Empirical model of optical thickness of stratospheric aerosol according to data of lidar measurements in Obninsk from 2012 to 2023

The main source of sulfur in the stratosphere is volcanic emissions of SO₂. In explosive type eruptions, they occur directly into the stratosphere. A certain contribution is made by other sources of anthropogenic and natural origin, the products of which enter the stratosphere as a result of vertical diffusion and tropospheric-stratospheric exchange. The final product of the chemical transformation of sulfur compounds is sulfuric acid, which condenses in the presence of water vapor to form a sulfuric acid aerosol. 

Lidar measurements of stratospheric aerosol have been carried out at STC “Typhoon” (Obninsk, 55.1°N, 36.6°E) since 2012. The average optical thickness of the stratospheric aerosol for the period from 2012 to 2023 in the layer from 13 to 30 km was 0.010. The report compares lidar measurements of optical thickness with data available in the literature on the entry of SO₂ into the stratosphere. A preliminary analysis showed that there is a similarity in the time dependence of the average annual values of τ_S(t_i) and the sulfur content of F(t_i) in the stratospheric aerosol associated with explosive volcanic eruptions (t_i is the year number). To build the model, it is assumed that the value of F(t_i)= 0.46[Q_T (t_i -1) + Q_T (t_i -2)exp(-1) + Q_T (t_i -3)exp(-2) +… Q_C (t_i) + Q_C (t_i -1)exp(-1) + Q_C (t_i -2)exp(-2)+…],where Q_T, Q_C are tropical and mid–latitude average annual emissions of explosive-type volcanoes, taken with a time shift of Δ=1 for tropical belt eruptions and Δ =0 for mid-latitude eruptions. The data on Q_T and Q_C are taken from [Carn et al., J. of Vols. and Geoth. Res., 311, 99-134; Schallock et al., Atmos. Chem. Phys., 23, 1169-1207], as well as from the NASA website [https://so2.gsfc.nasa.gov/measures.html]. Next, the linear regression equation τ_S (t_i) =         K₁F(t_i)+ K₂ is considered. The 2012-2023 lidar measurements were supplemented by data from early measurements at the Obninsk lidar station after the eruption of Mount Pinatubo in 1991. The average τ_S value for 1992 was 0.14 ± 0.01, and the value of 21300 Kt was assumed for Q_T based on available literature data. As a result, the coefficients K₁=1.38 x 10^-5 (Kt of sulfur)^-1 and K₂ =0.0038 were obtained. The K₂ value represents the background τ_S value over the city of Obninsk, not connected with explosive-type volcanic eruptions. Accordingly, for the average τ_S value associated with explosive volcanic eruptions, the value 0.01-0.0038=0.0062 is obtained. To interpret the background τ_S value, the contribution of alternative tropospheric sources to the sulfur content in the stratospheric aerosol, which include degassing volcanoes and other natural and anthropogenic sources, was taken into account. According to [Brodowsky et al., Atm. Chem. Phys., 24, 5513-5548] the annual averaged contribution of these sources is 156 Kt over the previous decade. When this addition is taken into account in F(t_i), the coefficient K₁ does not change, and the coefficient K₂ decreases by 1.38 x 10^-5 x 156 = 0.0022. The value 0.0022 refers to the background sulfuric acid aerosol, which is not associated with explosive volcanic eruptions. The residual difference of 0.0038 - 0.0022=0.0016 corresponds to a non-sulfuric acid aerosol present in the stratosphere (biomass burning aerosol and meteor aerosol). A satisfactory correspondence of the proposed model with independent data has been obtained, which makes it possible to extend the estimates obtained to overall latitudinal zone of 50-60 N.