## 2 Materials and Methods  
Two types of datasets are used in this study, namely satellite and ground-based data. Satellite datasets of MODIS, OMI and CALIPSO were used for Aerosol optical depth, Aerosol Types over Land and Vertical Feature Mask (VFM) products, respectively.  
The Moderate-resolution Imaging Spectro-radiometer [48] is in orbit on board Terra satellite since 2000 and Aqua satellite since 2002. In this study, we have used the Collection 6 (C6) product which features aerosol optical depth (AOD) and aerosol types over land derived from MODIS-Terra- observed spectral reflectance [49]. Moreover, the AOD MOD04 Level 2 collection 6 uses the Deep Blue algorithm with a spatial resolution of 10 km.  
The Dutch-Finnish Ozone Monitoring Instrument (OMI) aboard the Aura satellite launched in July 2004, is a nadir viewing imaging spectrograph. Backscattered solar radiance spectra are measured simultaneously in the wavelength range from 270 nm to 500 nm. It takes almost one day for global coverage in nadir viewing with a 2600 km wide swath. Spatial resolution is about 13 \(\times\) 24 km\({}^{2}\). OMI measures with a spectral resolution of 0.45 nm in the ultraviolet (UV-2 channel) and 0.63 nm in the visible [50] wavelength ranges. For the current study, UV Aerosol Index was calculated across a \(1^{\circ}\times 1^{\circ}\) resolution.  
Launched on 28 April 2006, the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite is a part of the \"A-train\" satellite constellation from NASA in a sun-synchronous orbit. Equipped with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), this earth observing satellite provides unprecedented high-resolution vertical profiles of both aerosols and clouds [51].  
Ground data for Lahore included two types of datasets, namely, the AErosol RObotic NETwork (AERONET) and the High Volume air sampler data. The AERONET Level 2 (cloud screened and other  
Figure 1: Location of the AERONET station along with three high volume samplers which were installed in Lahore city from December 2014 till February 2015 for continuous monitoring of PM\({}_{10}\) concentrations.  
quality flags) data was downloaded from AERONET website for the Lahore station. AERONET 500 nm was used, which was extrapolated to 550 nm using appropriate conversion formulas given by Equation (1):  
\[\text{AOD}\ aeronet_{550}=\text{AOD}\ aeronet_{500}\ (\ \frac{550}{500})^{-\alpha} \tag{1}\]  
where \(\alpha\) is the Angstrom exponent for wavelength 500-870 nm used in this study [52]. While PM\({}_{10}\) samplers were installed at three different sites in Lahore city and data was collected on daily basis for winter 2014-2015 only. Figure 1 shows the location of ground observation sites. These sites include:  
1. Lahore University of Management Sciences (LUMS), Lahore, Pakistan.
2. University of Engineering and Technology (UET), Lahore campus, Pakistan.
3. University of engineering and Technology (UET), Kala Shah Khaku campus, Pakistan.  
Before final analysis, MODIS data was pre-processed by performing geo-referencing and extraction of data over the study area, as indicated in Figure 2.  
From MOD04 data, Aerosol Optical Depth (AOD) and Aerosol Type over Land (ATL) were extracted. MODIS pre-processing was done in ENVI 5 using the MODIS reconversion Tool Kit (MCTK) and average maps were created using IDL scripts. After this analysis, the datasets were georeferenced and mapped by using ArcMap 10.2. Statistical analysis was performed using Microsoft Excel version 2013. The same procedure was applied towards extracting and processing OMI's Absorbing Aerosol Index (AAI) products. Vertical features of the sky using CALIPSO's Level 2 Vertical Feature Mask (VFM) product was extracted using IDL script.