# Analysis of Four Years of Global XCO\({}_{2}\) Anomalies as
## 4 Results
### Case Study: Collocated OCO-2 and TROPOMI Observations in the Highveld Region  
We analyse the daily XCO\({}_{2}\) anomalies over the Highveld industrial region in South Africa together with NO\({}_{2}\) tropospheric columns. The area includes several coal-fired power plants as well as pollution from urban sites. We concentrate the analysis on the month of July 2018, in order to reduce the effect of the CO\({}_{2}\) drawdown. We find six direct overlaps with the OCO-2 swath (including a sufficient number of XCO\({}_{2}\) retrievals) passing through NO\({}_{2}\) local enhancements (plumes). In Figure 10, we illustrate the TROPOMI NO\({}_{2}\) tropospheric columns overlaid with local OCO-2 XCO\({}_{2}\) anomalies (with background calculated from the values inside a latitude-longitude box \([40^{\circ}\)S, \(20^{\circ}\)S] \(\times\)\([15^{\circ}\)E, \(45^{\circ}\)E]). In all six cases, we note clear XCO\({}_{2}\) gradients between the area inside the NO\({}_{2}\) plume and the surrounding background. On 11 and 20 July 2018, we identify smaller plumes originating from Midmba coal-fired power plant, but also on 13 July 2018 we see fine structures with high XCO\({}_{2}\) anomalies over the NO\({}_{2}\) plumes. In order to analyze the cases further, we collocate the TROPOMI NO\({}_{2}\) observations to each OCO-2 footprint. In Figure S6, we illustrate the normalized histograms of OCO-2 XCO\({}_{2}\) anomalies labeled as \"anthropogenic\" and \"surrounding,\" discriminated when tropospheric NO\({}_{2}\) values are larger or smaller than \(5\times 10^{15}\) molec./cm\({}^{2}\), respectively. In all cases, we note that the anthropogenic histograms peak at higher positive XCO\({}_{2}\) anomalies than the histogram for the surrounding. We also note that on 4 July 2018, only a few data points are classified as \"anthropogenic\". In Figure 11, we illustrate the correlation between the median NO\({}_{2}\) and XCO\({}_{2}\) anomaly values inside and outside the NO\({}_{2}\) plume as well as the difference between the values inside and outside the plume for each of the six days. We note that the points outside the NO\({}_{2}\) plumes (red dots in Figure 11) fall into the bottom-left part of the scatter plot, corresponding to negative XCO\({}_{2}\) anomalies and relatively low NO\({}_{2}\) levels. Meanwhile, the points falling inside the plume with enhanced NO\({}_{2}\) levels (blue dots in Figure 11) show a positive linear correlation between the median NO\({}_{2}\) tropospheric columns and XCO\({}_{2}\) anomalies. A similar correlation is observed for the differences between the values inside and outside the plume (yellow dots in Figure 11). In both cases, the slope, or the CO\({}_{2}\)-to-NO\({}_{2}\) ratio, is about \(\sim\)1.5 \(\times 10^{-16}\) ppm/(molec./cm\({}^{2}\)). On 11 and 20 July 2018, the OCO-2 flight track passes right through the NO\({}_{2}\) plume generated from the Midmba power plant. Figure 12 shows the OCO-2 XCO\({}_{2}\) values (black dots) as a function of latitude during both days. Figure 12 (red dots) also includes the XCO\({}_{2}\) values obtained by empirically converting TROPOMI NO\({}_{2}\) tropospheric columns (collocated to the OCO-2 measurements) into XCO\({}_{2}\) using the results of the linear regression between the two data sets. The CO\({}_{2}\)-to-NO\({}_{2}\) ratio in both  
Figure 9: Case studies for the year 2015: Iberian peninsula, Highveld industrial region in South Africa, India, and Central America. OCO-2 XCO\({}_{2}\) anomalies and FLEXPART XCO\({}_{2}\) enhancements are overlapped to the Google map background. days was found to be \(\sim\)1.1 \(\times 10^{-16}\) ppm/(molec./cm\({}^{2}\)). Both XCO\({}_{2}\) cross-sections show a similar enhancement when passing through the emission plumes (latitude 24.1\({}^{\circ}\)S). Figure 11: Scatter plot between the median XCO\({}_{2}\) anomalies and tropospheric NO\({}_{2}\) columns inside (blue dots) and outside (red dots) the NO\({}_{2}\) plumes. The difference between the values inside and outside the plume are also shown (yellow dots).