Interpretation of ozone measurements by ground-based visible spectroscopy—a study of the seasonal dependence of airmass factors for ozone based on climatology data

Abstract

Interpretation of ozone absorption measurements by ground-based visible spectroscopy is highly dependent on how well the airmass factors for ozone are calculated. Total ozone values measured by SAOZ (Système d'Analyse par Observations Zenithales) and Brewer in Sodankylä have been compared in earlier papers. A systematic seasonal variation in the Brewer/SAOZ ratio has been observed. The ratio has a winter minimum and a summer maximum. In these calculations a standard set of airmass factors have been used to retrieve total ozone values from measured slant columns of ozone. The standard airmass factors have been calculated from radiative transfer calculations assuming a constant vertical distribution of atmospheric absorbers and scatterers. The seasonal variations of the Brewer/SAOZ ratio suggest that there is a seasonal dependence in the airmass factors that should be accounted for in the SAOZ algorithm. This paper focuses on the seasonal behaviour of the airmass factors for ozone. Computations of airmass factors using ozone, temperature and pressure profiles from sondes launched at Ny-Ålesund (79°N, 12°E), Sodankylä (67°, 27°E) and Hohen-peiβenberg (47 °N, 11 °E) reveal a clear seasonal dependency. At 90° solar zenith angle the computed airmass factors varied from summer to winter by 7.6, 6.5, and 6.0% at Ny-Ålesund, Sodankylä, and Hohenpeiβenberg, respectively. Airmass factors computed from sondes launched at Hohenpeiβenberg during the winter months (September–March) are 3% lower than those calculated for Ny-Ålesund in the same period. During the summer months (April–August) the difference is 1%. If the seasonal variation in the airmass factors is not accounted for, it will contribute to a seasonal dependency in the Brewer/SAOZ ratio of 4–6% for the three sites considered in this study. Monthly mean airmass factors that take into account the local climatology have been calculated for Sodankylä. When using monthly mean airmass factors the seasonal dependency of the Brewer/SAOZ ratio was reduced.