Fig. 1. Imaging mechanism of the SO₂ camera. (a) Normalized SO₂ absorption cross-section and solar scattering spectrum; (b) transmittance curves of filters at 310 nm and 330 nm

Fig. 2. Calibration curve obtained by the calibration cell

Fig. 3. Calibration curve obtained by DOAS

Fig. 4. Calibration curves obtained by spectral calibration method

Fig. 5. Scattering intensity curves of self-calibration method. (a) Variation of solar scattering spectral intensity with wavelength at different θ_SZA; (b) relative scattering intensity curves at different θ_SZA

Fig. 6. Intensity ratio of the two channels varying with θ_SZA at different atmospheric conditions

Fig. 7. Calculated calibration coefficients varying with logarithm of intensity ratio of the two channels

Fig. 8. Original images taken by the UV camera and artificial background images. (a) Original image of 310 nm channel; (b) original image of 330 nm channel; (c) background image of 310 nm channel; (d) background image of 330 nm channel

Fig. 9. Calibration curve in the validation experiment

Fig. 10. Experimental site in Shanghai Port. (a) Set-up of SO₂ camera, DOAS, and associated components; (b) target ship

Fig. 11. Original images of ship plume. (a) Original image obtained from 310 nm channel; (b) original image obtained from 330 nm channel; (c) intensity ratio of artificial background images of two channels

Fig. 12. SO₂ discharge capacity of ship exhaust obtained from UV camera

Fig. 13. Time series of relative errors obtained from DOAS approach and self-calibration mothed