## 3 Results
### Local and Regional Simulations of Formaldehyde Columns  
Here we examine the impact of drought stress on the temporal variability of isoprene fluxes and HCHO columns at the MOFLUX site. Both observed and calculated data are averaged biweekly (14 days) (Figure 5) and monthly (Table 1). As seen in Figure 3, isoprene fluxes exhibit great temporal variability at short timescales, but its evaluation using OMI HCHO observations cannot be performed due to the high noise and limited set of valid data (Section S2 of the Supplementary Materials). Observed biweekly OMI HCHO columns averaged over a 20-km radius around the site (Figure 5b) follow essentially the same pattern as in situ measured isoprene fluxes (Figure 5a), although with a dampened amplitude. This dampening is expected due to the presence of other HCHO precursors less affected by drought (e.g., methane, methanol and anthropogenic VOCs) and due to complex transport and chemical effects, as discussed for instance by [41]. MAGRITTE simulations show the same biweekly variability during summer 2011, with HCHO columns reduced when using the drought factor \(\gamma_{SM}^{OPT}\). Whereas the observed columns are relatively constant in July and early August, MAGRITTE simulations exhibit a greater biweekly variability with a mid-July peak, reflecting the temporal variability of isoprene fluxes. In 2012, OMI observations display a strong decline of HCHO column following the peak in early July, which is not reproduced by MAGRITTE simulations. Overall, application of the drought factor \(\gamma_{SM}^{OPT}\) reduces the positive biases in biweekly averages of HCHO columns estimated in the no-stress simulation as shown in Figure 5b. Note however that the OMI systematic errors are important, of the order of 50%, and encompass both MAGRITTE simulations, except in early August 2012. Figure 4: Observed and modelled diurnal cycles of isoprene fluxes normalized by the mean midday value (11–13 h) in years 2011 and 2012 over three periods: (i) early summer (May–June), (ii) mid-summer (July–August 10) and (iii) late summer (from August 10). \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**June**} & \multicolumn{3}{c}{**July**} & \multicolumn{3}{c}{**August**} \\ \cline{2-10}  & **2011** & **2012** & \begin{tabular}{c} **RC** \\ **(\%)** \\ \end{tabular} & **2011** & **2012** & \begin{tabular}{c} **RC** \\ **(\%)** \\ \end{tabular} & **2011** & **2012** & \begin{tabular}{c} **RC** \\ **(\%)** \\ \end{tabular} \\ \hline \hline \multicolumn{1}{l}{(a) Observed isoprene flux} & 7.3 & 10.7 & +47 & 14.4 & 9.0 & \(-\)38 & 7.2 & 3.4 & \(-\)53 \\ \hline \hline \multicolumn{1}{l}{(b) MEGAN, \(\gamma_{SM}=1\)} & 6.3 & 9.9 & +60 & 14.2 & 16.6 & +16 & 9.2 & 7.9 & \(-\)14 \\ \hline \multicolumn{1}{l}{(c) MEGAN, \(\gamma_{SM}^{OPT}\)} & 6.3 & 8.4 & +34 & 14.2 & 10.1 & \(-\)29 & 8.4 & 3.1 & \(-\)63 \\ \hline \multicolumn{1}{l}{(d) OMI HCHO column} & 16.2 & 17.5 & \begin{tabular}{c} +8 \\ (\(\pm\)22) \\ \end{tabular} & 21.9 & 17.7 & \
\begin{tabular}{c} \(-\)19 \\ (\(\pm\)13) \\ \end{tabular} & 16.2 & 12.5 &
\begin{tabular}{c} \(-\)23 \\ (\(\pm\)20) \\ \end{tabular} \\ \hline \hline \multicolumn{1}{l}{(e) MAGITITE HCHO column with \(\gamma_{SM}=1\)} & 16.9 & 17.6 & +4 & 24.5 & 22.2 & \(-\)10 & 20.6 & 16.3 & \(-\)21 \\ \hline \multicolumn{1}{l}{(f) MAGITITE HCHO column with \(\gamma_{SM}^{0.04}\)} & 16.5 & 17.5 & +6 & 24.1 & 22.1 & \(-\)8 & 20.1 & 16.0 & \(-\)20 \\ \hline \multicolumn{1}{l}{(g) MAGITITE HCHO column with \(\gamma_{SM}^{OPT}\)} & 15.9 & 16.9 & +6 & 23.0 & 18.4 & \(-\)20 & 19.3 & 13.9 & \(-\)28 \\ \hline \hline \end{tabular}
\end{table}