Estimates of CO<sub>2</sub> fluxes over the city of Cape Town, South Africa, through Bayesian inverse modelling
We present a city-scale inversion over Cape Town, South Africa. Measurement sites for atmospheric CO<sub>2</sub> concentrations were installed at Robben Island and Hangklip lighthouses, located downwind and upwind of the metropolis. Prior estimates of the fossil fuel fluxes were obtai...
| Main Authors: | , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-04-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/4765/2018/acp-18-4765-2018.pdf |
| Summary: | We present a city-scale inversion over Cape Town, South Africa. Measurement
sites for atmospheric CO<sub>2</sub> concentrations were installed at Robben
Island and Hangklip lighthouses, located downwind and upwind of the
metropolis. Prior estimates of the fossil fuel fluxes were obtained from a
bespoke inventory analysis where emissions were spatially and temporally
disaggregated and uncertainty estimates determined by means of error
propagation techniques. Net ecosystem exchange (NEE) fluxes from biogenic
processes were obtained from the land atmosphere exchange model CABLE
(Community Atmosphere Biosphere Land Exchange). Uncertainty estimates were
based on the estimates of net primary productivity. CABLE was dynamically
coupled to the regional climate model CCAM (Conformal Cubic Atmospheric
Model), which provided the climate inputs required to drive the Lagrangian
particle dispersion model. The Bayesian inversion framework included a
control vector where fossil fuel and NEE fluxes were solved for separately.</br></br>Due to the large prior uncertainty prescribed to the NEE fluxes, the current
inversion framework was unable to adequately distinguish between the fossil
fuel and NEE fluxes, but the inversion was able to obtain improved estimates
of the total fluxes within pixels and across the domain. The median of the
uncertainty reductions of the total weekly flux estimates for the inversion
domain of Cape Town was 28 %, but reach as high as 50 %. At the pixel level,
uncertainty reductions of the total weekly flux reached up to 98 %, but these
large uncertainty reductions were for NEE-dominated pixels. Improved
corrections to the fossil fuel fluxes would be possible if the uncertainty
around the prior NEE fluxes could be reduced. In order for this inversion
framework to be operationalised for monitoring, reporting, and verification (MRV) of emissions from Cape Town, the NEE component of the CO<sub>2</sub>
budget needs to be better understood. Additional measurements of
Δ<sup>14</sup>C and <i>δ</i><sup>13</sup>C isotope measurements would be a
beneficial component of an atmospheric monitoring programme aimed at MRV of
CO<sub>2</sub> for any city which has significant biogenic influence, allowing
improved separation of contributions from NEE and fossil fuel fluxes to the
observed CO<sub>2</sub> concentration. |
|---|---|
| ISSN: | 1680-7316 1680-7324 |