Multiple linear regression model. For this study, we built a multiple linear regression (MLR) model to find the optimal combination of the sectoral emissions simulated by CAMx that matches the GCAS tropospheric vertical columns. CAMx simulations were made in Source Apportionment mode to separate the NO2 vertical column densities associated with the 15 individual sources (e.g., EGU) and groups of sources (on-road mobile) (Table ). In practice, some of the emission sources in OSAT are too close together to be able to be clearly distinguished from each other. We therefore merged the following: 1.