Calibration of laser induced fluorescence (LIF) instruments

that measure OH is challenging because it is difficult to

reliably introduce a known amount of this reactive radical into a measurement apparatus. In a recent paper, Hofzumahaus et al., [1996] describe a novel and seemingly simple technique to accomplish this goal: they dissociate trace quantities of water vapor in air with a low pressure mercury (Hg) lamp to produce low concentrations (10^5 - 10^9 cm^(-3)) of OH (R1)

Donahue, N. M.

Hanisco, T. F.

Lanzendorf, E. J.

Wennberg, P. O.

Caltech Authors - Main

GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 23, PAGES 3037-3038, DECEMBER 1, 1997 
Comment on: "The measurement of tropospheric OH radicals 
by laser-induced fluorescence spectroscopy during the 
POPCORN field campaign" by Hofzumahaus et al. and 
"Intercomparison of tropospheric OH radical measurements 
by multiple folded long-path laser absorption and laser induced 
fluorescence" by Brauers et al. 
E. J. Lanzendorf, T. F. Hanisco, N.M. Donahue and P.O. Wennberg 
Department of Chemistry, Harvard University, Cambridge, MA. 
Calibration of laser induced fluorescence (L•) instruments 
that measure OH is challenging because it is difficult to 
reliably introduce a known amount of this reactive radical into 
a measurement apparatus. In a recent paper, Hofzumahaus et 
al., [1996] describe a novel and seemingly simple technique 
to accomplish this goal: they dissociate trace quantities of 
water vapor in air with a low pressure mercury (Hg) lamp to 
5 9 -3 
produce low concentrations (10 - 10 cm ) of OH (R1). The 
lamp fluence is then calibrated by ozone actinometry (R2): 
H 20 + hv (1849.5 • --> OH + H (R 1 ) 
0 2+hv(1849.5• •O+O 
O + 0 2 --> 0 3 . (R2) 
The OH concentration is inferred from the ambient water 
vapor and 02 mixing ratios, the measured 03 concentration, 
and the relevant cross sections: 
[OH]= 0.H20 [H20][O3  (EQ1) 
20'02 [0 2 ] 
In Brauers et al., [1996], an OH measurement calibrated 
with this technique was compared with an OH measurement 
by differential optical absorption spectroscopy (DOAS) [Dorn 
et al., 1996]. Excellent agreement (within 1%) was obtained, 
leading the authors to conclude that the agreement "is even 
better than what is expected from the estimated accuracies 
(DOAS better than 7%, L• 16% (lo))" [Brauers et al., 
1996]. This L• calibration technique is also used by others 
and has been discussed in the literature [Aschmutat et al., 
1994, Reiner et al., 1997, Schultz et al., 1996]. 
Accurate absorption cross ections for 02 and H20 at the 
1849.53• Hg emission wavelength are required to infer the 
OH density from the measured 03 and H20 concentration. I  
the studies described above the cross sections were taken 
from the literature: the H20 cross ection ((5H2o) from Hudson 
and Kieffer, [1975]; the oxygen cross section ((502) from 
Washida et al., [1971]. The measurement by Washida et al. 
is difficult to interpret because the absorption coefficients are 
given in units of (atm4xcm -•) without discussion of base or 
temperature. The actual value is 2.2x10 © cm2/molec (N.
Washida personal communication, 1997). This is 2.3 times 
larger than the value used by Hofzumahaus et al., [1996]. 
Thus, the agreement between the L• and DOAS 
Copyright 1997 by the American Geophysical Union. 
measurements [Brauers et al., 1996] appears to depend upon 
an erroneous 0 2 cross section. This error translates directly 
into an error in the calibration of the LIF experiment (EQ 1). 
Because of the above discrepancy wemeasured the 02 
cross ection using a pen ray Hg lamp with a flowing N 2 
purge. We used a CsI photomultiplier tube (which is 
