Fig. 1. Relative spectral intensity of measured spectrum (dashed line) and solar scattering spectrum (solid line) at 7 km away from the source. Solar scattered light with wavelength less than 300 nm cannot be applied to remote sensing due to ozone effect, which is not shown here

Fig. 2. SO₂ absorption cross-section (dotted line) and normalized solar scattering spectrum (solid line) in the band of 280-360 nm, and relative transmittance curves of filter A and filter B are also shown

Fig. 3. Variation of α_AMF with monitoring distance for different extinction coefficients

Fig. 4. Topographic and image information used to fit the extinction coefficient. (a) A 3D topographic map of measured volcano, with straight lines and cross indicating the pixel points selected for fitting the extinction coefficient, and small rectangular boxes indicating the location of the camera; (b) raw image collected by SO₂ UV camera, with the pixel points selected for fitting the extinction coefficients mark with black lines and cross, and the rectangle represents the area where the background intensity of the sky is calculated

Fig. 5. Results of extinction coefficient fitting, the data indicated by asterisks in the figure correspond to the distance information and intensity information in Fig. 4. (a) Channel A; (b) channel B

Fig. 6. Images of the relative positions of calibration cell and the camera and optical thickness before and after the correction of light dilution effect. (a)(c) Position relationship between calibration cell and the camera before and after the correction of light dilution effect; (b)(d) optical thickness images of calibration cell before and after the correction of light dilution effect

Fig. 7. SO₂ calibration curves obtained by cell method. (a) Calibration curves without correction for light dilution effect on the cell method; (b) calibration curves considering the effect of light dilution

Fig. 8. Retrieval results of SO₂ column density image. (a)(b) SO₂ column density image retrieved by cell calibration curves before and after correction of light dilution effect; (c) SO₂ column density image retrieved by DOAS method; (d) data comparison in column 200 of SO₂ column density images in Fig. 8(a)-(c)

Fig. 9. Schematic of SO₂ UV camera optical structure and light reflection path

Fig. 10. Fitting results of cell calibration curve after error correction

Fig. 11. Retrieval results of SO₂ column density. (a) SO₂ column density image retrieved by cell calibration curve after error correction; (b) data comparison in column 200 of SO₂ column density images

Fig. 12. Difference between observation fields of view (FOV) of SO₂ UV camera and DOAS method

Fig. 13. SO₂ emission rate. (a) Evolution of SO₂ emission rate with time for DOAS method, cell method, and cell method after error correction; (b) statistical comparison of emission rates obtained by cell method after error correction with those obtained by DOAS method, where the horizontal axis represents the emission rates obtained by DOAS method and the vertical axis represents the emission rates obtained by cell method after error correction

Fig. 14. Calibration error varies with extinction coefficient fitting error and background measurement error. (a) Extinction coefficient fitting error; (b) background measurement error

Fig. 15. Variation of calibration error with filter reflectance measurement error and cell windows reflectance measurement error. (a) Filter reflectance measurement error; (b) cell window reflectance measurement error