# Summertime Aerosol Radiative Effects and Their Dependence on Temperature over the Southeastern USA
## 3 Results and Discussion  
### Temperature Dependence of Summertime AOD over the Southeastern USA  
As the first step, we investigated the relationship between AOD and LST using the AATSR observations. The anomalies of the regional mean LST and AOD were calculated for the summers (JJA) of the years 2005-2011 by subtracting the average of all the summers from the yearly summer averages. Summer averages were calculated from monthly averages to ensure that each month had an equal weight in the seasonal average. Figure 1 presents a scatterplot of the observed AOD versus LST anomalies for the summers 2005-2011 and apparently, there is no clear correlation between these anomalies for these years. This lack of correlation seems to be in contradiction with the results of [PERSON] et al. [18], who presented a linear correlation between AOD (retrieved from MISR data) and temperature anomalies (from Goddard Institute for Space Studies (GISS)) for the years 2000-2005 averaged over the same region. Figure 1: Summertime anomalies (JJA) of aerosol optical depth (AOD) vs. regional mean land surface temperature (LST) over the southeastern USA for the years 2005–2011. Pentagons represent averages over the whole domain. LST and AOD are from the L3 AATSR. The error bars represent the uncertainty of the observations (one standard deviation). To further investigate this apparent discrepancy, we compared the time series of AOD, LST and other quantities that might influence the AOD in this region (e.g., tropospheric NO\({}_{2}\) column densities, total column water vapour [70], soil moisture [71], fire radiative power [72], and meteorological parameters [65]) for the summers of 2005-2011. Examples of these time series are shown in Figure 2. Their comparison shows that LST and AOD have some similar features with high values in 2007 and 2011 and a minimum in 2009, but their correlation coefficient is only 0.16. The temporal variations of tropospheric NO\({}_{2}\) column densities and AOD are in better agreement with similar features and a much higher correlation (\(r=0.92\)). Finally, the comparison between AOD and southerly wind speed at 10 m (SW10) shows no correlation (\(r=-0.02\)). The other quantities not shown in Figure 2 did not exhibit any clear relationships with AOD but that does not mean that AOD could not be affected by those quantities during certain summers. To conclude, a significant correlation is observed only between summertime AOD and tropospheric NO\({}_{2}\) column densities over the southeastern USA. As tropospheric NO\({}_{2}\) is predominantly generated from anthropogenic sources [73], Figure 2 indicates that the summertime AOD levels in this region are mainly related to the amount of anthropogenic emissions. The NO\({}_{2}\) column densities have decreased between 2005 and 2011 (see Table S2 for details) and this decrease is in line with the reductions of anthropogenic emissions in the studied region (e.g., [24; 25; 47; 74; 75]). The emissions have decreased for several years due to emission control measures, but the economic crisis in 2007-2009 augmented the reduction rate of the emissions further [76], which is also visible in Figure 2.  
The AOD can be affected by anthropogenic emissions of both primary aerosol particles and precursor gases and also by enhancing SOA formation from biogenic precursors (e.g., [16; 19; 20]). [PERSON] et al. [73] showed that over the southeastern USA summertime AOD values and formaldehyde (used as an indicator for non-methane VOCs) column densities are strongly correlated, which implies that SOA formation could have a more important contribution to AOD than primary emissions. Correspondingly, [PERSON] et al. [77] found that monoterpene SOA accou  
Figure 2: Summer averages of LST (K), AOD, tropospheric NO\({}_{2}\) column densities (molecules/cm\({}^{2}\)) and the southerly wind speed at 10 m (m/s) over the southeastern USA, for the years 2005–2011.