## 3 Results and Discussion  
According to the (IPCC, [10]) report, aerosols are recognized as an important factor affecting global climate change and play a key role in processes related to solar and thermal radiative transfer in the atmosphere, water cycle and precipitation processes. During winter seasons in Pakistan, fog becomes very dominant in the months of December, January and February (DJF) [1]. Monthly mean maps of the aerosol optical depth (AOD) retrieved from MODIS observations [53] during the DJF months of each winter (2012-2013, 2013-2014 and 2014-2015) over the study area are shown in Figure 3.  
Figure 2: Methodology: flow diagram exhibits the datasets, procedures, and tools used in this study. Enhanced AOD over the River Indus basin can be clearly identified, as stated by previous studies [34; 54]. Its aerosol loading reached a maximum during December, followed by January and February. Continual increasing behaviour in AOD is observed over Pakistan. Especially, the winter of 2014-2015 exhibited anomalous enhanced levels of AOD over Pakistan. Supporting these findings, further, Figure 4 represents results of the average AOD for each winter period selected for the current study (2012-2013, 2013-2014 and 2014-2015). A similar temporal increase in AOD is observed over Pakistan. The overall analysis shows that, as winters are on peak in the months of December and January, this provides favourable conditions for fog formation, as the temperature is low and relative humidity is high. So, the fog intensity and frequency is high over regions along the Indus basin and the major cause is high loads of air pollutants. This area hosts most of the population, industry, dense road networks and vehicular fleet of Pakistan. AOD levels during February exhibit lowered values compared to the December-January months. The overall mean map of February also shows a lower AOD compared to December and January. As stated earlier, the key reason for decreased intensity in February is unfavourable conditions for the fog formation, including lower air pollution levels and comparatively higher air temperature. Figure 3: Shows average AOD evolution over Pakistan for each month of the study period. Data is extracted from the MOD04 Level 2 Collection 6 Product. AOD is studied by using deep blue algorithm [49] over Pakistan. Red polygons exhibit the provincial boundaries and the location of the major cities of Pakistan is indicated by small circles in the maps. In order to investigate the types of aerosols present over the study area, the MODIS product named MOD4 Level 2 Collection 6 with subset Aerosol Type over land (ATL) was used. MODIS aerosol types were categorized into four types, which include: Continental, Weak, Moderate Absorption Fine and Strong Absorption Fine. According to this classification, the most dominant aerosol types over Pakistan are Strong and Moderate Absorption Fine, as shown in Figure 4 (lower panel--average ATL for each winter period). To further investigate the types of aerosols, the OMI UV aerosol Index (OMI-UVAI) was also used. It is a residual quantity resulting from the comparison between measured and calculated radiances in the range 330-390 nm, where gas absorption effects are negligible [55]. It is particularly sensitive (even when the aerosols are over clouds) to carbonaceous and mineral aerosols and represents the presence of absorbing aerosols (smoke and dust [56]). For clear sky/Rayleigh scattering conditions, UVAI values are near zero, positive values for absorbing aerosols (smoke, dust), and negative values for non-absorbing aerosols (sea salt, sulfate/nitrates _etc._). UVAI calculated for winter months and averaged over all winter periods (2012-2015) is presented in Figure 5b. Results indicate positive UVAI over Pakistan, thus representing the presence of absorbing aerosols. The results are further confirmed by the MODIS ATL product (Figure 5c) averaged over Pakistan during the same time period.