## 4 Discussion and Conclusions  
Previous studies conducted on the MOFLUX site noted that the low value of wilting point used in the default parameterization of MEGANv2.1 at this site (0.084 m\({}^{3}\) m\({}^{-3}\), [103]) prevent the stress factor from capturing the effect of drought on isoprene emissions [30; 32; 52; 68]. In this study, we focused on optimizing the empirical parameter \(\Delta\theta\) of the MEGANv2.1 soil moisture stress algorithm. The parameter \(\Delta\theta\) was originally set to 0.06 [48] and was later re-evaluated to a lower, more conservative, value of 0.04 m\({}^{3}\) m\({}^{-3}\)[5] so that the emissions are shut off only in extreme drought conditions. Here we re-evaluated it (\(\Delta\theta=0.12\)) based on isoprene flux measurements at the MOFLUX site conducted in mild and severe drought conditions, and GLEAM soil moisture. The adjusted parameterization leads to an improved prediction of daily isoprene emissions rates during the severe drought at the MOFLUX site (r = 0.90, RMSE = 2.02 mg m\({}^{-2}\) h\({}^{-1}\)) compared to the no-stress simulation (r = 0.76, RMSE = 4.98 mg\({}^{-2}\) h\({}^{-1}\)) and to the simulation adopting the MEGANv3 parameterization (r = 0.79, RMSE = 3.13 mg m\({}^{-2}\) h\({}^{-1}\)). Analysis of OMI and modelled tropospheric HCHO column over the MOFLUX site supports this conclusion. At a regional scale, the relative changes in simulated formaldehyde between the two years show some improvement when including the adapted parameterization over certain regions in the U.S., but clearly not over all areas. Nonetheless, global long-term HCHO simulations using \(\gamma_{SM}^{OPT}\) lead to heterogeneous results, with extended areas where the correlation with OMI observations worsens compared to the use of the default parameterization. The study is subject to limitations and uncertainties. Foremost, the adjustment of the parameter \(\Delta\theta\) is influenced by many factors including the \(\theta\) dataset, the soil depth of reference, and the MEGAN parametrized response functions to environmental conditions other than soil moisture. For instance, using in situ measurements of the \(\theta\) at 100 cm and the local wilting point (0.23 m\({}^{3}\) m\({}^{-3}\)), the optimized \(\Delta\theta\) would be about twice higher (0.25 m\({}^{3}\) m\({}^{-3}\)) than the value derived using GLEAM \(\theta\) (0.12 m\({}^{3}\) m\({}^{-3}\)). In addition, the high  
Figure 7: Difference in correlation coefficients of simulated HCHO columns with OMI HCHO columns at 2\({}^{\circ}\)\(\times\) 2.5\({}^{\circ}\) spatial resolution: R\({}_{2}\) – R\({}_{1}\), where R\({}_{1}\) and R\({}_{2}\) are the correlation coefficients from IMAGES, \(\gamma_{SM}^{0.04}\) and IMAGES, \(\gamma_{SM}^{OPT}\) simulations (displayed in Figure S7), respectively. sensitivity of isoprene emissions to soil properties using the MEGANv2.1 parameterization has been demonstrated in numerous studies and motivated the adoption of higher values of \(\Delta\theta\) despite the earlier rationale to only apply the isoprene reduction for extreme droughs, that led to its downward revision. Also, soil moisture should be seen as relative, i.e., framed by the soil properties the model uses. In particular, the high heterogeneity of porosity renders soil properties inaccurate since they are a function of porosity, such that the comparison of soil moisture in absolute terms is of limited relevance. Consistency in the use of soil characteristics dataset is also crucial as already pointed out in [49]. For year 2012, [PERSON] et al. (2015) [32] used the MEGANv2.1 drought activity factor algorithm with the measured \(\theta\) data at 10 cm jointly with the local wilting point (0.23 m\({}^{3}\) m\({}^{-3}\)) and found a substantial improvement in the overall model results against the MOFLUx isoprene flux measurements. However, the wilting point of 0.13 m\({}^{3}\) m\({}^{-3}\) for the top 30 cm (Section 2.1) would be more appropriate when using the 10 cm \(\theta\) data.