## 3 Results
### Local and Regional Simulations of Formaldehyde Columns

Table 1: Monthly averages of **(a-c)** local isoprene fluxes (in mg m\({}^{-2}\) h\({}^{-1}\)) from observations and MEGAN estimations (MEGAN, \(\gamma_{SM}=1\) and MEGAN, \(\gamma_{SM}^{OPT}\)); **(d-g)** formaldehyde columns (in \(10^{15}\) molecules cm\({}^{-2}\)) observed by OMI sensor around the MOFLUX site and calculated by MAGRITTE with biogenic emissions in no-stress conditions (MAGRITTE, \(\gamma_{SM}=1\)) and applying the drought stress factor (MAGRITTE, \(\gamma_{SM}^{0.04}\) and MAGRITTE, \(\gamma_{SM}^{OPT}\)). Relative changes (RC) are provided between monthly averages in years 2011 and 2012 as percentages. The relative changes in OMI columns are provided along with their uncertainty estimates at 1\(\sigma\)-level calculated based on random uncertainties of columns retrievals. Figure 5: Biweekly averaged (**a**) observed isoprene fluxes (in mg m\({}^{-2}\) h\({}^{-1}\)) in June-August 2011 and 2012 (with standard deviations \(\sigma\) of the daily variability shown as orange shading) and MEGAN estimates in no-stress conditions and with \(\gamma_{SM}^{OPT}\) (with \(\sigma\) shown as error bars); **(b)** HCHO columns (\(10^{15}\) molecules cm\({}^{-2}\)) from OMI observations within a 20-km radius of the MOFLUX site (orange) (with systematic error bars from the retrievals) and from MAGRITTE simulations using biogenic emissions neglecting (blue) or accounting for \(\gamma_{SM}^{OPT}\)(green). Monthly isoprene fluxes were enhanced in June 2012 compared to the previous year (Figure 5a and Table 1). More favourable (i.e., warm and sunny) environmental conditions at the onset of the severe drought explain this increase, as shown by the model ability to match the large emission enhancement in June (+47% in the observations and 34-60% in the model). In subsequent months, observed isoprene fluxes were about 40% and 50% lower than in the previous year, representing drastic decreases relative to the model expectation ignoring drought stress effects (+16% and \(-14\%\) in July and August, respectively). The updated MEGAN model based on \(\gamma_{SM}^{OPT}\) reproduces fairly well the seasonal variation (Figure 5a) as well as the inter-annual variability of summer monthly averages (Table 1). A peak was observed in OMI HCHO in July 2011 at about 22 \(\times\) 10\({}^{15}\) molecules cm\({}^{-2}\) mainly due to high temperatures entailing fast VOC oxidation and strong biogenic emissions. About half this column was observed in August 2012 (12.5 \(\times\) 10\({}^{15}\) molecules cm\({}^{-2}\)) when severe drought took its toll on biogenic emissions. While the small increase observed in June 2012 with respect to the previous year (+7%) is well simulated in all runs, the substantial drop in July (\(-\)19%) is very well represented in MAGRITTE, \(\gamma_{SM}^{OPT}\) (\(-\)20%), whereas MAGRITTE, \(\gamma_{SM}=1\) and MAGRITTE, \(\gamma_{SM}^{0.04}\) barely account for half of that reduction (Table 1). Yet in August, the relative impact of drought in MAGRITTE, \(\gamma_{SM}^{OPT}\) is slightly overestimated (\(-\)28% vs. \(-\)23% in OMI). While those comparisons point to a better performance of MAGRITTE, \(\gamma_{SM}^{OPT}\) at reproducing the interannual variability of monthly averaged HCHO columns, uncertainties in OMI relative changes (given between brackets in Table 1) do not allow for a robust conclusion to be drawn. In particular, in June, the uncertainty (\(\pm\)22%) exceeds the relative change (+8%). Further, we assess the absolute difference in HCHO columns of 2012 and 2011 observed by OMI and modelled by MAGRITTE with \(\gamma_{SM}=1\) and \(\gamma_{SM}^{OPT}\) (Figure 6). Two regions depicted in Figure 6, where MAGRITTE predicts significant impacts of the drought stress effect, namely Texas (hereafter TX) and southeast U.S. (hereafter SEUS), are examined. Grid cells that are marked by a stippling are those for which the 2011 and 2012 columns are not statistically different (at the 68% confidence level), based on the random error estimates provided with the HCHO product.