The Two Column Aerosol Project (TCAP) investigates uncertainties in the aerosol direct effect in the northern hemisphere mid-latitudes. The DOE Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) and Mobile Aerosol Observing System (MAOS) provide an opportunity for 1) atmospheric radiation closure studies, and 2) test retrievals of aerosol optical properties in the presence and absence of clouds. We explore column and profile information about aerosol optical properties and trace gases in the lower atmosphere by means of the University of Colorado 2D Multi AXis Differential Optical Absorption Spectroscopy (CU 2D-MAX-DOAS) instrument.