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
Sensitivity of Ozone FormationTo Photons
Sasha MadronichNational Center for Atmospheric ResearchBoulder Colorado USA
Mexico City, 14 August 2009
Tropospheric Ozone Formation:
Urban ozone is generated when air containing hydrocarbons and nitrogen oxides is exposed to ultraviolet radiation
- Haagen-Smit (1950s)
3
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
NOx-VOC Regimes NOx-limited
Very low NOx:O3 ~ J0.5 [NOx]
VOC-limited
NOx-inhibited
Very high NOx:O3 ~ J [VOC] / [NOx]
Sensitivity (%/%) of O3 in Mexico City
NCAR Master Mechanism box model
Madronich, unpubl..
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
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:
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.
Solar Spectrum
UNEP, 2002
O2 and O3 absorball UV-C (l<280 nm)before it reaches the troposphere
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..
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..
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..
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)
JNO2 Observed/Model_cleanMarch 2006 T1 supersite
thin curves = individual daysthick blue curve = average
Madronich, unpubl..
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)
O3 Suppression from Aerosol (Mexico City)
Castro et al. 2001
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
Aerosol Single Scattering AlbedoMexico City
UV-MFRSR (T1)AERONET (T1)Barnard et al. (CENICA)
Corr et al., 2009
DIURNAL CYCLE OF AEROSOL OPTICS550 nm
Paredes-Miranda et al., 2008
Clouds
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
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
SPECTRAL EFFECTS OF PARTIAL CLOUD COVER
Crafword et al., 2003
PARTIAL CLOUD COVERBiomodal distributions
Crafword et al., 2003
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
Correlation coefficient - R
G.A. Grell et al. / Atmospheric Environment 39 (2005) 6957–6975
scatter mostly from clouds not modeled correctly !?
Mean Bias
G.A. Grell et al. / Atmospheric Environment 39 (2005) 6957–6975
bias mostly from aerosols not modeled correctly !?
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)
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
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.