## 4 Conclusions  
Our findings show that during the 2012-2015 winter periods in Pakistan, December and January months experience more fog incidents and relatively higher AOD. As the winter season goes on, the intensity of foggy weather decreases with the aerosol load decreasing as well, which is supported by the observations. The lowest mean AOD was found during December 2012 and highest AOD in December 2014 with an overall increasing behaviour in AOD observed over Pakistan. The continual increase in the frequency of dense fog occurrences is also observed. Anomalous incidents of dense and moderated fogs were identified during February 2015. It shows that the span of winter fog was extended during the winter season of 2014-2015. The dominant type of aerosol is medium and strong absorption fine retrieved from the MODIS ATL product, which was further confirmed by OMI UVAI data (categorized as smoke and dust). MODIS AOD, compared well with the AERONET AOD in Lahore city and exhibited a significant correlation of r = 0.86. Active fire locations from MODIS were  
Figure 9: Shows spatial distribution of aerosol, clouds along the overpass of CALIPSO Satellite by using the vertical feature mask: (**a**) on the night of 13 December 2013, with the location of the overpass (red line) provided in the inset image; (**b**) on 23 January 2015, with the location of the overpass (red line) provided in the inset image. Aerosol in both cases is represented by orange colour, while clouds are depicted in cyan colour.  
strongly co-located with aerosol types of smoke. It revealed that agriculture fires and biomass burning are predominant sources of aerosols and its precursor gases in the Indus delta, in addition to increased used of biofuel and industrial activities. Reanalysis Back trajectories have shown that increasing AOD over Pakistan, is also caused by transboundary pollution on selected days. It was further supported by the CALIPSO overpasses over the Pakistan region and indicated the presence of low-lying aerosol layers, in addition to clouds. Mass concentrations of PM\({}_{10}\) measured at all sampling sites within Lahore city were found to be higher than Pakistan's NEQS Level for PM\({}_{10}\) on most occasions.  
Authors gratefully acknowledge the CALIPSO, MODIS, OMI teams for providing the Level-2 data products. We also acknowledge Data of Firm active fire from NASA (National Aeronautics and Space Administration). Our special gratitude goes to ICMOD-Neppal and NUST for providing partial financial support from postgraduate R&D fund to conduct this study and our collaborating partner from LUMS, UET Lahore, and Kala Shah Kaku Campuses.  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] analyzed results over all Pakistan and [PERSON] was the part of the team who performed PM\({}_{10}\) sampling in Lahore City. [PERSON] wrote the paper and supervised this study.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} IGP & Indo-Gangetic Plains \\ km\({}^{2}\) & Square Kilometer \\ AOD & Aerosol optical depth \\ nm & Nanometer \\ UV-Vis-NIR-SWIR & Ultraviolet, Visible, Near Infrared, Short Wavelength Infrared \\ MODIS & Moderate Resolution Imaging Spectroradiometer \\ OMI & Ozone monitoring instrument \\ CALIPSO & Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations \\ AERONET & AErosol RObotic NETwork \\ \end{tabular}