Airborne
Measurements of Spectrally Resolved Actinic Flux and Photolysis
Frequencies:Instrumentation description
R. E. Shetter and B. L. Lefer
Atmospheric Chemistry Division
National Center for Atmospheric Research
Boulder, Colorado
Spectrally
resolved upwelling and downwelling actinic flux measurements
from 280 to 420 nm were provided on the NASA DC-8 aircraft for
the TRACE P mission using scanning actinic flux spectroradiometer
(SAFS) instruments [Shetter and Muller, 1998]. The
technique is based on hemispherical quartz light collectors,
double monochromators, low dark current photomultipliers, and
instrument control and data acquisition computers.
The SAFS instrument package on the aircraft
will include 2 independent spectroradiometer systems. Since an
individual spectroradiometer system collects the 2p steradian
hemisphere above or below the aircraft, addition of the actinic
fluxes will give the spherically integrated total actinic flux. The
instrument configuration employs 2400 G/mm gratings which produce
a 1 nm FWHM spectral resolution. Similar
spectroradiometers have been deployed on the NASA DC-8 for the
PEM Tropics A, PEM Tropics B, and SONEX missions. Performance
on these missions was quite good with >90% data return. The
instruments will have response and detection characteristics represented
in Table 1.
Table 1.
Measurement
|
Detection Limit
|
Time Response
|
Accuracy
|
Precision
|
Actinic Flux 282-422nm
|
<0.05mW/cm2/nm
|
<10 sec
|
~6%
|
~3%
|
The
time response of the instrument depends on the number of wavelength
intervals sampled. In the instrument configuration for TRACE
P included 140 wavelength intervals can be sampled in <10
sec per scan.
The
spectral response of instruments is calibrated in our laboratory
using an optical calibration facility equipped with precision
radiometric power supplies and multiple NIST traceable 1000W
quartz tungsten halogen lamps. Secondary
lamp standards are employed in the field to calibrate the systems
before each aircraft flight. Wavelength
stability calibrations are performed every day using a mercury
lamp.
The
wavelength dependent actinic fluxes were used to calculate photolysis
frequencies of 11 atmospheric molecules important to the local
photochemistry. The molecules include O3, NO2,
CH2O, HONO, HNO3, CH3NO3,
CH3CH2NO3, H2O2,
CH3OOH, CH3COCH3, and PAN. Total
actinic fluxes and photolysis frequencies for the 11 molecules
will be reported to the data archive.
Figure
1
|