LECTURE 20
Atmospheric Odd Hydrogen,
HOx
AOSC 637 Spring 2010
Atmospheric Chemistry
Russell R. Dickerson
Copyright © 2010 R. R. Dickerson
1
Odd Hydrogen: Outline
Importance
Chemistry
Sources
Sinks
Reservoirs and Ratios
Detection Techniques
Fluorescence
FAGE
DOAS
Chem Amplification
Global Budget Calculations
Remaining Challenges
Bibliography
Odd Hydrogen
•
•
•
•
Importance:
Ozone destruction in both the stratosphere and trospsphere.
Removal of NOx, ClOx, CO, VOC’s, SOx, HCFC’s
The most important species for transformations.
Many pollutants have no other sink.
O3  h  O2  O(1D)
Chemistry
Sources
(remember what
HCO and H do)
(  318 nm)
H 2O  O(1D)  2OH
H 2  O(1D)  OH  H
CH 4  O(1D)  CH 3  OH
CH 3OOH  h  CH 3O  OH
CH 3O  O2  CH 2O  HO2
CH O  h  CHO  H
Copyright © 20102 R. R. Dickerson
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HYDROCARBRONS REACTIVITY
FOR URBAN SMOG (OZONE) FORMATION
HYDROCARBON
k(O)
k(O₃)
k(OH)
(All units: cm³s⁻¹)
Methane, CH₄
1.1x10⁻¹⁷
SLOW
7.9x10⁻¹⁵
Ethane, C₂H₆
9.6x10⁻¹⁶
SLOW
2.7x10⁻¹³
Propane, C₃H₈
1.5x10⁻¹⁴
SLOW
1.2x10⁻¹²
Butane, C₄H₁₀
3.1x10⁻¹⁴
SLOW
2.3x10⁻¹²
Hexane, C₆H₁₄
9.5x10⁻¹⁴
SLOW
5.7x10⁻¹²
2,3 Dimethyl butane
(C₆H₁₄)
2.1x10⁻¹³
SLOW
6.3x10⁻ ¹²
Ethene, C₂H₄
8.4x10⁻¹³
1.8x10⁻¹⁸
8.0x10⁻¹²
Propene, C₃H₆
3.6x10⁻¹²
1.1x10⁻¹⁷
2.5x10⁻¹¹
Benzene, C₆H₆
1.6x10⁻¹⁴
SLOW
1.2x10⁻¹²
5.9x10⁻¹⁴
SLOW
6.4x10⁻¹²
Toluene, C₇H₈
Copyright © 2010 R. R. Dickerson
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Odd Hydrogen
To calculate OH we need: j(O3), [O3], [H2O]
Sinks
OH  OH  H 2O  O
HO2  OH  H 2O  O2
M
NO2  OH  HNO3 (het _ rem oval)
HO2  HO2  H 2O2  O2
OH  HCl  H 2O  Cl ( strat)
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Reservoir species, control ratios of OH/HO2
O2
OH  CO 
CO2  HO2
HO2  NO  NO2  HO
HO2  O3  2O2  OH
2 / H 2O
SO2  OH O

 H 2 SO4  HO2
OH  H 2O2  HO2  H 2O
OH  H 2CO  HCO  H 2O
OH  VOC ' s  HO2  RO2
HO2  RO2  HOOR  O2
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RONO
Copyright © 2010 2
R. R. Dickerson
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Steady State Approximations:
2k2 [O(1D)][H 2O]  [ HO2 ](k6 [O3 ]  k7 [ NO])  2 j9 [ H 2O2 ]
[OH ] 
k3[CO]  k 4 [CH 4 ]
1/ 2
 ( j9  k10 [OH ]  k11 )k2 [O( D)][H 2O] 

[ HO2 ]  
k8 (k10 [OH ]  k11 )