insensitive to the strong 2537• Hg line) equipped with an 
interference filter to measure the attenuation of the 1849.5• 
Hg line as a function of 02 pressure. As we increased the 
optical depth, however, the calculated cross section decreased 
continuously. Similar behavior has been observed elsewhere 
[Canttell et al., 1997]. 
To investigate this further, we took a high resolution 
(0.013•) spectrum of the Hg lamp on the Harvard- 
Smithsonian 6.65 meter vacuum-ultraviolet spectrometer. 
Details of the experimental apparatus are given in Yoshino et 
al., [1983]. The lamp was purged with N2 for cooling and to 
prevent ozone formation. The spectrometer provides an 
absorption path -1300 cm long. During the experiment we 
varied the amount of 02 in the spectrometer, and the lamp 
operating conditions. Figure 1 (a) shows an expanded portion 
of (502 in the (8,0) Schumann-Runge (S-R) band [Yoshino et 
al., 1992] superposed with the Hg lamp spectrum measured 
with the spectrometer evacuated. The spectrum shows that 
the pen ray lamp is not a line source. The transition is self- 
reversed, significantly broadened, and asymmetric, with a 
long tail to the red. The saturation effects observed are 
clearly understandable: (502 is significantly higher for the 
portion of the lamp spectrum at shorter wavelength. This is 
19 
illustrated in Figure l(b). With the addition of--4.2x10 
molec/cm 2 0 2 to the spectrometer (--1 torr), most of the 
attenuation is at the center of the S-R lines. Addition of 
-3.4x10 TM molec/cm 2 oxygen (about 8 torr) shows depletion 
of the lamp spectrum in the region of the S-R bands to 
essentially the dark count base line of the photomultiplier 
tube. However, absorption due to the unsaturated 02 lines 
between 1849.5• and 1850.0• is still visible. Note that the 
absorption seen in the 1 torr data allows us to align the 
wavelength of the Hg lamp source with the oxygen absorption 
data of Yoshino et al., [1992] to within 0.003•. 
The effective 02 cross ection for the lamp is determined 
from the 0 2 absorption spectrum of Yoshino et al., [1992] 
weighted by the Hg emission spectrum observed at the 
spectrometer exit: 
Paper number 97GL02899. 
0094-8534/97/97 GL-02899505.00 
3037 
0'O2 (eff) : JI(v)0'o2 (v)dv / JI(v)dv. (EQ2) 
•0 
10000 
1 ooo 
½• lOO 
o,• lO 
• 1 
1849.5 1850.0 1850.5 
Wavelength (•)
Figure 1. (a) Expanded portion of the oxygen absorption 
cross section [Yoshino et al., 1992] superposed with the Hg 
spectrum measured with the spectrometer evacuated. (b) 
Spectrum for three different amounts of oxygen added to the 
spectrometer. The intensity is plotted on a log scale and the . . 2 
02 concentration shown as optical path (molec/cm). 
lJo2(eff ) depends on 02 column and lamp operating 
conditions, which will change the Hg emission spectrum. In 
addition, any error in Oo2(V) will translate directly into an 
error in lJo2(eff ). These ffects are shown in Figure 2. 
The inset of Figure 2 illustrates the effect of lamp 
conditions on the observed spectrum. Additional cooling 
added to the Hg lamp (trace B) results in a reduction in 
spectral width by approximately 1/4 relative to the lamp with 
2.0 
1.6 :' ? ............... : ')i 
I • 1849.45 o 1849.55 
r •' Wavelength (A) 
0.8 -T- A 
'"'----...• -o- B 
0.4 
0 1 2 3 
0 2 Column ( x l0 20 molec/cm 2) 
Figure 2. IJo2(eff ) ¾S 02 column abundance for two different 
Hg lamp operating characteristics (A and B). The inset shows 
the Hg spectra superposed on the 02 absorption cross section 
[Yoshino et al., 1992]. The narrower linewidth seen in (B) is 
from the lamp with the plasma voltage reduced by-3 and the 
N2 purge flow over the lamp dramatically increased relative to 
