORM CLOUDS ON ACTINIC FLUX
340 nm, sza = 0 deg., cloud between 4 and 6 km
02468
10
0.E+00 4.E+14 8.E+14
Actinic flux, quanta cm-2 s-1
Altit
ude,
km od = 100
od = 10od = 0
Madronich, 1987
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SPECTRAL EFFECTS OF PARTIAL CLOUD COVER
Crafword et al., 2003
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PARTIAL CLOUD COVERBiomodal distributions
Crafword et al., 2003
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WRF-Chem Regional O3 Prediction
Observed daily 1-h maximum O3 for all EPA AIRNOW surface stations in the model domain, 21 July - 4 August 2002.
G.A. Grell et al. / Atmospheric Environment 39 (2005) 6957–6975
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Correlation coefficient - R
G.A. Grell et al. / Atmospheric Environment 39 (2005) 6957–6975
scatter mostly from clouds not modeled correctly !?
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Mean Bias
G.A. Grell et al. / Atmospheric Environment 39 (2005) 6957–6975
bias mostly from aerosols not modeled correctly !?
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Problems and Opportunities
O3 productions is– sometimes NOx limited– sometimes VOC limited– always photon limited
Pollution affects photon availability (10-30% reductions are not uncommon).
Aerosols and clouds change the vertical gradient of photochemistry– usually brighter above, dimmer below (but not always)
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UV properties of aerosols are poorly known– Composition– Size distributions– Morphologies, mixing states– Vertical distribution
VOC-NOx-photon interactions: Photon availability may change NOx-limited transition point:
NOx
O3High J
Low J
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Delay of reactivity: slower urban photochemistry allows more export of precursors for regional oxidants.
Regional photochemistry may be accelerated by scattering aerosols (Dickerson et al., 1997)
Clouds: need improved cloud statistics for parameterizing optical properties. Also, how to deal with model vs. real clouds?
Need evaluation of model J-values with in situ measurements under realistic conditions. Need to demonstrate closure through the vertical extent.