Multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations of formaldehyde and nitrogen dioxide at three sites in Asia and comparison with the global chemistry transport model CHASER

Formaldehyde (HCHO) and nitrogen dioxide (NO.sub.2) concentrations and profiles were retrieved from ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations during January 2017-December 2018 at three sites in Asia: (1) Phimai (15.18.sup." N, 102.5.sup." E), Thailand; (2) Pantnagar (29.sup." N, 78.90.sup." E) in the Indo-Gangetic Plain (IGP), India; and (3) Chiba (35.62.sup." N, 140.10.sup." E), Japan. Retrievals were performed using the Japanese MAX-DOAS profile retrieval algorithm ver. 2 (JM2). The observations were used to evaluate the NO.sub.2 and HCHO partial columns and profiles (0-4 km) simulated using the global chemistry transport model (CTM) CHASER (Chemical Atmospheric General Circulation Model for Study of Atmospheric Environment and Radiative Forcing). The NO.sub.2 and HCHO concentrations at all three sites showed consistent seasonal variation throughout the investigated period. Biomass burning affected the HCHO and NO.sub.2 variations at Phimai during the dry season and at Pantnagar during spring (March-May) and post-monsoon (September-November). Results found for the HCHO-to-NO.sub.2 ratio (R.sub.FN ), an indicator of high ozone sensitivity, indicate that the transition region (i.e., 1 R.sub.FN 2) changes regionally, echoing the recent finding for R.sub.FN effectiveness. Moreover, reasonable estimates of transition regions can be derived, accounting for the NO.sub.2 -HCHO chemical feedback.