1
O3  h  O2  O(1D)....(1)
H 2O  O( D)  2OH .............(2)
1
HO2  CO  CO2  O2 .......(3)
CH 4  OH  CH 3  H 2O....(4)
CH 3O  O2  CH 2O  HO2 ..(5)
2 HO2  H 2O2  O2 ............(8)
H 2O2  h  2OH ..............(9)
HO2  OH  H 2O  O2 ........(10)
H 2O2 Heterog
 P roducts.......(11)
HO2  O3  OH  2O2 .......(6)
HO2  NO  OH  NO2 ......(7)
From Logan, JGR (1981)
Copyright © 2010 R. R. Dickerson
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Calculated mean OH
(x10-6 cm-3) in a CH4,
CO, O3 atmosphere,
from Crutzen’s model at
MPI.
Why does het max occur
in the LFT in the
tropics?
Copyright © 2010 R. R. Dickerson
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Potential Energy Curves for OH
Chem. Phys. 237(1-2), 123-138 (1998) • DOI:10.1016/S0301-0104(98)00219-5
Copyright © 2010 R. R. Dickerson
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Solar flux induced fluorescence was successful for the
detection of OH in the Stratosphere (Anderson JGR,
7820, 1971).
Anderson put a scanning
spectrometer on the nose of a
rocket and measured the
emission at 308 nm due to solar
excitation.
In situ resonance fluorescence
with a microwave discharge
lamp worked in the strat
(Anderson GRL, 1976), but not
in the trop.
Copyright © 2010 R. R. Dickerson
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FAGE, Fluorescence Assay by Gas Expansion, is
essentially laser-induced fluorescence at low pressure.
Copyright © 2010 R. R. Dickerson
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Early attempts to measure OH via fluorescence failed – why?
Radiation at 308 nm photolyzes O3 to O(1D).
Copyright © 2010 R. R. Dickerson
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Diurnal variation of
OH measured using
LIF (o) and DOAS (•)
during POPCORN
(Adapted from
Hofzumahaus et al.,
1998).
Copyright © 2010 R. R. Dickerson
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Correlation plot of all
LIF OH data versus the
photolysis frequency of
ozone, j(O1D).
(Adapted from Holland
et al., 1998).
Copyright © 2010 R. R. Dickerson
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Copyright © 2010 R. R. Dickerson
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Atmospheric absorption
spectra measured using
DOAS as a function of
time of day (UT). Solid
lines are reference
absorption spectra of OH
radicals fitted to the
measurements (Adapted
from Dorn et al., 1996).
Copyright © 2010 R. R. Dickerson
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Copyright © 2010 R. R. Dickerson
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Dependence of the measured OH concentration on NO2 during the
POPCORN field campaign. To make this behavior visible, the OH data were
first normalized with respect to j(O1D) and then plotted versus equal
log(NO2)-intervals of 0.1. Full curve corresponds to the model-calculated
dependence. [Adapted from Ehhalt, 1999].
Copyright © 2010 R. R. Dickerson
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Comparison of observed and calculated OH concentrations
versus NOX during the 1993 Idaho Hill experiment (THOPE).
The different model calculations account for different amounts
of unmeasured biogenic hydrocarbons [Adapted from McKeen et
al., 1997].
Copyright © 2010 R. R. Dickerson
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Altitude profiles of measured
(open circles) and modeled OH
for 10 May 1996 during
SUCCESS. Measurements and
models are averaged into 0.5 km
altitude bins. Models with (dashdot line) and without (dashed
line) acetone are compared.
(Adapted from Brune et al.,
1998).
CH3C(O)CH3 + hv → 2CH3 + CO
Copyright © 2010 R. R. Dickerson
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Calculated mean OH
(x10-6 cm-3) in a CH4,
CO, O3 atmosphere,
from Crutzen’s model at
MPI.
Why does het max occur
in the LFT in the
tropics?
Copyright © 2010 R. R. Dickerson
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Horowitz et al.,
JGR 2003.
Copyright © 2010 R. R. Dickerson
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Global mean OH can be calculated from the
rate of loss of methyl chloroform, CCl3CH3
Prinn et
Copyright © 2010 R. R. Dickerson
al., Science, 1995.
26
Concentrations of CCl3CH3 continue to fall.
Copyright © 2010 R. R. Dickerson
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From NOAA Earth System
Research Laboratory
http://www.esrl.noaa.gov/gmd/
odgi/
Copyright © 2010 R. R. Dickerson
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Calculating Global Mean OH from CH3CCl3
Concentrations
Because OH is so hard to measure, we would like to get at the
concentration another way [Prinn et al., 1987; Prinn et al., 1995].
Let’s designing a good experiment – a good OH tracer must have:
1. Only one sink – reaction with OH
2. A lifetime » inter-hemispheric mixing
» seasonal variations in OH
1. Well known atmospheric burden
2. Well known production rate
3. Well known rate const, kOH
4. A reliable, precise measurement technique.
Copyright © 2010 R. R. Dickerson
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In the original work, Prinn performed a simple global burden calculation
of mean [OH].
1. Assume steady state, i.e., production = loss.
2. Measured or calculated production rate.
3. Loss (= production) = kOH [CH3CCl3][OH]
4. Assume [CH3CCl3] is constant in time and space (we’ll revisit this
later).
Production
Prod
[OH] 

