## 3 Results and Discussion
### Co\({}_{2}\): WRF-Chem vs. Observations  
To assess the efficacy of the WRF-chem model-based CO\({}_{2}\) values, we compared them against the observations at particular CO\({}_{2}\) instrument sites. As stated in Section 2.3, observations during our Summertime 2016, Wintertime 2017, and Springtime 2017 periods of interest were available from four sites: OHP, CAV, SME, and ERSA. Of these, three sites are within the innermost highest-resolution 1 km WRF domain. The enhancement values  
Figure 5: Boundary layer height comparison for WRF-Chem simulations with and without the urban heat island effect. The top, middle, and bottom row are the three stations: OHP, CAV, and SME, respectively, while the first, second, and third column are the Summertime 2016, Wintertime 2017, and Springtime 2017 periods, respectively. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Station** & **Summertime 2016** & **Wintertime 2017** & **Springtime 2017** \\ \hline OHP & 19.85\% & 8.49\% & 7.59\% \\ CAV & 5.01\% & 5.68\% & 6.41\% \\ SME & 22.02\% & 4.24\% & 9.55\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mean difference in boundary layer height between simulations with and without the urban heat island effect module, expressed as percentages, at each of the measurement sites across each of the periods. shown here are calculated as described in the Methods section. Hours when the wind was blowing from the NE at ERSA were considered to be potentially contaminated by urban and industrial plumes coming off of Italy, and were consequently left out of the analysis [40]. Table 4 shows a comparison of the background values from the model and from the observations (mean and one standard deviation) for Summer 2016 and Winter 2017; note that no background observations were available for Springtime 2017. For each season, the model background value is similar at the three stations, showing that the modeling framework has very little regional background variability. This point is expanded upon in the next section. In Summertime 2016, the model background is 3 to 3.2 ppm above the observation background and is less variable by about 40%. In Wintertime 2017, the model background is lower than the ERSA background value by about 3.5 ppm and is less variable by about 70%. These differences are taken into account when comparing the modeled and observed enhancements. The remaining enhancement values were then compared against the WRF-based mixing ratio values, which are composed of a combination of four CO\({}_{2}\) components: ODIAC2017 as the anthropogenic component, NEE as the biogenic component, our manually-derived ECMWF-based ocean product for the sea component, and the FINN fire component. The values are taken from the latitude/longitude/altitude pixel which encompasses the measurement site in question at the appropriate sampling height. Although (1 km)\({}^{2}\) is high-resolution, these sites are significantly smaller than the pixels, which will necessarily introduce some unknown amount of uncertainty due to representation error (e.g., [94]). Figure 6 shows this comparison with the component breakdown for three of the sites: OHP at 100 m sampling height in the top row, CAV in the middle row, and SME in the bottom row. The left-hand column shows the Summertime 2016 comparisons, the middle column contains the Wintertime 2017 comparisons, and the right-hand column shows the Springtime 2017 comparisons. The total simulated CO\({}_{2}\) value is included as a blue line. We also include the total CO\({}_{2}\) value for the simulations that did not include the urban heat island effect, shown as a dotted brown line, although the component breakdowns are left out of the figure. We limit our comparison to only the daytime hours of between 12:00 and 18:00 UTC, where the model's boundary layer is expected to be sufficiently developed and well-mixed to provide reasonable results.