Comment on amt-2022-288

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Solar actinic radiation in the ultraviolet and visible range (UV/VIS) perpetuates atmospheric photochemistry by inducing photolysis processes which form reactive radical species. Photolysis frequencies are rate constants that quantify the rates of photolysis reactions and therefore constitute important parameters for quantitative analyses. Photolysis frequencies are usually calculated from modelled or measured solar spectral actinic flux densities. Suitable measurement techniques are available, but measurement accuracy can suffer from non-ideal 2<span class="inline-formula"><i>π</i></span> or 4<span class="inline-formula"><i>π</i></span> solid-angle reception characteristics of the usually employed 2<span class="inline-formula"><i>π</i></span> optical receivers or receiver combinations. These imperfections, i.e. deviations from an angle-independent response, should be compensated for by corrections of the measured data. In this work, the relative angular sensitivities of four commonly used 2<span class="inline-formula"><i>π</i></span> quartz receivers were determined in the laboratory in a range 280–660 nm. Based on this information, the influence of the non-ideal responses on measured spectral actinic flux densities for ground-based and airborne applications was investigated for a wide range of atmospheric conditions. Spectral radiance distributions and contributions of direct, diffuse downward and diffuse upward spectral actinic flux densities were calculated with a radiative transfer model to derive the corrections. The intention was to determine the ranges of possible corrections under realistic measurement conditions and to derive simple parametrizations with reasonable uncertainties. For ground-based 2<span class="inline-formula"><i>π</i></span> measurements of downward spectral actinic flux densities, corrections typically range <span class="inline-formula">&lt;10</span> % dependent on wavelength and solar zenith angle, with 2 %–8 % uncertainties covering all atmospheric conditions. Corrections for 4<span class="inline-formula"><i>π</i></span> airborne measurements were determined for the platforms Zeppelin NT (New Technology) and HALO (High Altitude and Long Range Research Aircraft) in altitude ranges 0.05–2 and 0.2–15 km, respectively. Total, downward and upward spectral actinic flux densities were treated separately. In addition to various atmospheric conditions, different ground albedos and small (<span class="inline-formula">&lt;5</span><span class="inline-formula">^∘</span>) aircraft attitude variations were considered in the uncertainties, as well as aircraft headings with respect to the sun in the case of HALO. Corrections for total and downward spectral actinic flux densities again typically range <span class="inline-formula">&lt;10</span> % dependent on wavelength, solar zenith angle and altitude, with 2 %–10 % uncertainties covering all atmospheric conditions for solar zenith angles below 80<span class="inline-formula">^∘</span>. For upward spectral actinic flux densities, corrections were more variable and significantly greater, up to about <span class="inline-formula">−50</span> % at low altitudes and low ground albedos. A parametrization for corrections and uncertainties was derived using uncorrected ratios of upward / downward spectral actinic flux densities as input, applicable independent of atmospheric conditions for a given wavelength, solar zenith angle and altitude. The use was limited to conditions with solar zenith angles <span class="inline-formula">&lt;80</span><span class="inline-formula">^∘</span> when direct sun radiation cannot strike upward- and downward-looking receivers simultaneously. Examples of research flights with the Zeppelin and HALO are discussed, as well as other approaches described in the literature.