# Summertime Aerosol Radiative Effects and Their Dependence on Temperature over the Southeastern USA
## 2 Methodology
### Spaceborne Observations  
#### 2.1.1 Aatsr  
The core datasets for this study were provided by the Advanced Along-Track Scanning Radiometer (AATSR), which flew on the ESA polar orbiting Environmental Satellite ENVISAT (2002-2012). It was a dual view imaging spectrometer with seven wavebands, four of them in the visible and near-infrared (0.555, 0.659, 0.865, and 1.6 \(\upmu\)m) and three in the shortwave infrared--thermal infrared (3.7, 11, and 12 \(\upmu\)m). AATSR had a swath width of 512 km and the spatial resolution at nadir view was 1 \(\times\) 1 km\({}^{2}\). The nadir view and the forward view at 55\({}^{\circ}\) incident angle to the surface allowed for near-simultaneous observation of the same area on the Earth's surface through two different \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Product** & **Usage** & **Instrument** & **Product Type** \\ \hline Aerosol optical depth & Proxy for aerosol load, & AATSR & Level 3, \\ (AOD) & 2005–2011 & (Aerosol\_cci/ESA) & 1 \(\times\) 1 deg, daily \\ \hline Land surface temperature & Temperature, & AATSR & Level 3, \\ (LST) & 2005–2011 & (GlobTemperature/ESA) & 0.01 \(\times\) 0.01 deg, daily \\ \hline Nitrogen dioxide & Proxy for anthropogenic emissions, & OMI & Level 3, \\ (NO\({}_{2}\)) & 2005–2011 & (ACDISC/NASA) & 0.25 \(\times\) 0.25 deg, daily \\ \hline IGBP Land cover type & Proxy for vegetation type, & MODIS & Level 3, \\  & 2005–2011 & (LPDAAC/NASA) & 0.05 \(\times\) 0.05 deg, yearly \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Satellite products used in the project. atmospheric columns within ~2 min. The overpass time for the southeastern USA was approximately 10:00 a.m. local solar time. From the AATSR, clear-sky AOD data available from the ESA Aerosol_cci project and LST data from ESA's DUE GlobTemperature project were used. More specifically, daily Level 3 AOD data (version 1.42) with a spatial resolution of 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) were chosen because of the similarity in their spatial resolution to the resolution of the climate model, which enabled a comparison between the observations and simulations on a similar spatial scale. The Level 3 LST data with higher resolution (0.01\({}^{\circ}\)\(\times\) 0.01\({}^{\circ}\)) were re-gridded to 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) resolution to match the resolution of the AOD data. Several algorithms have been developed for the retrieval of AOD from the AATSR observations. In this work we used the AOD retrieved with the AATSR Dual-View (ADV) algorithm, which was developed at the Finnish Meteorological Institute [36]. Over land, the algorithm uses both nadir and forward view measurements of top-of-the-atmosphere (TOA) reflectance to decouple atmospheric and surface contributions from the observed signal. The main product of the retrieval is AOD, including pixel-level uncertainties. Validation results have shown that the ADV AOD values agree well with sun photometer measurements (\(r\) = 0.85, RMSE = 0.09 over land). The Level 3 AOD product is an average of the Level 2 AOD product (10 \(\times\) 10 km\({}^{2}\)), which is retrieved from cloud-cleared observations. After the retrieval, the Level 2 AOD values are post-processed to remove the remaining cloud contamination [37]. For a more detailed description of the algorithm see [36] and the algorithm's Algorithm Theoretical Basis Document [38]. In addition to the average AOD values, the Level 3 data product includes pixel specific standard deviations and they were used as uncertainty estimates in the analysis. We assumed that the uncertainties are strongly correlated temporally; thus, the seasonal uncertainty estimates were calculated as simple averages. To ensure that this relatively new AOD product was suitable for our analysis over the studied region, it was compared with AOD retrievals available from Multi-angle Imaging SpectroRadiometer (MISR) observations and with previously published AOD trends for this region.