# Summertime Aerosol Radiative Effects and Their Dependence on Temperature over the Southeastern USA
## 3 Results and Discussion
### Temperature Dependence of Summertime AOD over the Most Common Land Cover Types  
The largest source of isoprene emissions in the southeastern USA is broadleaf trees [57; 83], thus if the observed temperature dependence of AOD is caused by biogenic emissions the dependence should be larger in the vicinity of forests than in locations with fewer trees. To study this, we used the MODIS land cover type classification data and limited the observed datasets to the three most common land cover types: woody savannas (22 pixels out of 82), mixed forests (13 pixels out of 82) and cropland/natural mosaic (10 pixels out of 82) (see Figure S6 for details). Then, the same analysis we did to estimate the temperature-dependent AOD component for the whole domain (described in Section 3.1) was also performed for each of the land cover types separately. None of the three land cover types exhibited a distinguishable relationship between AOD and LST anomalies (see Figures S7-S9 for details) whereas AOD and tropospheric NO\({}_{2}\) column density anomalies exhibited positive linear relationships over all of them (see Figures S10-S12 for details). The ODR analysis revealed that over mixed forests AOD was related to tropospheric NO\({}_{2}\) column density anomalies and temperature anomalies but not to southerly wind speed anomalies (with 95% confidence intervals given in parentheses):  
\[AOD_{anom,MF}=a~{}NO_{2,trop,anom}+b~{}LST_{anom}+c~{}SWV10_{anom}+d, \tag{2}\]  
where  
\(a=2.07\mathrm{e}^{-16}\pm 5.11\mathrm{e}^{-17}~{}\mathrm{cm}^{2}~{}\mathrm{ molec}^{-1}\),  
\(b=0.027\pm 0.013~{}\mathrm{K}^{-1}\),  
\(c=0.022\pm 0.041~{}\mathrm{s}~{}\mathrm{m}^{-1}\), and  
\(d=0.001\pm 0.012\)  
Over woody savannas AOD was related to tropospheric NO\({}_{2}\) column density anomalies and southerly wind speed anomalies but not to temperature anomalies:  
\[AOD_{anom,WS}=a~{}NO_{2,trop,anom}+b~{}LST_{anom}+c~{}SWV10_{anom}+d, \tag{3}\]  
where  
\(a=2.07\mathrm{e}^{-16}\pm 4.94\mathrm{e}^{-17}~{}\mathrm{cm}^{2}~{}\mathrm{ molec}^{-1}\),  
\(b=-0.005\pm 0.011~{}\mathrm{K}^{-1}\),  
\(c=0.056\pm 0.033~{}\mathrm{s}~{}\mathrm{m}^{-1}\), and  
\(d=-0.000\pm 0.012\).  
Over cropland/natural mosaic AOD was clearly related only to tropospheric NO\({}_{2}\) column density anomalies:  
\[AOD_{anom,CM}=a~{}NO_{2,trop,anom}+b~{}LST_{anom}\mp c~{}SWV10_{anom}+d, \tag{4}\]  
where  
\(a=2.25\mathrm{e}^{-16}\pm 5.12\mathrm{e}^{-17}~{}\mathrm{cm}^{2}~{}\mathrm{ molec}^{-1}\),  
\(b=0.003\pm 0.010~{}\mathrm{K}^{-1}\),  
\(c=-0.016\pm 0.033~{}\mathrm{s}~{}\mathrm{m}^{-1}\), and  
\(d=-0.000\pm 0.013\).  
These regression results indicate that the positive temperature dependence of AOD was statistically significant only over mixed forests where the AOD increases as a function of temperature by approximately (\(27\pm 13\)) \(\times\) 10\({}^{-3}~{}\mathrm{K}^{-1}\). The slope of this linear fit is almost four times larger than that for the whole domain. This is in line with the notion that forests are the main BVOC source in the studied domain [57; 83]. As the other land cover types are emitting significantly smaller amounts of BVOCs, this finding supports our assumption that the temperature-dependent AOD component derived from the observations is most likely caused by SOA formed from biogenic VOC emissions. Furthermore,AOD depends on southerly wind speed anomalies only over woody savannas, which are mainly located at the southeastern corner of the southeastern USA close to the coasts (see Figure S6 for details), which is a region most likely affected by transport of sea salt and Saharan dust. Finally, the AODs over all land cover types exhibited a clear dependence on tropospheric NO\({}_{2}\) column density anomalies, which underlines the significance of anthropogenic emissions on AOD levels in this region.