Sensitivity of GEMS formaldehyde vertical columns to a priori profile for air mass factor during ASIA-AQ

Formaldehyde (HCHO), a byproduct of volatile organic compound (VOC) oxidation, is commonly used to constrain VOC emissions in chemical transport model (CTM) simulations through its vertical column density (VCD). While satellite observations provide extensive pollution maps of HCHO VCDs, they often involve significant uncertainties in vertical column retrievals, requiring continuous evaluation against ground-based observations. This study evaluates long-term GEMS HCHO products (version 3) by comparing them with Pandora HCHO VCDs from 2022 to 2024. GEMS HCHO VCDs are 10–40% lower but present good agreement (r=0.6–0.76) with Pandora observations, except for regions (Sapporo, Kobe, and Dhaka), where few samples are available due to the limited GEMS scan schedule. We examine the sensitivity of HCHO VCDs using air mass factors (AMFs) with a priori profiles from a five-CTM ensemble model with a fine spatial resolution (9 km) and updated emissions during the ASIA-AQ campaign, instead of using GEMS a priori profile. The ensemble model demonstrates improved agreement (r=0.83) with DC-8 observations compared to the GEMS a priori profile (r=-0.54), reducing overestimations of surface HCHO concentrations. Using vertical shape factors from ensemble model profiles increases AMFs by 20% in the morning at Suwon, improving agreements with MAX-DOAS observations (NMB= 86% to 33%). Our work highlights the need for continuous updates to a priori profile simulations for precise HCHO retrievals.