Springtime Decoupling Between Column and Surface CO over China's Greater Bay Area Associated with Indochina Biomass Burning.

Carbon monoxide (CO) is an important tracer of combustion emissions and regional atmospheric transport. Over the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), column CO exhibits a distinct spring maximum, whereas near-surface CO follows a contrasting seasonal cycle with a winter maximum. We combined TROPOMI CO observations, MOPITT vertical profiles, and ground-based measurements for March-April 2019-2023 to investigate this column-surface decoupling. Relative to the 2022 low-fire reference for the core Indochina source region, the mean March-April column CO enhancements over the GBA were 22.7 ± 9.5% (2019), 30.0 ± 30.6% (2020), 26.4 ± 20.8% (2021), and 32.2 ± 16.1% (2023), with maxima of 43-64% in late March to early April. Weekly fire radiative power (FRP) anomalies were positively associated with GBA CO-column anomalies, while sensitivity tests showed that the enhancement estimates remained comparable. MOPITT profiles indicated that the springtime CO enhancement was most apparent at 700-800 hPa, while trajectories arriving at 500 m and 2500 m showed a pronounced vertical contrast in source connectivity. The results indicate that springtime Indochina biomass-burning influence over the GBA is expressed more strongly in the lower free troposphere and column CO burden than in near-surface CO, highlighting the complementary information provided by satellite column and surface observations.