In this thesis, the effect of the spatial resolution of the aerosol optical depth (AOD) onthe linear relationship between satellite retrieved AOD and ground level concentrationsof fine particulate matter (PM2.5) is investigated. 946 PM2.5 monitoring stationsacross the US are used together with AOD at three different spatial resolutions (1 km,3 km and 10 km) to investigate the linear relationship at different spatial scales (urbanscale, meso-scale and continental scale).The AOD data used for the study originates from the Moderate-Resolution ImagingSpectroradiometer (MODIS) and is derived by the MODIS standard algorithm andthe Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm. Theanalysis is done separately for the two sets of AOD data.The results supports patterns in the linear relationship presented by previous researchersand provides new conclusions related to the spatial resolution effect as well as the significanceof the size and characteristics of the study region. The main conclusions are:(1) the linear correlation between AOD and PM2.5 increases with higher spatial resolutionat all spatial scales, (2) the correlation decreases with enlarged study region inthe eastern part of the US, but increases with enlarged study region in the western partand (3) the AOD-PM2.5 correlation is higher and more stable over the eastern part ofthe US due to dissimilarities in surface characteristics, meteorological conditions andthe aerosol fine mode fraction.