# Summertime Aerosol Radiative Effects and Their Dependence on Temperature over the Southeastern USA
## 2 Methodology
### Spaceborne Observations
#### 2.1.2 Omi  
The Dutch-Finnish-built Ozone Monitoring Instrument (OMI) is a nadir-viewing pushbroom UV/Visible instrument. It is onboard NASA's EOS-Aura satellite and part of the A-Train satellite constellation. OMI measures backscattered radiances in three wavelength intervals: 270-310 nm (UV-1), 310-365 nm (UV-2), and 350-500 nm (visible) at spectral resolutions of 0.42-0.63 nm [41]. It has a swath width of 2600 km and spatial resolution of 13 \(\times\) 48 km\({}^{2}\) at nadir for the UV-1 channel, and 13 \(\times\) 24 km\({}^{2}\) for the UV-2 and visible channels. OMI measures ozone, trace gases (e.g., NO\({}_{2}\), SO\({}_{2}\), HCHO, BrO, CHOCHO, OCIO), aerosols and clouds.  
In this work, the Level 3 tropospheric NO\({}_{2}\) retrievals from the OMMO2d product were used [42]. It consists of only good-quality pixel-level data that are averaged into a 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) global grid, and thus we re-gridded the data into 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) resolution to match the AATSR observations. The data contain NO\({}_{2}\) column densities for all atmospheric conditions where the cloud fraction is less than 30%. For cloud fractions larger than 30% it becomes impossible to distinguish pollution from natural variation because high clouds mask the tropospheric contribution. For more information, see the documents provided by NASA GES DISC [43; 44]. The NO\({}_{2}\) data do not include information on the uncertainty of the retrieved values. Therefore, we used an uncertainty estimate of \(\pm\)20%, as suggested by [PERSON] et al. [45], who evaluated the NO\({}_{2}\) product over the eastern USA. The standard deviation of the data was estimated in a similar manner as for the AATSR LST data. In this research, the tropospheric NO\({}_{2}\) data were used as a proxy for anthropogenic emissions. NO\({}_{2}\) can be considered a proxy for anthropogenic influence since the main sources of nitrogen oxides in the studied region are combustion in transportation and electricity generating units, with some emissions also originating from industrial, commercial and residential sources [25]. Fertilized croplands could also be important NO\({}_{2}\) sources [46], and in our analysis they can be considered as anthropogenic emissions. Although NO\({}_{2}\) is not emitted from all anthropogenic sources, it exhibits a similar decreasing trend to SO\({}_{2}\) in this region [47]. Thus it is a suitable proxy for estimating the level of anthropogenic contribution to seasonally averaged AOD. To confirm that the tropospheric NO\({}_{2}\) retrievals from OMI represent anthropogenic pollution levels, we compared them with in situ observations of sulphate particle mass performed at the Interagency Monitoring of PROtected Visual Environments (IMPROVE) sites in the southeastern USA [48]. The comparison was done using summertime averages of tropospheric NO\({}_{2}\) column densities covering the entire study region and seasonal averages of sulphate particle mass combined from 12 IMPROVE sites within the region. This comparison showed a strong positive correlation between the observations (\(r\) = 0.98), which further supports the assumption that the tropospheric NO\({}_{2}\) observations can be used as a proxy for anthropogenic influence (see Figure S3 for more information). Lightning also produces NO\({}_{2}\), thus to evaluate the effect of lightning to the level of tropospheric NO\({}_{2}\) we used observations of lightning flash rates [49]. Based on the flash rates, summers 2010 and 2011 had the highest lightning activity during the studied period. As these summers had low tropospheric NO\({}_{2}\) concentrations, it indicates that anthropogenic sources have a larger effect on the seasonal level of tropospheric NO\({}_{2}\) than lightning.