Fig. 3: Instrument Accuracy Verification Diagram.

a1, a2 are the standard gas verification results of NO2 and HCHO, respectively. We established three distinct concentration levels for HCHO and NO2 using a gas blending device, achieving observed values that closely matched the intended levels. Figures (b1) and (b2) compare and correlate the long-term NO2 observation data between the AHCTIS and iterative cavity-enhanced DOAS (ICAD) systems, both positioned at the same monitoring site. Figures (c1) and (c2) present a similar analysis for O3, comparing long-term observation data between the AHCTIS and Thermo instruments. The O3 data for Thermo were obtained from the China National Environmental Monitoring Centre (CNEMC) monitoring station, situated less than 2 km from the AHCTIS instrument in a direct line. The information of the instruments used in the verification experiment is shown in the Supplementary Table 2-6.