## 3 Results and Discussion
### Case Study-Winter 2014-2015  
This study also had a special focus on the 2014-2015 winter in terms of field data collection for winter months in Lahore--one of the most polluted city in Pakistan [11; 41]. High volume samplers were installed for continuous monitoring of PM\({}_{10}\) concentrations at three different locations within Lahore city. Figure 7 displays the measured PM\({}_{10}\) concentration in Lahore city at three different sites: University of Engineering and Technology (UET), Kala Shah Khaku (KSK) campus, University of Engineering and Technology (UET), Lahore campus and Lahore University of Management Science (LUMS), Lahore, Pakistan. PM\({}_{10}\) concentration was measured for 24 h duration by using High Volume PM\({}_{10}\) samplers (Wedding and Associates Inc., Fort Collins, CO, USA) during the time period of January 2015-February 2015. Quartz filters (QMA by Whatman International Ltd., Maidstone, England), because of their high collection efficiency as well as their ability to withstand high temperatures, were used to collect the samples on a daily basis. Filters were pre-processed at 120 \({}^{\circ}\)C and weighed before and after the filter was removed from the sampler. The red line in the graph represents the National Environmental Quality Standards (NEQS) of 150 \(\upmu\)g/m\({}^{3}\) for 24 hours average concentrations of PM\({}_{10}\) defined by the Pakistan Environmental Protection Agency (Pak-EPA, [88]).It is observed that PM\({}_{10}\) levels at all monitoring sites exceeded the Pak-NEQs level on most of the occasions. Data gaps were caused by both the frequent power outages in Pakistan and malfunctioning of the instruments. Especially, the number of samples collected at both campuses of UET are less because these sites \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Month** & **Winter (2012–2013)** & **Winter (2013–2014)** & **Winter (2014–2015)** & **Average** \\ \hline December & 0.37 & 0.90 & 1.08 & 0.67 \(\pm\) 0.15 \\ January & 0.54 & 0.43 & 0.84 & 0.62 \(\pm\) 0.09 \\ February & 0.39 & 0.64 & 0.63 & 0.56 \(\pm\) 0.06 \\ Average & 0.43 & 0.57 & 0.84 & 0.62 \(\pm\) 0.09 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Temporal evolution of AOD over Lahore City retrieved from MODIS observations. Figure 6: **(a)** Time series of daily MODIS-derived AOD over Lahore city during all winters (2012–2015); **(b)** Total number of fog incidents (dense fog = grey colour, moderate fog = orange colour, shallow fog = blue colour) in Lahore city during respective winter seasons and each month as well; **(c)** Comparison of daily AOD between MODIS and AERONET observations for all winters; **(d)** Correlation measured between MODIS and AERONET observations for Lahore city. Data is extracted from the MOD04 Level 2 Collection 6 Product by using deep blue algorithm [49]. did not have a power backup system like the one available at the LUMS site. Correlation between mass concentrations of PM\({}_{10}\) and MODIS-retrieved AOD is presented in Figure 7d. Limited data was available as all monitoring sites in Lahore city were not having the filter samples on a daily basis because of aforementioned constraints. So, the correlation was calculated for mean PM\({}_{10}\) mass concentration from all sites. Additionally, MODIS AOD was also not available on certain days because of cloud cover. The observed correlation of r = 0.67 is quite good and is in agreement with the range stated in previous studies [89; 90; 91; 92] for similar kinds of comparison. Furthermore, the correlation depends on various factors such as seasonal variation, meteorological parameters, spatial resolution of the satellite pixel (10 km in this case) and aloft aerosol layer [89; 90; 91; 92].