UV/Vis Stratospheric Air Mass Factors considering photochemistry at two Antarctic stations

Abstract

Abstract. The molecules NO2, O3, OClO and BrO play a major role in the photochemistry of stratospheric ozone, notably in the formation of the springtime Antarctic ozone hole. For this reason, these species have been monitored by Differential Optical Absorption Spectroscopy (DOAS) instrumentation for many decades. In order to transform DOAS Slant Column Densities (SCDs) into Vertical Column Densities (VCDs), independent of the viewing geometry, the Air Mass Factors (AMFs) relating these quantities are needed. Ground-based stratospheric trace gas measurements are performed in zenith-viewing geometry at twilight, around and beyond 90° solar zenith angle (SZA). At those solar angles, the Earth’s sphericity and the rapid changes in photochemical parameters (e.g., photolysis rate coefficients) affect the calculation of the AMFs, particularly for photochemically active species such as NO2, OClO and BrO. This study presents a methodology to infer AMFs that account for sphericity and photochemical effects. We estimate stratospheric AMFs of NO2, O3, OClO and BrO for Belgrano and Marambio Antarctic stations using the MYSTIC [Mayer, 2009; Emde et al., 2010] Radiative Transfer Model (RTM). The photochemical changes taking place during twilight are considered using a photochemical box-model based on the SLIMCAT chemistry transport model [Chipperfield et al., 1999, 2006]. Vertical profile concentrations obtained by this model are averaged over the optical paths and used as an input for the MYSTIC RTM. The robustness of the proposed methodology is tested against measurements of NO2, O3, OClO and BrO SCDs obtained at Marambio and Belgrano. A good agreement is observed between modelled and measured values of NO2, O3 and OClO SCDs. For BrO bigger differences are obtained but they have been attributed to the tropospheric BrO contribution that has not been included in the model. Our results show that monthly averaged AMFs can be considered as a good approximation for O3 and BrO, but more temporally resolved sampling is recommended for NO2 and especially OClO during July. This work shows the large impact of photochemistry for both the magnitude and also the SZA dependency of the evolution of the AMFs during twilight.