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

Measured and modeled stratospheric filter sensitivity weighted ultraviolet/ visible (UV/vis) actinic fluxes - approximating the NO2 photolysis rate coe fficients (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 / Differenti al Optical Absorption Spectroscopy) gondola during a series of balloon flig hts. Since the actinometer's spectral sensitivity curve did not exactly mat ch the actinic spectrum of NO2 and the skylight's spectrum shape changes wi th atmospheric height and solar illumination, only proxies (proxy-)j(NO2) r ather than true j(NO2) values were monitored during balloon ascents (0-30 k m) for solar zenith angle (SZA) 75 degrees < SZA < 86 degrees, and at ballo on float altitude during solar occultation (86 degrees < SZA < 95 degrees). The measured direct and diffuse total proxy-j(NO2), values compare excelle ntly with radiative transfer (RT) modeling. That finding allows us to rule out uncertainties in computing UV/vis actinic fluxes as a significant facto r in the still insufficient modeling of stratospheric NO2 at large SZAs.