Abstract:
Formaldehyde (HCHO) and nitrogen dioxide (NO2) 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◦ N, 102.5◦ E), Thailand; (2) Pantnagar (29◦ N,
78.90◦ E) in the Indo-Gangetic Plain (IGP), India; and (3) Chiba (35.62◦ N, 140.10◦ E), Japan. Retrievals were
performed using the Japanese MAX-DOAS profile retrieval algorithm ver. 2 (JM2). The observations were used
to evaluate the NO2 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 NO2 and HCHO concentrations at all three sites showed consistent
seasonal variation throughout the investigated period. Biomass burning affected the HCHO and NO2 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-NO2 ratio (RFN), an indicator of high ozone sensitivity, indicate
that the transition region (i.e., 1 < RFN < 2) changes regionally, echoing the recent finding for RFN effectiveness.
Moreover, reasonable estimates of transition regions can be derived, accounting for the NO2–HCHO chemical
feedback.
The model was evaluated against global NO2 and HCHO columns data retrieved from Ozone Monitoring
Instrument (OMI) observations before comparison with ground-based datasets. Despite underestimation, the
model well simulated the satellite-observed global spatial distribution of NO2 and HCHO, with respective spatial
correlations (r) of 0.73 and 0.74. CHASER demonstrated good performance, reproducing the MAX-DOAS retrieved HCHO and NO2 abundances at Phimai, mainly above 500 m from the surface. Model results agree
with the measured variations within the 1-sigma (1σ) standard deviation of the observations. Simulations at
higher resolution improved the modeled NO2 estimates for Chiba, reducing the mean bias error (MBE) for the
0–2 km height by 35 %, but resolution-based improvements were limited to surface layers. Sensitivity studies
show that at Phimai, pyrogenic emissions contribute up to 50 % and 35 % to HCHO and NO2 concentrations,
respectively.