   Prod
k OH [CH3CCl 3 ]
k'
Copyright © 2010 R. R. Dickerson
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In the original work, Prinn performed a simple
global burden calculation of mean [OH].
1. Assume stready state, i.e., production = loss.
2. Measured or calculated production rate.
3. Loss (= production) = kOH [CH3CCl3][OH]
4. Assume [CH3CCl3] is constant in time and space.
First order estimate (box model)
CH3CCl3 + OH → H2O + CH2CCl3
kOH = 1.64x10-12e(-1520/T)
Mean middle trop temp ~ 255 K; k255 = 4.2x10-15 cm3 s-1
Copyright © 2010 R. R. Dickerson
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What was the mean [CH3CCl3]?
Latutude
Mixing Ratio (in 1981)
52°N
169 (ppt)
45°N
163
13°N
147
14°S
122
41°S
117
Lat weighted mean
144 ±25 ppt
Total tropospheric burden = mass of atmosphere x mean mixing ratio x
ratio of molecular weights.
4.0x1021 g x 1.44x10-10 x 133.5/29 = 2.65x1012 g
From Prinn et al., (1983).
Copyright © 2010
R. R. Dickerson
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What was the mean [CH3CCl3]?
The global production rate in 1981 was ~5.0x1011 (±0.5) g yr-1
We don’t know for sure that release = production.
Lifeteime 
Burden
2.65x1012


 5.3yr
11
Production 5x10


[OH]  k   4.2x1015 x5.3x365x25x3600  1.4x106 cm3
1
Copyright © 2010 R. R. Dickerson
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What was the uncertainty mean [OH] of 1.4x106 cm-3?
1. Rate Constant ±15%
2. Absolute concentration ± 20%
3. Production rate ± 10%
4. Mean global conc ± 25%
5. Annual variation ± 30%
RMS ±50%
Using a 12-box model, the global mean OH was estimated to be
9.7 ± 0.5x105 cm-3 in 1994 with little temporal change by Prinn
et al., 1995. They also derived a residence timje for
methylchloroform of 4.8 yr.
Copyright © 2010 R. R. Dickerson
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Remaining challenges
I.
II.
III.
What are the controlling factors in the upper trop.?
Is the mean OH derived from methyl chloroform
correct in light of recent discoveries about Cl
chemistry in the trop?
How will changes in the composition and climate
impact the atmosphere’s oxidizing capacity and what
unintended consequences await us?
Copyright © 2010 R. R. Dickerson
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Bibliography
Anderson, J.G., The absolute concentration of OH (X2P) in the earth's stratosphere, Geophys. Res. Lett. 3, 165-168, 1976.
Brune, W.H. et al., Airborne in-situ OH and HO2 observations in the cloud-free troposphere and lower stratosphere during
SUCCESS, Geophys. Res. Lett., 25, 1701-1704, 1998.
Ehhalt, D.H., Photooxidation of trace gases in the troposphere, Phys. Chem. Chem. Phys., 1, 5401-5408, 1999.
McKeen et al., Photochemical modeling of hydroxyl and its relationship to other species during the Tropospheric OH
Photochemistry Experiment, J. Geophys. Res., 102, 6467-6493, 1997.
Dorn, H. P., U. Brandenburger, T. Brauers, M. Hausmann, and D. H. Ehhalt, In-situ detection of tropospheric OH radicals
by folded long- path laser absorption. Results from the POPCORN field campaign in August 1994., Geophys.
Res. Lett., 23, 2537-2540, 1996.
Hard, T.M., L. A. George, and R. J. O'Brien, FAGE Determination of Tropospheric HO and HO2, J. Atmos. Sciences, 52,
3354-3372, 1995.
Hausmann, M., U. Brandenburger, T. Brauers, and H.-P. Dorn, Detection of tropospheric OH radicals by long-path
differential-optical-absorption spectroscopy: Experimental setup, accuracy, and precision, J. Geophys. Res.,
102, 16011-16022, 1997.
Logan, J. A., M. J. Prather, S. C. Wofsy, and M. B. McElroy (1981), Tropospheric chemistry: A global perspecvtive, J.
Geophys. Res., 86, 7210-7254.
Prinn, R., D. Cunnold, R. Rasmussen, P. Simmonds, F. Alyea, A. Crawford, P. Fraser, and R. Rosen (1987), Atmospheric
trends in methylchloroform and the global average for the hydroxyl radical, Science, 238, 945-950.
Prinn, R. G., R. F. Weiss, B. R. Miller, J. Huang, F. N. Alyea, D. M. Cunnold, P. J. Fraser, D. E. Hartley, and P. G.
Simmonds (1995), Atmospheric Trends and Lifetime of CH3CCl3 and Global OH Concentrations, Science,
269, 187-192.
Copyright © 2010 R. R. Dickerson
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