# Summertime Aerosol Radiative Effects and Their Dependence on Temperature over the Southeastern USA
## 3 Results and Discussion
### Radiative Impacts
Both methods produced similar estimates (\(-\)0.097 \(\pm\) 0.066 W/m\({}^{2}\)/K and \(-\)0.063 \(\pm\) 0.040 W/m\({}^{2}\)/K with remote sensing and in situ observations, respectively), which are three to five times smaller than our estimates for the southeastern USA. This difference in the magnitude of the estimates is in line with the findings of [PERSON] et al. [23] and [PERSON] and [PERSON] [87], who showed that the biogenic SOA mass concentrations are high in the southeastern USA because the particle partitioning potential of organic compounds is driven by anthropogenic pollution. Consequently, high levels of pollution enhance the formation of biogenic SOA, which leads to more pronounced radiative effects of biogenic aerosols over the southeastern USA than in regions with less pollution (e.g., boreal forests). Based on an equation similar to Equation (5) and AOD observations done with MISR, [PERSON] et al. (2018) estimated that the all-sky DRE of the summertime aerosols in this region would be \(-3.9\) W/m\({}^{2}\) (although they erroneously called it clear-sky). They estimated the radiative effect as the difference between summertime and wintertime AODs (resulting in an AOD difference of 0.18). For the clear-sky case (\(C_{c}\) = 0) their DRE estimate would be \(-9.75\) W/m\({}^{2}\). For comparison, we calculated seasonal differences (summer-winter (DJF) averages) from our datasets. Based on the AATSR data, the seasonal AOD difference was 0.23, which is in the same range as the seasonal AOD difference reported by [PERSON] et al. (2018). To compare how much the non-anthropogenic AOD could change due to the seasonal temperature difference we calculated the seasonal temperature difference (17.5 K) and multiplied it with the slope of the temperature dependence of the AOD (\((7\pm 6)\times 10^{-3}\) K\({}^{-1}\)), resulting in an AOD change of 0.12 \(\pm\) 0.10. This is only half of the total seasonal AOD difference. Assuming that our equation correctly represents the biogenic contribution to the AOD for the seasonal temperature range, this implies that the difference between the winter and summertime AODs cannot solely be explained with biogenic emissions. In a similar way, we estimated that the DRE of the temperature dependent AOD component due to the seasonal temperature change would be \(-6.0\)\(\pm\) 4.7 W/m\({}^{2}\) and \(-2.4\)\(\pm\) 1.9 W/m\({}^{2}\) for clear- and all-sky conditions, respectively. Furthermore, we estimated the clear-sky DRE caused by biogenic aerosols from the model simulations by subtracting the summertime net clear sky top-of-atmosphere solar radiation of the noBIOSOA simulation from the CONTROL simulation. This led to an average clear-sky DRE of \(-1.9\)\(\pm\) 0.7 W/m\({}^{2}\), which is at the lower limit of our observational estimate for the clear sky DRE of the temperature-dependent AOD component and significantly smaller than the DRE estimate of [PERSON] et al. (2018). This supports our previous conclusion that there are other factors in addition to the temperature-enhanced biogenic emissions that affect the seasonal AOD difference in this region and the radiative effects of biogenic aerosols cannot be estimated based on seasonal differences in the aerosol load. To highlight the importance of anthropogenic emissions on the seasonal AOD differences we calculated the following example. During the years 2005-2007 when the summertime tropospheric NO\({}_{2}\) column densities were larger than \(2.3\times 10^{15}\) molecules/cm\({}^{2}\), the seasonal AOD difference was 0.27. For the summers 2008-2011 with tropospheric NO\({}_{2}\) column densities lower than 2.3 \(\times\) 10\({}^{15}\) molecules/cm\({}^{2}\), the seasonal AOD difference was also lower, 0.20. However, for both periods the temperature change between the seasons was equivalent: 17.4 K and 17.8 K for 2005-2007 and 2008-2011, respectively. The significantly larger AOD difference corresponds to larger tropospheric NO\({}_{2}\) column densities while the temperature change does not seem to have a noticeable effect.