A set of photoelectric detectors for airborne measurements of the photolysis frequency of NO2, i.e., JNO2, 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 2&pgr; steradian (sr). The sensors are mounted on top and below the aircraft, respectively, to obtain a field of view of 4&pgr; 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 JNO2 can lead to significant errors in the determination of JNO2, 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 JNO2 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¿, JNO2 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, JNO2 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 JNO2 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. ¿ American Geophysical Union 1996