The Response Time of Neon's Soil CO2 Sensors and an Assessment of Its Impact on Soil CO2 Flux Calculations Using the Gradient Method.

The National Ecological Observatory Network (NEON) is measuring vertical profiles of soil CO₂ concentration in five soil plots at each of 47 sites to allow calculation of soil CO₂ fluxes using the gradient method. NEON designed a soil CO₂ assembly that allowed soil air to diffuse to the CO₂ sensor, while protecting the sensor from the soil environment and allowing sensor removal for calibration without disturbing the surrounding soil. However, the assembly’s relatively large volume slows the sensor’s response time. In this study, we quantify the response time of CO₂ sensors installed in these assemblies, as well as the response time’s impact on soil CO₂ flux calculations. Since both temperature and pressure influence the rate of CO₂ diffusion, the response time was determined across a range of temperatures and pressures that are representative of NEON site conditions using a controlled environmental chamber. As expected, the sensors had a relatively slow response time with a 75% response after 2.1±0.3 hours at 10 °C and 101 kPa. Due to their impacts on the rate of CO₂ diffusion, higher temperatures and lower pressures resulted in faster 75% response times (e.g., 1.0±0.1 hours at 50 °C and 72 kPa versus 2.2±0.4 hours at -29 °C and 101 kPa). Based on our tests we developed a model to predict the response time at any temperature and pressure combination within the range of -29 °C to 50 °C and 72 kPa to 101 kPa. We will assess the impact of these response times on soil CO₂ flux calculations.