(A). The lamp intensity is in arbitrary units. 
less cooling (A). As expected, the self reversal in B is also 
diminished. lJo2(eff ) obtained with the lamp operating under 
condition A shows a variation of a factor of 3 between when 
the spectrometer was evacuated and when a 1.0x102ø 
molec/cm 2 02 optical path was present. IJo2(eff ) measured 
with the Hg lamp operating under condition B shows a 
variation of 30% over the same range. In either case, it is 
clear that the use of a literature value for the oxygen cross 
section at 1849.5• is not appropriate for a pen ray lamp. 
The effective cross section depends upon both the lamp 
conditions and the 02 concentration between the lamp and the 
photolysis region. The lamp we used illustrates the general 
problem, and its characteristics may not be the same as lamps 
in other experiments. Thus, any attempt to determine an 
effective cross section using the results in Figure 2 is 
inappropriate; direct measurement of IJo2(eff ) under field 
conditions is essential. 
Acknowledgements. We thank Dr. Kouichi Yoshino of the 
Harvard-Smithsonian Center for Astrophysics (H-SCfA) for use of 
the vacuum ultraviolet spectrometer and for his expertise. The 02 
cross sections were obtained from the H-SCfA web site: http://cfa- 
www.harvard.edu/amp/data/S-R.html. We thank NASA's Upper 
Atmospheric Research Program for support. 
References 
Aschmutat, U., et al., A tunable source of hydroxyl (OH) and 
6 9 
hydroperoxy (HO2) radicals: In the range between 10 and 10 
crn -3, in Physico-Chemical Behaviour of Atmospheric Pollutants, 
edited by G. Angeletti and G. Restilli, pp.811-816, European 
Commission, Brussels, 1994. 
Brauers, T., et al., Intercomparison of tropospheric OH radical 
measurements by multiple folded long-path laser absorption and 
laser induced fluorescence, Geophys. Res. Lett., 23, 2545, 1996. 
Cantrell, C.A., et al., Absorption cross sectons for water vapor from 
183 to 193 nm, Geophys. Res. Lett., 24, 2195-2198, 1997. 
Dorn, H.-P., et al., 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. 
Hofzumahaus, A., et al., The measurement of tropospheric OH 
radicals by laser-induced fluorescence spectroscopy during the 
POPCORN field campaign, Geophys. Res. Lett., 23, 2541, 1996. 
Hudson, R. D., and L. J. Kieffer, Absorption cross sections of 
stratospheric molecules, The Natural Stratosphere of 1974, CIAP 
Monograph 1, pp. 5156-5194, 1975. 
Reiner, T., M. Hanke, and F. Arnold, Atmospheric peroxy radical 
measurements by ion molecule reaction-mass spectrometry: A 
novel analytical method using amplifying chemical conversion to 
sulfuric acid, J. Geophys. Res., 102, 1311-1326, 1997. 
Schultz M., et al., Calibration source for peroxy radicals with built-in 
actinometry using H20 and 02 photolysis at 185 nm, J. Geophys. 
Res., 100, 18,811-18,816, 1995. 
Washida, N., Y. Mori, and I. Tanaka, Quantum yield of ozone 
formation from the photolysis of the oxygen molecule at 1849 and 
1931 •, J. Chem. Phys., 54,1119-1122, 1971. 
Yoshino, K., et al., High resolution absorption cross section 
measurements and band oscillator strengths of the (1,0)-(12,0) 
Schumann-Runge bands of 02, Planet. Space Sci., 31,339, 1983. 
Yoshino, K., et al., High resolution absorption cross sections in the 
transmission window region of the Schumann-Runge bands and 
Herzberg continuum of 02, Planet. Space Sci., 40, 185-192, 1992. 
E. J. Lanzendorf, T. F. Hanisco, N.M. Donahue, and P.O. 
Wennberg, Department of Chemistry, Harvard University, 
Cambridge, MA 02138. (e-mail: lanzendorf@huarp.harvard.edu; ) 
(Received March 11, 1997; revised August 6, 1997; 
accepted October 8, 1997.) 


