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.