First Observation of Offshore Gradient of CO₂ and CH₄ Concentration in Southeast China from 21° N to 32° N Based on Shipborne Campaign

A shipborne campaign was conducted in China’s southeastern coastal waters (21° N–32° N) from 14 to 31 January 2024 to investigate atmospheric CO₂ and CH₄ concentrations and their offshore gradients. Advanced instrumentation enabled high-precision measurements, validated by canister sampling with strong correlations to reference data. The voyage employed a dual-route design: a northbound baseline along the mainland coast and a southbound route with offshore excursions up to 80 nm, facilitating the first quantification of GHG gradients in the continental shelf region. Baseline concentrations from the northbound route revealed regional variability: CO₂ levels ranged from 422.75 ± 9.96 ppm (Fujian) to 445.62 ± 1.51 ppm (Zhejiang), while CH₄ levels spanned 2005.78 ± 5.89 ppb (Fujian) to 2064.59 ± 13.93 ppb (Zhejiang). Southbound analysis at 10 nm intervals showed CO₂ gradients transitioning from positive to negative at ~30 nm and back to positive at ~70 nm, whereas CH₄ exhibited complex behavior, including a positive–negative–positive transition at 30–40 nm and consistent increase beyond 50 nm. Under winter monsoon conditions, transport flux analysis identified eastward CO₂ fluxes of 3819.55–6587.77 g·m⁻²·s⁻¹ and CH₄ fluxes of 6.42–11.42 g·m⁻²·s⁻¹. Southward transport diminished along the coast, with CO₂ fluxes declining from 5741.07 to 879.76 g·m⁻²·s⁻¹ and CH₄ fluxes from 9.84 to 1.49 g·m⁻²·s⁻¹ between Zhoushan and Hong Kong. The Taiwan Strait demonstrated a funneling effect, enhancing southward transport. These findings address data gaps in ocean regions and provide insights for future GHG monitoring.