MAX-DOAS observations of formaldehyde and nitrogen dioxide at three sites in Asia and comparison with the global chemistry transport model CHASER

Abstract

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 through 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. 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. The NO2 and HCHO concentrations at all three sites showed consistent seasonal variations throughout the investigated period. Biomass burning affected the HCHO and NO2 variation in Phimai during the dry season and in Pantnagar during spring (March–May) and post-monsoon (September–November). The results on the HCHO to NO2 ratio (RFN), an indicator of high ozone sensitivity, show that the transition region (i.e., 1< RFN < 2) changes regionally, echoing the recent finding on the effectiveness of RFN. Moreover, reasonable estimates of transition regions can be derived accounting for the NO2- HCHO chemical feedback. CHASER demonstrated good performances reproducing the HCHO and NO2 abundances at Phimai, mainly above 500 m from the surface. Model results agree with the measured variations, ranging within the one sigma standard deviation of the observations. Despite the complex terrain of Pantnagar (mountainous terrain), the modeled NO2 estimates between 1.8–2 km were reasonable. Simulations at higher resolution improved the modeled NO2 estimates in Chiba, reducing the mean bias error (MBE) in the 0–2 km height by 35 %. However, resolution-based improvements were limited to the surface layers. Sensitivity studies showed pyrogenic emissions in Phimai contribute to the HCHO and NO2 concentrations up to ~ 50 and ~35 %, respectively.