In:
Journal of Geophysical Research: Atmospheres, American Geophysical Union (AGU), Vol. 101, No. D13 ( 1996-08-20), p. 18613-18627
Abstract:
A set of photoelectric detectors for airborne measurements of the photolysis frequency of NO 2 , i.e., J NO 2 , was developed and integrated aboard the research aircraft Hercules C‐130 operated by the U.K. Meteorological Office. The instrument consists of two separate sensors, each of which provides an isotropic response over a solid angle of 2π steradian (sr). The sensors are mounted on top and below the aircraft, respectively, to obtain a field of view of 4π sr, and permit the discrimination of the upwelling and downwelling components of the actinic flux. From experimental tests and model calculations it is demonstrated that small differences between the spectral sensitivity of the sensors and the spectral response of J NO 2 can lead to significant errors in the determination of J NO 2 , especially under cloudy conditions. We present correction factors for clear sky conditions and suggest the use of a new filter combination in the sensors which requires only small corrections and provides acceptable accuracy, even under cloudy conditions. A climatology of J NO 2 values is presented from a series of flights made in 1993 at latitudes of 36°–59°N. For clear sky conditions and solar zenith angles of 33°–35°, J NO 2 was 8.3 × 10 −3 s −1 at sea level and increased with altitude to values of 13 × 10 −3 s −1 at 7.5 km altitude. Above clouds, J NO 2 reached maximum values of 24 × 10 −3 s −1 , and peak values of 29 × 10 −3 s −1 were observed for very short periods in the uppermost layers of clouds. Enhancement of the actinic flux due to light scattered from clouds was also observed at altitudes below 0.5 km. Comparison of the clear sky data with predictions from different radiative transfer models reveals the best agreement for models of higher angular resolution. The Delta Eddington method underpredicts the measurements significantly, whereas the J NO 2 values predicted by the discrete ordinate method and multidirectional model are only about 5% smaller than our measurements, a difference that is within the experimental uncertainties.
Type of Medium:
Online Resource
ISSN:
0148-0227
Language:
English
Publisher:
American Geophysical Union (AGU)
Publication Date:
1996
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