Andreas Hofzumahaus,
Alexander Kraus,
and Martin Müller
When this research was performed all the authors were with the Institut für Atmosphärische Chemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany.
M. Müller is now with the Fraunhofer Institut für Atmosphärische Umweltforschung, Garmisch-Partenkirchen, Germany.
Andreas Hofzumahaus, Alexander Kraus, and Martin Müller, "Solar actinic flux spectroradiometry: a technique for measuring photolysis frequencies in the atmosphere," Appl. Opt. 38, 4443-4460 (1999)
A spectroradiometer has been developed for direct measurement of
the solar actinic UV flux (scalar intensity) and determination of
photolysis frequencies in the atmosphere. The instrument is based
on a scanning double monochromator with an entrance optic that exhibits
an isotropic angular response over a solid angle of 2π
sr. Actinic flux spectra are measured at a resolution of 1 nm
across a range of 280–420 nm, which is relevant for most tropospheric
photolysis processes. The photolysis frequencies are derived from
the measured radiation spectra by use of published absorption cross
sections and quantum yields. The advantage of this technique
compared with the traditional chemical actinometry is its
versatility. It is possible to determine the photolysis frequency
for any photochemical reaction of interest provided that the respective
molecular photodissociation parameters are known and the absorption
cross section falls within a wavelength range that is accessible by the
spectroradiometer. The instrument and the calibration procedures
are described in detail, and problems specific to measurement of the
actinic radiation are discussed. An error analysis is presented
together with a discussion of the spectral requirements of the
instrument for accurate measurements of important tropospheric
photolysis frequencies (JO1D,
JNO2,
JHCHO). An example of measurements from
previous atmospheric chemistry field campaigns are presented and
discussed.
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Reproducibility at constant temperature
versus Hg line.
Wavelength error at the wavelength positions of
selected Hg lines during routine field operation (see text).
Amplitude of uncorrected sinusoidal wavelength
error having a period of 40 nm (see text).
Table 3
Wavelength-Independent Errors of the Measured Spectral
Actinic Fluxa
Error after correction for the nonideal angular response Zp
±1
Leveling of detector
±0.3
Maximum errors are given if not otherwise
stated.
The calibration concept is based on the
approximation that the inverse-square law holds for the entrance optic
(see Appendix A).
See Section 4.
Working distance z0 and
operating current of the calibration lamp.
Table 4
Systematic Error (Bias) of Calculated Photolysis
Frequencies for Three Selected Processes Owing to the Spectral Bandpass
of the Spectroradiometera
Process
Bandpass (FWHM)/nm
Bias (%)
ϑ0 = 30°
ϑ0 = 80°
Case A
Case B
Case A
Case B
O3 → O2 + O(1D)
0.5
+0.2
+0.2
+0.4
+0.4
1.0
+0.8
+0.8
+1.6
+1.6
2.0
+3.1
+3.2
+5.8
+6.0
HCHO → HCO + H
0.5
-0.5
-0.5
-1.1
-1.1
1.0
-1.6
-1.5
-3.2
-3.0
2.0
-2.9
-2.7
-5.5
-5.2
NO2 → NO + O
0.5
-0.03
-0.02
-0.04
-0.02
1.0
-0.08
-0.07
-0.10
-0.08
2.0
-0.14
-0.12
-0.16
-0.12
The error is given for two solar zenith
angles ϑ0 and two sampling grids (case A, 0.05-nm step
size; case B, step size = instrumental bandpass).
Table 5
Systematic Error (Bias) of Calculated Photolysis
Frequencies Owing to a Wavelength Error (Shift) of the Actinic
Spectrum Recorded with a Bandpass of 1.0 nm
(FWHM)a
The error is given for two solar zenith
angles ϑ0 and two different stray-light rejection ratios
R, where R = 104 is typical for
a single monochromator and R = 108 for a
double monochromator.
The error scales directly with the stray-light
rejection ratio R.
Tables (6)
Table 1
Important Photodissociation Processes in Tropospheric
Chemistry
Reproducibility at constant temperature
versus Hg line.
Wavelength error at the wavelength positions of
selected Hg lines during routine field operation (see text).
Amplitude of uncorrected sinusoidal wavelength
error having a period of 40 nm (see text).
Table 3
Wavelength-Independent Errors of the Measured Spectral
Actinic Fluxa
Error after correction for the nonideal angular response Zp
±1
Leveling of detector
±0.3
Maximum errors are given if not otherwise
stated.
The calibration concept is based on the
approximation that the inverse-square law holds for the entrance optic
(see Appendix A).
See Section 4.
Working distance z0 and
operating current of the calibration lamp.
Table 4
Systematic Error (Bias) of Calculated Photolysis
Frequencies for Three Selected Processes Owing to the Spectral Bandpass
of the Spectroradiometera
Process
Bandpass (FWHM)/nm
Bias (%)
ϑ0 = 30°
ϑ0 = 80°
Case A
Case B
Case A
Case B
O3 → O2 + O(1D)
0.5
+0.2
+0.2
+0.4
+0.4
1.0
+0.8
+0.8
+1.6
+1.6
2.0
+3.1
+3.2
+5.8
+6.0
HCHO → HCO + H
0.5
-0.5
-0.5
-1.1
-1.1
1.0
-1.6
-1.5
-3.2
-3.0
2.0
-2.9
-2.7
-5.5
-5.2
NO2 → NO + O
0.5
-0.03
-0.02
-0.04
-0.02
1.0
-0.08
-0.07
-0.10
-0.08
2.0
-0.14
-0.12
-0.16
-0.12
The error is given for two solar zenith
angles ϑ0 and two sampling grids (case A, 0.05-nm step
size; case B, step size = instrumental bandpass).
Table 5
Systematic Error (Bias) of Calculated Photolysis
Frequencies Owing to a Wavelength Error (Shift) of the Actinic
Spectrum Recorded with a Bandpass of 1.0 nm
(FWHM)a
The error is given for two solar zenith
angles ϑ0 and two different stray-light rejection ratios
R, where R = 104 is typical for
a single monochromator and R = 108 for a
double monochromator.
The error scales directly with the stray-light
rejection ratio R.