## 3 Results and Discussion
### Co\({}_{2}\): WRF-Chem vs. Observations
We note that because of the lack of data at ERSA in the Springtime 2017 period, no measurement enhancement time series is available, as this was the designated background site. However, the figures are included to show the component breakdown from the simulations and to allow for comparison against the simulation when the urban heat island module was turned off. Table 5 shows the mean and one standard deviation of the modeled and observed enhancements for Summertime 2016 and Wintertime 2017. [PERSON] et al. [50] found wintertime enhancements in a similar range for Indianapolis, which has a comparable population. Their winter modeled values were \(-0.52\) ppm compared to their measured enhancements, which is broadly consistent with what we find here. Their analysis did not extend to the summer. Los Angeles has a much stronger CO\({}_{2}\) enhancement that can range from 2-8 ppm [95], which during the day is almost entirely due to fossil fuel emissions [25]. Although the former is to be expected due to the larger population and number of emission sources, which work with the local geography to create a persistent urban dome [23], the latter point is consistent across all three seasons, as seen in Figure 6. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Station** & **Summer Model** & **Summer Obs** & **Winter Model** & **Winter Obs** \\ \hline OHP (ppm) & 403.6 \(\pm\) 1.6 & 400.4 \(\pm\) 2.7 & 409.3 \(\pm\) 0.7 & 412.8 \(\pm\) 2.5 \\ CAV (ppm) & 403.4 \(\pm\) 1.8 & 400.4 \(\pm\) 2.7 & 409.5 \(\pm\) 0.7 & 412.8 \(\pm\) 2.5 \\ SME (ppm) & 403.4 \(\pm\) 1.6 & 400.4 \(\pm\) 2.7 & 409.5 \(\pm\) 0.7 & 412.8 \(\pm\) 2.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mean modeled and observed background values and one standard deviation of the mean for Summertime 2016 and Wintertime 2017 in the simulations with the urban heat island module. First, we note that the modeled and measured enhancement values at OHP are negative and are even stronger in the summertime than in the wintertime. This is because of increased biogenic CO\({}_{2}\) uptake, which is logical because OHP is mostly surrounded by vegetation. The modeled enhancement is 74% more pronounced than the observed value, and almost matches the NEE component. As Table 4 shows, the model provides a background value that is 3 ppm higher than the measured marine background. This is not expected, as the biosphere signal in the summer would normally bring the continental background value below that of the marine background. This difference in background values between the model and the observations is partially compensated for by the larger negative enhancement provided by the model. However, the observations do not perfectly follow the NEE component, indicating that the site is likely partly influenced by anthropogenic fluxes, as we be analyzed further below. We note that in Figure 6 the model provides FF fluxes at OHP that are about ten times lower than the NEE ones during the summer (top left-hand panel). In the winter, the enhancement values provided by the model and the observations agree quite well, although the background value is 3.6 ppm lower in the model than in the ERSA data. This is also somewhat surprising, as the continental background would be expected to be higher than the marine background in the winter, when vegetation is dormant and anthropogenic emissions are at their maximum. Here too the modeled enhancement at OHP mostly follows the NEE variations (Figure 6, top middle panel), while the observations show some peaks that are likely due to the influence of anthropogenic emissions, as analyzed further below. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Station** & **Summer Model** & **Summer Obs** & **Winter Model** & **Winter Obs** \\ \hline OHP (ppm) & \(-4.0\pm 2.8\) & \(-2.3\pm 3.5\) & \(-1.4\pm 1.5\) & \(-1.1\pm 2.3\) \\ CAV (ppm) & \(-1.6\pm 2.7\) & \(1.2\pm 5.5\) & \(3.3\pm 3.2\) & \(5.7\pm 7.5\) \\ SME (ppm) & \(-3.3\pm 3.4\) & \(0.1\pm 4.7\) & \(1.7\pm 2.4\) & \(2.5\pm 6.3\) \\ \hline \hline \end{tabular}
\end{table}