Our understanding of the significance of dimethylmercury (DMHg) to the mercury (Hg) global ocean biogeochemical cycle is unclear because of the lack of detailed DMHg measurements in the water column. To our knowledge, 30 years of published studies have generated no more than 200 DMHg data points in the ocean surface waters and marine boundary layer (MBL). To improve the precision and reduce the uncertainty in determining DMHg in surface seawater, we developed a simple and robust DMHg automatic analyzer (DAA). This DAA system couples the main sampling and analytic steps, including a continuous flow chamber, with dual Carbotrap preconcentration, a gas chromatographic column, a cold vapor atomic fluorescence spectrometry, and a data logger for signal integration. We compared the operation, performance, and reproducibility between our DAA and the traditional manual analytic method. Its advantages include the ease of operation, the high time resolution and precision (30 min sampling and <5% relative variation), and long-term stability (2 weeks). The DAA can determine DMHg in both the MBL and surface seawater. The estimated detection limits for DMHg with the DAA in the atmosphere and in surface seawater are 10 pg/m3 and 0.2 fM, respectively. The successful DAA field measurement in coastal waters indicates that it can help detect the low DMHg concentration in surface seawater, and the time series DMHg data helped our understanding of the DMHg behavior (sources and sinks) and its flux into the MBL. The comparison of DMHg concentration in various oceans also suggests that the coastal region had the lowest averaged DMHg, up to an order of magnitude lower than other ecosystems.
Keywords: automatic analyzer; coastal waters; dimethylmercury; dissolved gaseous mercury; high time resolution; marine boundary layer.