Between marine and continental air: atmospheric CO₂ variability and local wind associations in coastal Taizhou, China

Atmospheric carbon dioxide (CO₂) observations at Hongjia, Taizhou, from April 2023 to November 2024 were analyzed using statistical methods, backward-trajectory clustering, the weighted potential source contribution function, and cross-tabulation of trajectory over-ocean fractions and local winds. The mean mole fraction was (440.61 ± 14.0) × 10⁻⁶, lower than at the inland Lin’an station. Concentrations were higher in winter and spring, lower in summer and autumn, and highest around dawn and lowest in the afternoon. Wind speed was negatively correlated with CO₂ in all seasons; W–WSW winds were associated with higher concentrations. The 72 h trajectories were classified as continental (over-ocean fraction f ≤ 0.2), mixed (0.2 < f < 0.8), or marine (f ≥ 0.8). The over-ocean fraction was negatively correlated with CO₂ (r = − 0.367). Marine pathways combined with SE–S winds and speeds ≥ 3 m s⁻¹ yielded a mean of 425.8 × 10⁻⁶, whereas continental-pathway means ranged from 448.1 to 449.2 × 10⁻⁶ under different wind filters. Weak westerlies across pathways corresponded to 451.5 × 10⁻⁶, increasing to 459.3 × 10⁻⁶ in winter. Within pathway classes, weak-westerly means exceeded those under other valid wind conditions by 6.9–12.8 × 10⁻⁶. These contrasts cannot independently quantify local anthropogenic contributions. Potential source regions included the Zhejiang–Fujian coast, the Jiangsu–Anhui border, and North China. The findings characterize coastal CO₂ variability associated with regional transport, local transport and dispersion, and terrestrial ecosystem exchange.