Fig. 1. Basic principle for measurement of trace gas concentrations in the open atmosphere.

Fig. 2. Principle of DOAS: separate broadband and narrowband spectral structures.

Fig. 3. Schematic diagram of DOAS system: (a) traditional DOAS, (b) DOAS based on Y-type optical fiber. 1, xenon arc lamp; 2, UV-enhanced reflector; 3, lamp house; 4, lens coupler; 5, Y-type fiber; 5A, flat reflecting mirror; 5B, flat reflecting mirror; 5C, receiving fiber; 6, main mirror of telescope; 7, absorption cell; 8, retro-reflector; 9, spectrometer; 10, computer.

Fig. 4. Fiber structure: (a) Y-type fiber bundle, (b) end plane structure of fiber.

Fig. 5. Schematic diagram of calibration experiment. 1, SO2 standard gas; 2, NO2 standard gas; 3, N2 balance gas; 4, 5, 6, pressure relief valve; 7, 8, 9, mass flowmeter; 10, mixture chamber; 11, absorption cell; 12, pressure meter; 13, thermometer; 14, exhaust; 15, light source; 16, telescope; 17, retro-reflector; 18, spectrometer; 19, computer.

Fig. 6. Data analysis process.

Fig. 7. Absorption cross section of NO2 and SO2.

Fig. 8. Convolution of absorption cross section: (a) SO2, (b) NO2.

Fig. 9. Separation of absorption cross section: (a) SO2, (b) NO2.

Fig. 10. Measured spectrum (top) and denoising results (bottom): (a) SO2, (b) NO2.

Fig. 11. Differential optical density (top) and differential absorption cross section (bottom): (a) SO2, (b) NO2.

Fig. 12. Calibration with standard gas: (a) NO2, (b) SO2.

Fig. 13. Measurement of mixture gas: (a) calibration with standard NO2 in mixture of SO2 and NO2, (b) calibration with standard SO2 in mixture of SO2 and NO2.

Fig. 14. Field measurement of DOAS: (a) main body of DOAS system, (b) retro-reflector array.

Fig. 15. Comparison of the measured concentration from DOAS and monitoring station: (a) SO2, (b) NO2.