Construction and Deployment of a Portable, Low-Cost Sensor Unit for Measurement of CO₂ Fluctuations

Introduction

• CO₂ flux data is necessary to understand greenhouse gas balances and how the global carbon cycle functions which can be used for:

  • Climate impacts (how humans impact the environment, how the environment reacts, and how we can reduce our footprint)

  • Optimizing agricultural practices (carbon sequestration through reduced tilling, optimizing fertilizer use, planting cover crops, etc.) to annually maximize efficiency, reduce costs, and increase soil resilience

• CO₂ flux data is gathered in various locations, but with more long-term datasets from a wider range of areas comes a greater global understanding of climate systems
• Current methods primarily use sensor units that are both permanent and cost-prohibitive installations (providing high quality data with little interference but at the cost of having a larger network of sensor units to cover more ground)
• This research investigates an alternative method which solves the problem of both cost and permanence by making a portable and inexpensive sensor unit 
• CO₂ flux is defined as the direction and rate of flow between two CO₂ sinks4 and can be calculated using
  • F = (dC/dt)((P_amb x V)/(R_gas x T x A))
• Variables:
  • F = Calculated flux (μmol m⁻² hr⁻¹)
  • dC/dt = Concentration change of CO₂ over time (ppm hr^-1)
  • P_amb = Ambient pressure (Pa)
  • V = Chamber volume (m³)
  • R_gas = Ideal gas constant (8.314 m³ Pa K⁻¹ mol⁻¹)
  • T = Temperature (K)
  • A = Chamber surface area (m²)
• To calculate fluctuations, the change in concentration of CO₂ over time can be measured using a sensor unit

Project Objectives

• Construct a low-cost, portable carbon dioxide flux sensor unit able to repeatedly measure soil fluctuations
• Deploy in the field and gather ambient data
• Code a functional program to run the system
• Code a functional data analysis method

Methods

• Two separate versions of the CO₂ flux sensor unit were built
• Version 1 (V1) – a simplified “test” unit for indoor use (not portable) (see Figs. 1 and 2)
• Version 2 (V2) – a complete automated unit capable of field deployment (semi-portable) (see Figs. 1, 3, and 4)

Results

• Indoor, overnight data from commercial potting mix collected with V1 confirmed successful sensor measurements and flushing system for the unit (see Figs. 5 and 6)
• The cycle in Fig. 6 represents a CO₂ flux of approximately 38 mg CO₂ hr-1 m-2
• Unknown malfunctions with electronic components used for V2 prevented the creation of an entirely self-dependent circuit for the system, resulting in a semi-portable sensor unit (see Fig. 3)

Conclusions

• The CO₂ sensor unit system is capable of measuring ambient CO₂ fluxes from soil. 
• Fully portable sensor units could be deployed in remote areas to gather more global CO₂ flux data

Future Directions

• Deploy sensor unit in the field at Albert Schweitzer Institute
• Code a comprehensive data analysis script
• Create a fully portable sensor unit
• Include an ambient pressure sensor in the unit