Fig. 1. MAX-DOAS regional station location in Qianxian, Xi'an and the instrument observation principle
(a): Observation location; (b): Observation principle

Fig. 2. Ultraviolet and visible blue H₂O and O₄ absorption cross section

Fig. 3. Water vapor optimal retrieval band test
(a): Ultraviolet; (b): Visible

Fig. 4. Examples of DOAS fitting retrieval in ultraviolet (a) and visible bands (b)

Fig. 5. The DSCD in the ultraviolet and visible bands (α=5°)
(a): O₄ DSCD; (b): H₂O DSCD; (c): H₂O DSCD/O₄ DSCD

Fig. 6. Correlation analysis of O₄ and H₂O DSCD in in the ultraviolet and visible bands
(a): H₂O DSCD; (b): O₄ DSCD; (c): H₂O DSCD/O₄ DSCD

Fig. 7. The absorption cross section of H₂O and other trace gases (O₄, HONO and HCHO) in the ultraviolet band

Fig. 8. Example of DOAS fitting retrieval for O₄, HONO and HCHO
(a): The water vapor absorption reference cross section was not included in the fitting process;(b): The water vapor absorption reference cross section was included in the fitting process