Abstract
Abstract. Chronic high surface ozone (O3) levels and the increasing sulfur oxides (SOx = SO2+SO4) 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, full-chemistry and adjoint simulations using the STEM atmospheric chemistry model are conducted to assess the contribution of local emission sourcesto SC O3 and to evaluate the impacts of transported sulfur and local emissions on the SC sulfur budgetduring the ARCTAS-CARB experiment period in 2008. Sensitivity simulations quantify contributions of biogenic and fire emissions to SC O3 levels. California biogenic and fire emissions contribute 3–4 ppb to near-surface O3 over SC, with larger contributions to other regions in CA. During a long-range transport event from Asia starting from 22 June, high SOx levels (up to ~0.7 ppb of SO2 and ~1.3 ppb of SO4) is observed above ~6 km, but they did not affect CA surface air quality. 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 SO4 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.
Highlights
In the past 20 years, California population has increased by 33% and the economy has grown rapidly (Cox et al, 2009)
We focus this study on the South Coast (SC) area because it had the largest number of flight-collected air samples, analysis was done over the San Francisco (SF) and the Central Valley (CV) area
The wind vectors are overlaid on the WRF-predicted mixing layer height (PBLHT)
Summary
In the past 20 years, California population has increased by 33% and the economy has grown rapidly (Cox et al, 2009). Most Californians live in areas that are designated as nonattainment for the state (about 99%) and national (about 93%) health-based O3 and/or PM standards. The US National Ambient Air Quality Standard (NAAQS) for daily maximum 8h average O3 has recently been lowered to 75 ppb, and is likely to be lowered further to between 60 ppb and 70 ppb in future regulatory reviews of its direct impacts on human health. Despite the continued precursoremission reductions, limited improvement in O3 has been achieved over the last decade. Local production from both natural and anthropogenic emission sources, together with inter-continental and in-state transport contributes to the O3 levels over both urban and rural areas
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