Airborne Measurements of Spectrally Resolved Actinic Flux and Photolysis Frequencies:

Instrumentation description

Richard E. Shetter and Christopher A. Cantrell

Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado



Spectrally resolved actinic flux measurements from 290-420 nm and atmospheric photolysis frequency measurements will be provided on the NASA DC-8 and P3-B aircraft for the PEM-Tropics B mission using actinic flux spectroradiometry [Shetter and Muller, 1998]. The technique is based on hemispherical quartz light collectors, double monochromators, and low dark current photomultipliers. The instrument is represented schematically in Figure 1.

Figure 1. Spectroradiometer Schematic

The instrument package on each aircraft will be identical and 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 actinic flux. The instruments will have response and detection characteristics represented in Table 1.

 

Table 1.

Measurement

Detection Limit

Time Response

Accuracy

Precision

Actinic Flux 280-420nm

<0.05m W/cm2/nm

<15 sec

~10%

~5%

JO(1D)

1x10-8 sec-1

<15 sec

~10%

~5%

J(NO2)

3x10-8 sce-1

<15 sec

~10%

~5%