## 5 Discussions  
Lahore remained \"very unhealthy\" 26% of the time, and \"unhealthy for everyone\" 54% of the time, during the last three years (July 2019-April 2021). For 88.4% of the study period (July 2019-April 2021) the AQI values of Lahore remained greater than 100 for 88.4% of the time, and greater than 150 for 54% of the time, which is stated as unhealthy for sensitive groups (children, elderly, and patients) by the WHO. It remained \"good\" for 12 and \"moderate\" for only 104 days during the last 3 years (2019-2021). The average PM concentration in Lahore in 2010 was 121.8 ug/m\({}^{3}\)[9]. The value increased to 167 ug/m\({}^{3}\) in 2020 and 175 ug/m\({}^{3}\) in 2021, as determined through real-time data. The AQI value reached a maximum of 478 and 466, in 2019 and 2020, respectively, during the smog season from October to December every year. The average AOD value in Lahore was 0.98 um. The maximum value was 2.27 um from Jia Bagga, and the minimum was 0.46 um. According to the U.S. EPA standard, the safe range is (0.1 um-0.4 um) [36]. The ambient air of Lahore did not meet this standard, not for a single time, at any location. The maximum value was 5.6 times higher than the upper limit of the safe range. It can be concluded that during the entire three year period, roads have played a vital role in pollution and smog. Quaid-e-Azam Industrial Estate more vigorously contributed to CO emissions, whereas Sundar Industrial Estate and brick kilns contributed more vigorously to SO\({}_{2}\). The indoor environment, particularly hospitals and schools, must use air purifiers with a relevant filter. Variations in the sum of the concentrations of CO, SO\({}_{2}\), and NO\({}_{2}\) determined through remote sensing coincided considerably with the respective AQI plot, and therefore, that NO\({}_{2}\), CO, and SO\({}_{2}\) can be used as an indicator of the AQI in Lahore. Similar results were reported by [PERSON] for NO\({}_{2}\) in the drastic smog season of 2016 [32]. Additionally, it was further inferred that any of the NO\({}_{2}\), CO and SO\({}_{2}\) concentrations obtained through remote sensing can be used for initial surveillance of AQI in Lahore. This paper has validated the results of previous work for the period 2007 to 2011 [22]. Through plotting, it was observed that in the period when AQI rose, cumulative values of each pollutant did not rise correspondingly. Similarly, when AQI dropped, the actual value of each pollutant did not drop rapidly. Neither HCHO, CH\({}_{4}\), nor AOD had a similar relationship with AQI values. Although AOD values were higher during the smog season than the rest of the year, they were maximum during the monsoon season, when AQI values were comparatively low. The findings of this work also included that AOD, which has been reported as an indicator  
Figure 8: The daily trend of AQI in Lahore to identify the difference in weekday and weekend trends. of AQI in several previous kinds of research, is a parameter which, in the case of Lahore, is little misleading [41; 42]. On-ground observational data were obtained, with the following results. After the analysis of 14 Lahore sites and reduction of data into three zones, i.e., industrial estates, roads, and brick kilns, it was concluded that within industrial estates, peak values of CO, NO\({}_{2}\), HCHO, and CH\({}_{4}\) occurred at the Quaid-e-Azam Industrial Estate. The peak value of CO that occurred in February 2021, when the complete lockdown was lifted, is shown in Figure 4. The Sundar Industrial Estate was a heavy participant in SO\({}_{2}\) contribution. Heavily trafficked roads, followed by Quaid-e-Azam Industrial Estate, were the main contributors of CO, formaldehyde, and NO\({}_{2}\). Within roads, NO\({}_{2}\) values were highest on Jail Road and Main Boulevard, as seen in Figure 5. Formaldehyde should be the main concern at Yateem Khana (Road) and Main Boulevard (Road). Brick kilns were the main contributor of SO\({}_{2}\), particularly Main Lahore and Jia Bagga, due to the type of fuel used.