The goal of the project is to deploy a standard Thermal Desorption Aerosol GC/MS (TAG) instrument at the Los Angeles ground site during CalNex 2010 for in-situ measurements of particulate organic species at hourly time resolution. The temporal variation profiles of organic marker compounds will be obtained. Multivariate analysis will be performed to determine OA source categories and their relative contributions to the total OA loading (in conjunction with AMS). The source-resolved time trends will be examined for correlations with aerosol optical and physical properties important to climate. Finally, the OA chemical information generated from this study will be contrasted to that acquired at the San Joaquin Valley ground sites during CalNex 2010 (funded by CARB), ICARTT, SOAR and BEARPEX to evaluate differences in aerosol sources, source contributions, and atmospheric transformations among sites.