English

Comment on: “The measurement of tropospheric OH radicals by laser-induced fluorescence spectroscopy during the POPCORN Field Campaign” by Hofzumahaus et al. and “Intercomparison of tropospheric OH radical measurements by multiple folded long-path laser absorption and laser induced fluorescence” by Brauers et al.

GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 23, PAGES 3037-3038, DECEMBER 1, 1997 
Comment on: "The measurement of tropospheric OH radicals 
by laser-induced fluorescence spectroscopy during the 
POPCORN field campaign" by Hofzumahaus et al. and 
"Intercomparison of tropospheric OH radical measurements 
by multiple folded long-path laser absorption and laser induced 
fluorescence" by Brauers et al. 
E. J. Lanzendorf, T. F. Hanisco, N.M. Donahue and P.O. Wennberg 
Department of Chemistry, Harvard University, Cambridge, MA. 
Calibration of laser induced fluorescence (L•) instruments 
that measure OH is challenging because it is difficult to 
reliably introduce a known amount of this reactive radical into 
a measurement apparatus. In a recent paper, Hofzumahaus et 
al., [1996] describe a novel and seemingly simple technique 
to accomplish this goal: they dissociate trace quantities of 
water vapor in air with a low pressure mercury (Hg) lamp to 
5 9 -3 
produce low concentrations (10 - 10 cm ) of OH (R1). The 
lamp fluence is then calibrated by ozone actinometry (R2): 
H 20 + hv (1849.5 • --> OH + H (R 1 ) 
0 2+hv(1849.5• •O+O 
O + 0 2 --> 0 3 . (R2) 
The OH concentration is inferred from the ambient water 
vapor and 02 mixing ratios, the measured 03 concentration, 
and the relevant cross sections: 
[OH]= 0.H20 [H20][O3  (EQ1) 
20'02 [0 2 ] 
In Brauers et al., [1996], an OH measurement calibrated 
with this technique was compared with an OH measurement 
by differential optical absorption spectroscopy (DOAS) [Dorn 
et al., 1996]. Excellent agreement (within 1%) was obtained, 
leading the authors to conclude that the agreement "is even 
better than what is expected from the estimated accuracies 
(DOAS better than 7%, L• 16% (lo))" [Brauers et al., 
1996]. This L• calibration technique is also used by others 
and has been discussed in the literature [Aschmutat et al., 
1994, Reiner et al., 1997, Schultz et al., 1996]. 
Accurate absorption cross ections for 02 and H20 at the 
1849.53• Hg emission wavelength are required to infer the 
OH density from the measured 03 and H20 concentration. I  
the studies described above the cross sections were taken 
from the literature: the H20 cross ection ((5H2o) from Hudson 
and Kieffer, [1975]; the oxygen cross section ((502) from 
Washida et al., [1971]. The measurement by Washida et al. 
is difficult to interpret because the absorption coefficients are 
given in units of (atm4xcm -•) without discussion of base or 
temperature. The actual value is 2.2x10 © cm2/molec (N.
Washida personal communication, 1997). This is 2.3 times 
larger than the value used by Hofzumahaus et al., [1996]. 
Thus, the agreement between the L• and DOAS 
Copyright 1997 by the American Geophysical Union. 
measurements [Brauers et al., 1996] appears to depend upon 
an erroneous 0 2 cross section. This error translates directly 
into an error in the calibration of the LIF experiment (EQ 1). 
Because of the above discrepancy wemeasured the 02 
cross ection using a pen ray Hg lamp with a flowing N 2 
purge. We used a CsI photomultiplier tube (which is 
insensitive to the strong 2537• Hg line) equipped with an 
interference filter to measure the attenuation of the 1849.5• 
Hg line as a function of 02 pressure. As we increased the 
