Atmospheric measurement of point source fossil CO2 emissions

We use the Kapuni Gas Treatment Plant to examine methodologies for atmospheric monitoring of point source fossil fuel CO2 (CO(2)ff) 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 similar to 0.1 Tg carbon per year. The plant is located in a rural dairy farming area, with no other significant CO(2)ff sources nearby, but large, diurnally varying, biospheric CO2 fluxes from the surrounding highly productive agricultural grassland. We made flask measurements of CO2 and (CO2)-C-14 (from which we derive the CO(2)ff 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 CO(2)ff content averaged over several weeks from the C-14 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 CO(2)ff in 15 min averaged surface flask samples and in similar to one week integrated CO(2)ff averages from grass samples. In this pilot study, we found that using grass samples, the modeled and observed CO(2)ff 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 C-14 sampling strategies.