Chronic ozone (O<sub>3</sub>) problems and the increasing sulfur oxides (SO<sub>x</sub>=SO<sub>2</sub>+SO<sub>4</sub>) ambient concentrations over South Coast (SC) and other areas of California (CA) are affected by both local emissions and long-range transport. In this paper, multi-scale tracer and full-chemistry simulations with the STEM atmospheric chemistry model are used to assess the contribution of local emission sources to SC O<sub>3</sub> and evaluate the impacts of transported sulfur and local emissions on the SC sulfur budget during the ARCTAS-CARB experiment period in 2008. Sensitivity simulations quantify contributions of biogenic and fire emissions to SC O<sub>3</sub> levels. California biogenic and fire emissions contribute 3–4 ppb to near-surface O<sub>3</sub> over SC, with larger contributions to other regions in CA. Long-range transport from Asia is estimated to enhance surface SO<sub>4</sub> over SC by ~0.5 μg/sm<sup>3</sup>, and the higher SO<sub>x</sub> levels (up to ~0.7 ppb of SO<sub>2</sub> and ~6 μg/sm<sup>3</sup> of SO<sub>4</sub>) observed above ~6 km did not affect surface air quality in the study region. Enhanced near-surface SO<sub>x</sub> levels over SC during the flight week were attributed mostly to local emissions. Two anthropogenic SO<sub>x</sub> emission inventories (EIs) from the California Air Resources Board (CARB) and the US Environmental Protection Agency (EPA) are compared and applied in 60 km and 12 km chemical transport simulations, and the results are compared with observations. The CARB EI shows improvements over the National Emission Inventory (NEI) by EPA, but generally underestimates surface SC SO<sub>x</sub> by about a factor of two. Maritime (mostly shipping) emissions contribute to the high SO<sub>2</sub> levels over the ocean and on-shore, and fine SO<sub>4</sub> over the downwind areas is impacted by maritime sources. Maritime emissions also modify the NO<sub>x</sub>-VOC limitations over coastal areas. These suggest an important role for shipping emission controls in reducing fine particle and O<sub>3</sub> concentrations in SC.