Comparison of measured and modeled stratospheric UV/visible actinic fluxes at large solar zenith angles

Measured and modeled stratospheric filter sensitivity weighted ultraviolet/visible (UV/vis) actinic fluxes - approximating the NO2 photolysis rate coefficients (j(NO2)) - are compared. The measurements were performed with two calibrated a 2 pi -actinometers assembled on the azimuth angle-controlled LPMA/DOAS (Laboratoire de Physique Moleculaire et Applications / Differential Optical Absorption Spectroscopy) gondola during a series of balloon flights. Since the actinometer's spectral sensitivity curve did not exactly match the actinic spectrum of NO2 and the skylight's spectrum shape changes with atmospheric height and solar illumination, only proxies (proxy-)j(NO2) rather than true j(NO2) values were monitored during balloon ascents (0-30 km) for solar zenith angle (SZA) 75 degrees < SZA < 86 degrees, and at balloon float altitude during solar occultation (86 degrees < SZA < 95 degrees). The measured direct and diffuse total proxy-j(NO2), values compare excellently with radiative transfer (RT) modeling. That finding allows us to rule out uncertainties in computing UV/vis actinic fluxes as a significant factor in the still insufficient modeling of stratospheric NO2 at large SZAs.