Optimizing the spectral analysis of SO2 MAX-DOAS measurements near coal-fired power plants

Ground-based MAX-DOAS (Multi-Axis Differential Optical Absorption Spectroscopy) measurements are widely used to monitor atmospheric pollutants like e.g. NO₂, HCHO or SO₂ (nitrogen dioxide, formaldehyde or sulfur dioxide). However, measurements of SO₂ in plumes from strong emission sources like coal-fired power plants remain challenging due to difficulties in choosing the optimum spectral range for the data analysis. Fit windows at short wavelengths cover the strongest SO₂ absorption bands, but suffer from low intensity signals and spectral interference with the strong O₃ absorption. Alternative fit windows have higher intensity signals, but cover weaker SO₂ absorptions. This study presents a systematic investigation of the SO₂ data analysis in different spectral ranges for MAX-DOAS measurements performed close to power plant plumes. In the early plume, the SO₂ concentrations can vary strongly and can reach extremely high values close to the stack. The focus lies on improving the quality of the resulting SO₂ dSCDs (differential slant column densities) through an optimized selection of spectral fitting windows for SO₂ and NO₂ under highly polluted conditions.