optical depth, however, the calculated cross section decreased 
continuously. Similar behavior has been observed elsewhere 
[Canttell et al., 1997]. 
To investigate this further, we took a high resolution 
(0.013•) spectrum of the Hg lamp on the Harvard- 
Smithsonian 6.65 meter vacuum-ultraviolet spectrometer. 
Details of the experimental apparatus are given in Yoshino et 
al., [1983]. The lamp was purged with N2 for cooling and to 
prevent ozone formation. The spectrometer provides an 
absorption path -1300 cm long. During the experiment we 
varied the amount of 02 in the spectrometer, and the lamp 
operating conditions. Figure 1 (a) shows an expanded portion 
of (502 in the (8,0) Schumann-Runge (S-R) band [Yoshino et 
al., 1992] superposed with the Hg lamp spectrum measured 
with the spectrometer evacuated. The spectrum shows that 
the pen ray lamp is not a line source. The transition is self- 
reversed, significantly broadened, and asymmetric, with a 
long tail to the red. The saturation effects observed are 
clearly understandable: (502 is significantly higher for the 
portion of the lamp spectrum at shorter wavelength. This is 
19 
illustrated in Figure l(b). With the addition of--4.2x10 
molec/cm 2 0 2 to the spectrometer (--1 torr), most of the 
attenuation is at the center of the S-R lines. Addition of 
-3.4x10 TM molec/cm 2 oxygen (about 8 torr) shows depletion 
of the lamp spectrum in the region of the S-R bands to 
essentially the dark count base line of the photomultiplier 
tube. However, absorption due to the unsaturated 02 lines 
between 1849.5• and 1850.0• is still visible. Note that the 
absorption seen in the 1 torr data allows us to align the 
wavelength of the Hg lamp source with the oxygen absorption 
data of Yoshino et al., [1992] to within 0.003•. 
The effective 02 cross ection for the lamp is determined 
from the 0 2 absorption spectrum of Yoshino et al., [1992] 
weighted by the Hg emission spectrum observed at the 
spectrometer exit: 
Paper number 97GL02899. 
0094-8534/97/97 GL-02899505.00 
3037 0'O2 (eff) : JI(v)0'o2 (v)dv / JI(v)dv. (EQ2) 
•0 
10000 
1 ooo 
½• lOO 
o,• lO 
• 1 
1849.5 1850.0 1850.5 
Wavelength (•)
Figure 1. (a) Expanded portion of the oxygen absorption 
cross section [Yoshino et al., 1992] superposed with the Hg 
spectrum measured with the spectrometer evacuated. (b) 
Spectrum for three different amounts of oxygen added to the 
spectrometer. The intensity is plotted on a log scale and the 
. . 2 
02 concentration shown as optical path (molec/cm). 
lJo2(eff ) depends on 02 column and lamp operating 
conditions, which will change the Hg emission spectrum. In 
addition, any error in Oo2(V) will translate directly into an 
error in lJo2(eff ). These ffects are shown in Figure 2. 
The inset of Figure 2 illustrates the effect of lamp 
conditions on the observed spectrum. Additional cooling 
added to the Hg lamp (trace B) results in a reduction in 
spectral width by approximately 1/4 relative to the lamp with 
2.0 
1.6 :' ? ............... : ')i 
I • 1849.45 o 1849.55 
r •' Wavelength (A) 
0.8 -T- A 
'"'----...• -o- B 
0.4 
0 1 2 3 
0 2 Column ( x l0 20 molec/cm 2) 
Figure 2. IJo2(eff ) ¾S 02 column abundance for two different 
Hg lamp operating characteristics (A and B). The inset shows 
the Hg spectra superposed on the 02 absorption cross section 
[Yoshino et al., 1992]. The narrower linewidth seen in (B) is 
from the lamp with the plasma voltage reduced by-3 and the 
N2 purge flow over the lamp dramatically increased relative to 
(A). The lamp intensity is in arbitrary units. 
less cooling (A). As expected, the self reversal in B is also 
