# Summertime Aerosol Radiative Effects and Their Dependence on Temperature over the Southeastern USA
## 4 Conclusions  
By using satellite remote sensing observations from AATSR and OMI and aerosol-climate model ECHAM-HAMMOZ simulations in concert, we quantified the observed temperature dependence of the AOD and the corresponding radiative effects over the southeastern USA. The satellite observations lead to the conclusion that anthropogenic emissions are the main driver of summertime AOD levels in this region. There is also a temperature-dependent component in the summertime AOD over the southeastern USA but its impact on the total AOD is considerably smaller than the impact of anthropogenic emissions and, therefore, it is not clearly visible due to the changing level of anthropogenic emissions. Furthermore, it appears that anthropogenic emissions have a more dominant role in the seasonal cycle of AOD than biogenic emissions, although biogenic emissions are a prerequisite for the cycle. To quantify the temperature-dependent AOD component, we used the Orthogonal Distance Regression method. Based on this analysis, AOD appears to be influenced by tropospheric NO\({}_{2}\) column densities, LST and southerly wind speed but not by the other parameters used in our analysis. The dependence on tropospheric NO\({}_{2}\) column densities could be explained by anthropogenic emissions, whereas the dependence on southerly wind speed is likely a result of transported sea salt and Saharan dust. This analysis shows that the AOD has a small dependence on temperature ((7 \(\pm\) 6) \(\times\) 10\({}^{-3}\) K\({}^{-1}\)). Our model simulations produced a similar temperature dependence of the biogenic AOD over the southeastern USA. The model showed that the increase in AOD due to BVOC emissions and the subsequent SOA formation was \((4\,\pm\,1)\times 10^{-3}\) K\({}^{-1}\), which is within the uncertainty range of the observed change in the temperature-dependent AOD component. To evaluate the effect of the vegetation type on the observed temperature dependence of AOD we used the MODIS land cover type classification. The data showed that the three most abundant land cover types in this region are woody savannas, mixed forests and cropland/natural mosaic. When the analysis was limited to 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) pixels covered mainly with the above mentioned land cover types, only pixels with mixed forests exhibited a clear temperature dependence of the AOD. For the pixels covered mainly by mixed forests the biogenic contribution increases non-anthropogenic AOD by approximately (31 \(\pm\) 13) \(\times\) 10\({}^{-3}\) K\({}^{-1}\), which is over four times larger than for the whole domain. As the largest source of isoprene emissions in the southeastern USA are broadleaf trees [57; 83], the increased temperature dependence of AOD in the vicinity of forests supports our assumption that the temperature dependence is most likely caused by SOA formed from biogenic VOC emissions that increase with increasing temperature. The corresponding clear-sky direct radiative effect (DRE) of the observation-based biogenic AOD is \(-\)0.33 \(\pm\) 0.29 W/m\({}^{2}\)/K and \(-\)1.3 \(\pm\) 0.7 W/m\({}^{2}\)/K for the whole domain and over mixed forests only, respectively. The model estimate of the regional clear-sky DRE for biogenic aerosols is in the same range as the observational estimate: \(-\)0.29 \(\pm\) 0.09 W/m\({}^{2}\)/K. All these DRE values are significantly larger than the values reported for other forested regions [15; 86]. Most likely, the more pronounced radiative effects of biogenic aerosols over the southeastern USA are caused by high levels of pollution that enhance the formation of biogenic SOA. Furthermore, the model simulations show that biogenic emissions have a significant effect on the indirect radiative forcing in this region. The approximated effective radiative forcing (ERF) for the biogenic aerosols was \(-\)1.0 \(\pm\) 0.5 W/m\({}^{2}\)/K, which is larger than the values reported for other forested regions [15].