## 3 Results and Discussion
### Major Sources of Aerosols  
Major sources of aerosols in Pakistan are seasonal biomass-burning smoke and dust storms in addition to industrial and vehicular activities [54; 59; 34]. Winds are the carrier of these pollutants as cold winds (western disturbances [60]) that arrive from west Asia, northwest India and then change direction to the eastern part, are considered as a major sources of aerosol movement and spread over Bangladesh and the Ganges-Brahmaputra delta [61; 62; 63]. Previous studies show that the major causes of fog during the winter season are secondary aerosols and biomass burning, including agriculture fires [64; 65]. The IGP region is the most fertile region of the subcontinent and agriculture activities are spread over about 400 million hectares of land. In Pakistan, the agriculture sector contributes about 21 percent to GDP and provides livelihood for 60 percent of the population. Most of the agriculture depends on a huge amount of water coming from Hindukush-Karakorum-Himalaya (HKH) glaciers through the river-canal system, ground water pumped through deep wells and heavy precipitation, especially during the monsoon season. Groundwater use is about 35 percent for agriculture and 42 percent of the area in the Indus basin has a ground water table less than 3 m deep [66]. Therefore, soil moisture content is largely available for extensive agriculture practices. According to [67], winds coming from India provide favourable conditions for dense fog formation over the IGP, with low temperature and high relative humidity conditions which also enhance aerosol radiative forcing. Farmers in Pakistan light their fields to make them ready for the next crop, as post-harvest fires are an easy and cost-effective way to get rid of crop residue, weeds, and pests and to get nutrients for the next crop. Large numbers of agriculture fires are observed across the Indus plains during crop harvesting seasons. Mainly, fires are intense during the months of April-May for clearing wheat residue and during October-November for rice paddies [54; 59], followed by January-February months for the sugarcane crop. Agricultural fires emit aerosol particles, greenhouse gases, and various trace gases into the atmosphere [68]. Active fire data plotted as red circles in Figure 5c,d over the study area are taken from MODIS active fire data (MCD14 DL) downloaded from the website [69]. It can be seen that regions with extensive vegetation fires are spread along the Indus plains in Pakistan consistent with enhanced levels of AOD (Figure 5a). The agriculture plains of the Sindh and Punjab provinces along the River Indus have the largest fire density. Similar patterns are observed in AOD and aerosol types retrieved from both MODIS (Figure 5c--aerosol type over land) and OMI (Figure 5d--smoke). The analysis presented in Figure 5 revealed that absorbing aerosols are predominant and originated from fire activities (both agriculture and biomass burning) in addition to vehicular and industrial emissions. Especially, during the winter season use of biofuel (char wood, dung cake, and coal) is increased for space heating and cooking due to power and natural gas outages in Pakistan. It results in enhanced emission of aerosols, smoke, black carbon and other gases such as NO\({}_{2}\), SO\({}_{2}\) and CO [13; 68; 70]. Recently, several studies used satellite monitoring of various trace gases such as SO\({}_{2}\), NO\({}_{2}\) and CO in order to identify the source region of aerosols, as these originate from the same sources as these tiny particles [71; 72; 73; 74]. In order to investigate such a relationship between AOD and aerosol precursor gases, the Indus plains were divided into two different regions: Upper Indus Plains (UIP--light green polygons in Figure 5c, d) and Lower Indus Plains (LIP--dark green polygons in Figure 5c, d). UIP includes the parts of Khyber Pakhtunkhwa (KPK), northern and central Punjab provinces while LIP consists of regions of southern Punjab and Sindh province (excluding Karachi).