Multi-scale modeling study of the source contributions to near-surface ozone and sulfur oxides levels over California during the ARCTAS-CARB period

The elevated SOx observed at 1-4 km is estimated to enhance surface SOx over SC by ∼0.25 ppb (upper limit) on ∼24 June. The near-surface SOx levels over SC during the flight week are attributed mostly to local emissions. Two anthropogenic SOx 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 withobservations.

The CARB EI shows improvements over the National Emission Inventory (NEI) by EPA, but generally underestimates surface SC SOx by about a factor of two. Adjoint sensitivity analysis indicated that SO₂ levels at 00:00 UTC (17:00 local time) at six SC surface sites were influenced by previous day maritime emissions over the ocean, the terrestrial emissions over nearby urban areas, and by transported SO₂ from the north through both terrestrial and maritime areas.

Overall maritime emissions contribute 10-70% of SO₂ and 20-60% fine SOSO₄ on-shore and over the most terrestrial areas, with contributions decreasing with in-land distance from the coast. Maritime emissions also modify the photochemical environment, shifting O₃ production over coastal SC to more VOC-limited conditions. These suggest an important role for shipping emission controls in reducing fine particle and O₃ concentrations in SC.