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 NO 2 photolysis rate coefficients (j NO 2 )— are compared. The measurements were performed with two calibrated 2π‐actinometers assembled on the azimuth angle‐controlled LPMA/DOAS (Laboratoire de Physique Moléculaire 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 NO 2 and the skylight's spectrum shape changes with atmospheric height and solar illumination, only proxies (proxy‐ )j NO 2 rather than true j NO 2 values were monitored during balloon ascents (0–30 km) for solar zenith angle (SZA) 75° < SZA < 86°, and at balloon float altitude during solar occultation (86° < SZA < 95°). The measured direct and diffuse total proxy ‐j NO 2 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 NO 2 at large SZAs.