diminished. lJo2(eff ) obtained with the lamp operating under 
condition A shows a variation of a factor of 3 between when 
the spectrometer was evacuated and when a 1.0x102ø 
molec/cm 2 02 optical path was present. IJo2(eff ) measured 
with the Hg lamp operating under condition B shows a 
variation of 30% over the same range. In either case, it is 
clear that the use of a literature value for the oxygen cross 
section at 1849.5• is not appropriate for a pen ray lamp. 
The effective cross section depends upon both the lamp 
conditions and the 02 concentration between the lamp and the 
photolysis region. The lamp we used illustrates the general 
problem, and its characteristics may not be the same as lamps 
in other experiments. Thus, any attempt to determine an 
effective cross section using the results in Figure 2 is 
inappropriate; direct measurement of IJo2(eff ) under field 
conditions is essential. 
Acknowledgements. We thank Dr. Kouichi Yoshino of the 
Harvard-Smithsonian Center for Astrophysics (H-SCfA) for use of 
the vacuum ultraviolet spectrometer and for his expertise. The 02 
cross sections were obtained from the H-SCfA web site: http://cfa- 
www.harvard.edu/amp/data/S-R.html. We thank NASA's Upper 
Atmospheric Research Program for support. 
References 
Aschmutat, U., et al., A tunable source of hydroxyl (OH) and 
6 9 hydroperoxy (HO2) radicals: In the range between 10 and 10 
crn -3, in Physico-Chemical Behaviour of Atmospheric Pollutants, 
edited by G. Angeletti and G. Restilli, pp.811-816, European 
Commission, Brussels, 1994. 
Brauers, T., et al., Intercomparison of tropospheric OH radical 
measurements by multiple folded long-path laser absorption and 
laser induced fluorescence, Geophys. Res. Lett., 23, 2545, 1996. 
Cantrell, C.A., et al., Absorption cross sectons for water vapor from 
183 to 193 nm, Geophys. Res. Lett., 24, 2195-2198, 1997. 
Dorn, H.-P., et al., 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. 
Hofzumahaus, A., et al., The measurement of tropospheric OH 
radicals by laser-induced fluorescence spectroscopy during the 
POPCORN field campaign, Geophys. Res. Lett., 23, 2541, 1996. 
Hudson, R. D., and L. J. Kieffer, Absorption cross sections of 
stratospheric molecules, The Natural Stratosphere of 1974, CIAP 
Monograph 1, pp. 5156-5194, 1975. 
Reiner, T., M. Hanke, and F. Arnold, Atmospheric peroxy radical 
measurements by ion molecule reaction-mass spectrometry: A 
novel analytical method using amplifying chemical conversion to 
sulfuric acid, J. Geophys. Res., 102, 1311-1326, 1997. 
Schultz M., et al., Calibration source for peroxy radicals with built-in 
actinometry using H20 and 02 photolysis at 185 nm, J. Geophys. 
Res., 100, 18,811-18,816, 1995. 
Washida, N., Y. Mori, and I. Tanaka, Quantum yield of ozone 
formation from the photolysis of the oxygen molecule at 1849 and 
1931 •, J. Chem. Phys., 54,1119-1122, 1971. 
Yoshino, K., et al., High resolution absorption cross section 
measurements and band oscillator strengths of the (1,0)-(12,0) 
Schumann-Runge bands of 02, Planet. Space Sci., 31,339, 1983. 
Yoshino, K., et al., High resolution absorption cross sections in the 
transmission window region of the Schumann-Runge bands and 
Herzberg continuum of 02, Planet. Space Sci., 40, 185-192, 1992. 
E. J. Lanzendorf, T. F. Hanisco, N.M. Donahue, and P.O. 
Wennberg, Department of Chemistry, Harvard University, 
Cambridge, MA 02138. (e-mail: lanzendorf@huarp.harvard.edu; ) 
(Received March 11, 1997; revised August 6, 1997; 
accepted October 8, 1997.) 


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