Atmospheric measurement of point source fossil CO₂ emissions

Abstract. We use the Kapuni Gas Treatment Plant to examine methodologies for atmospheric monitoring of point source fossil fuel CO₂ (CO₂ff) emissions. The Kapuni plant, located in rural New Zealand, removes CO₂ from locally extracted natural gas and vents that CO₂ to the atmosphere, at a rate of ~0.1 Tg carbon per year. The plant is located in a rural dairy farming area, with no other significant CO₂ff sources nearby, but large, diurnally varying, biospheric CO₂ fluxes from the surrounding highly productive agricultural grassland. We made flask measurements of CO₂ and ¹⁴CO₂ (from which we derive the CO₂ff component) and in situ measurements of CO₂ downwind of the Kapuni plant, using a Helikite to sample transects across the emission plume from the surface up to 100 m above ground level. We also determined the surface CO₂ff content averaged over several weeks from the ¹⁴C content of grass samples collected from the surrounding area. We use the WindTrax plume dispersion model to compare the atmospheric observations with the emissions reported by the Kapuni plant, and to determine how well atmospheric measurements can constrain the emissions. The model has difficulty accurately capturing the fluctuations and short-term variability in the Helikite samples, but does quite well in representing the observed CO₂ff in 15 min averaged surface flask samples and in ~ one week integrated CO₂ff averages from grass samples. In this pilot study, we found that using grass samples, the modeled and observed CO₂ff emissions averaged over one week agreed to within 30%. The results imply that greater verification accuracy may be achieved by including more detailed meteorological observations and refining ¹⁴C sampling strategies.