## 3 Results
### Local and Regional Simulations of Formaldehyde Columns
A strong enhancement in 2012 columns over TX is observed by OMI, particularly in June but also in August. Since the area is primarily covered by prairie grasslands, HCHO columns are lower over central and west TX compared to the SEUS. The severe drought that hit Texas during the summer of 2011 ([[https://www.drought.gov/states/texas](https://www.drought.gov/states/texas)]([https://www.drought.gov/states/texas](https://www.drought.gov/states/texas)), accessed on 1 September 2021) led to enhanced temperatures and PAR (Figure S3) and to lower humidity levels in comparison with 2012. The strong regional enhancement over TX observed in OMI, +21% of relative change (RC) on average (Table 2), is largely underestimated by the run ignoring drought stress (MAGRITTE, \(\gamma_{SM}=1\)) (+3%) because the faster VOC oxidation rate in 2012 (due to higher OH concentrations) and higher HCHO photolysis rate in 2011 (due to high radiation levels) were compensated by lower isoprene emissions in 2012 (due to lower temperatures and PAR). The model agreement with OMI greatly improves MAGRITTE, \(\gamma_{SM}^{OPT}\) run 3, when accounting for \(\gamma_{SM}^{OPT}\). The activity factor decreased biogenic emissions over east Texas in June 2012, leading to enhanced differences (14%) in HCHO columns over the TX region. In subsequent months, the activity factor also improves the prediction of the column differences between 2011 and 2012, from \(-\)10% in MAGRITTE, \(\gamma_{SM}=1\) to small positive changes. While the average change has been improved due to the inclusion of stress, its spatial distribution has not. For instance, MAGRITTE, \(\gamma_{SM}=1\) run 1 is in better agreement with OMI observations than MAGRITTE, \(\gamma_{SM}^{OPT}\) in East Texas (Piney Woods), pointing to an overestimated drought stress effect in this region. This region exhibits relatively high HCHO columns due to the important source of isoprene oxidation stemming from the presence of a mixed evergreen-deciduous forestland and the 2012-2011 absolute difference in temperatures (Figure S3) plays a dominant role in this interannual variation of isoprene emissions. Figure 6: Absolute difference in monthly HCHO columns (\(10^{15}\) molecules cm\({}^{-2}\)) at \(0.5^{\circ}\) spatial resolution in the eastern U.S. between 2012 and 2011 during the summer (June–August) for OMI (**top row**), MAGRITTE, \(\gamma_{SM}=1\) (**middle row**) and MAGRITTE, \(\gamma_{SM}^{OPT}\) (**bottom row**). The black boxes mark the areas selected for the regional assessments in Texas (TX: 26–34\({}^{\circ}\)N, 94–100\({}^{\circ}\)W) and southeastern U.S. (SEUS: 29–39\({}^{\circ}\)N, 75–93\({}^{\circ}\)W). The stippling indicates the grid cells where the differences in OMI HCHO columns is not statistically different from zero at the 68% confidence level (1\(\sigma\)), based on estimated random errors of the OMI retrieval. The southeastern forests, a dense temperate trees coverage, extend from east Texas (Piney Woods) to North Carolina. HCHO columns in SEUS are primarily attributed to the oxidation of non-methane VOC due to the high biogenic isoprene and monoterpene emissions [41; 42; 62]. The region experienced lower temperatures and PAR in June and August 2012 (Figure S3) which explains to a great extent the negative 2012-to-2011 HCHO differences observed by OMI and captured by the model, whereas opposite patterns are found in July. OMI observations in the SEUS show slight decreases in 2012 averaged columns in June (\(-3\%\)) and July (\(-2\%\)) followed by a larger drop of \(-14\%\) in August (Table 2). In June and August, the relative changes simulated by MAGRITTE, \(\gamma_{SM}^{OPT}\) in SEUS amount to \(-3\%\) and \(-20\%\), respectively, i.e., in overall better agreement than MAGRITTE, \(\gamma_{SM}^{0.04}\) using the default parameter. The impact of the drought stress factor (Figure S4) is mainly localized in Louisiana-Arkansas-Missouri and Georgia.