Sea surface carbon dioxide during early summer at the Tuandao nearshore time series site

To investigate air-sea CO₂ flux at the Qingdao nearshore site and its temporal variations, a high-resolution continuous observation of surface carbon dioxide partial pressure (pCO₂) was carried out at Zhongyuan Pier near Tuandao from May 25 to July 8, 2019. It was observed that during this period, surface pCO₂ varied between ∼490 and ∼690 μatm, mainly associated with sea surface temperature. Surface pCO₂ also displayed substantial diurnal variations, with an average amplitude of 64 ± 21 μatm, largely dominated by biological activities. During the observational period, this site acted as a source of atmospheric CO₂, releasing 361 mmol CO₂ m^-2. The notable diurnal variations in air-sea CO₂ flux, such as the observed average amplitude of 10.9 mmol m^-2 d^-1 in this study, pose a challenge for accurately estimating the air-sea CO₂ flux in coastal regions without high-resolution observations.