## 3 Results and Discussion
### Persistent Winter Fog over Lahore City  
Lahore is the second largest metropolitan city of Pakistan and is the 16 th most populated city in the world. As shown earlier in the overall view of Pakistan, this study exhibited a temporal increase in AOD across Pakistan, including Lahore city. A higher value of AOD was observed in December than that in February in Lahore city, as February is warmer than December (see Table 2). However, the overall increase in AOD is observed over Lahore city. In January 2014, a declining tendency is observed for AOD for Lahore city. The discrepancy was apparently caused as the study region experienced warmer temperatures in January 2014 (temp. anomaly = \(-\)0.07 \({}^{\circ}\)C) as compared to January months during the year 2013 (temp. anomaly = \(-\)0.9 \({}^{\circ}\)C) and 2015 (temp. anomaly = \(-\)0.49 \({}^{\circ}\)C). Temperature anomaly was calculated by using CAMS monthly gridded surface air temperature [81] with respect to the base years of 1970-2000. As warmer temperature leads to unstable atmospheric conditions, rapid movement of air masses might have dispersed the aerosols over larger areas. Consequently, it resulted in less accumulation of aerosols over the study region. On the other hand, warmer temperature also results in a relatively smaller number of fog incidents and, thus, reduced aerosol emissions caused by space heating and other activities. This can be clearly seen from Figure 6b, which depicts a significantly smaller number of overall fog incidents during the month of January 2014 as compared to January 2013 and 2015. Figure 6a displays the temporal evolution of daily mean AOD over Lahore city for the past 3 winters and exhibited a relative increase of 61% with respect to the winter of 2012-2013. An increase in anthropogenic aerosol over South Asia is also evident from previous studies [82; 83; 84; 85]. Various studies reported the increase in aerosol load [34] and precursor gases [12; 75] over Lahore city. Figure 6b shows the trend in fog incidents/days observed in Lahore city during all winters. Fog was categorized into three different types on the basis of visibility (i) dense fog--when visibility is less than 200 m; (ii) moderate fog--when visibility is less than 500 m; and (iii) shallow fog--when visibility is less than 1000 m [86]. Visibility data was acquired from the following online source [87] and it uses data from the national weather service of the respective country. Fog incidents were also verified with MODIS true colour RGBs and daily news bulletins in both electronic and print media. A continual increase in dense fog incidents was observed for all winter periods. A maximum number of dense fog incidents were observed during the winter of 2014-2015 followed by 2013-2014 and 2012-2013. It is worth mentioning that February 2015 experienced anomalous incidents of moderate and dense fogs. It shows that the span of winter fog was extended during the winter season of 2014-2015. AERONET data was used to validate MODIS satellite-based observations, as shown in Figure 6(c). It exhibited an overall good agreement between daily satellite and ground-based AOD observation for all winters over Lahore city. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Region** & **Povines** & **CO** & **NO\({}_{\mathbf{2}}\)** & **SO\({}_{\mathbf{2}}\)** \\ \hline Upper Indus Plains (UIP) & KPK, Northern and Central Punjab & \(-\)0.07 & 0.38 & 0.47 \\ Lower Indus Plains (LIP) & Southern Punjab and Sindh & 0.6 & \(-\)0.37 & 0.09 \\ Indus Plains & KPK, Punjab, Sindh & 0.37 & 0.11 & 0.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Correlation between AOD and different trace gases during the time period of winter 2012–2015. This comparison only includes the dates on which both MODIS and AERONET data were available and AERONET data was extrapolated to 550 nm wavelength [52]. A significant correlation of 86% is observed in Figure 6d for the complete study period.