The procedure for determining the total content of chlorine nitrate (ClONO₂) from ground-based measurements of the solar radiation spectra on a Bruker 125HR Fourier spectrometer at the St. Petersburg station (59.88^oN, 29.82^oE, 20 m above sea level) of the international observational network NDACC is examined. The method was applied to spectra measured in the period from 2009 to 2019, and the results were compared with calculations by the EMAC chemistry-climate model. Good qualitative and quantitative agreement was obtained between the experimental data and the results of numerical modeling. For the period 2009–2017 the average mismatch between model and experiment was 3%, the standard deviation was 43%, and the correlation coefficient was 0.79 ± 0.02, which indicates an adequate description of the variability of the total ClONO₂ content by the model. Assessment of the linear trend of the total ClONO₂ content showed a significant decrease in the total chlorine nitrate content in the atmosphere over St. Petersburg according both to the ground-based measurements (–2.3 ± 1.9% per year) and to the model (–1.2 ± 0.4% per year).