An enhanced bubble size sensor for long-term ebullition studies

Methane ebullition from freshwater sediments is a significant source of greenhouse gas to the atmosphere. Methane bubble size determines the fraction of methane that dissolves into the water column instead of entering the atmosphere, but current methods to measure bubble sizes in situ are limited. Previously, we reported the design for a novel optical bubble size sensor capable of measuring methane bubble sizes in situ. Here, we report sensor enhancements to allow for continuous long-term sensor deployment, uninterrupted data collection under high ebullition flux, and improved bubble volume measurement accuracy. The enhanced design includes a data and power cable running from the sensor to a custom data buoy housing batteries and an SD memory card. Batteries and SD cards are replaced every 2-4 weeks through the data buoy, allowing for continuous, undisturbed sensor deployment. In addition, sensor parameters have been tuned to reduce error from particle intrusion and fluctuations in raw signals that were observed during transit of some bubbles through the sensor. Field data show that on average the sensor measures cumulative bubble volumes equaling approximately 86% of collected gas volumes. We discuss potential mechanisms of remaining error, some of which may be due to zooplankton intrusion, and suggest directions for future sensor enhancements.