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

To investigate air-sea CO flux at the Qingdao nearshore site and its temporal variations, a high-resolution continuous observation of surface carbon dioxide partial pressure (pCO) was carried out at Zhongyuan Pier near Tuandao from May 25 to July 8, 2019. It was observed that during this period, surface pCO varied between ∼490 and ∼690 μatm, mainly associated with sea surface temperature. Surface pCO 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 CO, releasing 361 mmol CO m. The notable diurnal variations in air-sea CO flux, such as the observed average amplitude of 10.9 mmol m d in this study, pose a challenge for accurately estimating the air-sea CO flux in coastal regions without high-resolution observations.