Suggested Citation:
Zhao, Yusheng, Yin Li, Yiting Li, Anikender Kumar, Qi Ying, Michael J. Kleeman (2024)

Reducing Southern California Ozone Concentrations in the Year 2050 Under a Low Carbon Energy Scenario

Recent studies predict that ozone (O3) concentrations within major California cities in the year 2050 will continue to violate the 8-h O3 standard despite the adoption of low-carbon energy. This O3 penalty largely stems from persistent NOx-rich chemistry within urban cores that causes O3 concentrations to increase when traditional combustion sources are replaced by renewable energy sources. Here we employ a Chemical Transport Model (CTM) equipped with a novel O3 source apportionment technique to better understand O3 formation and to design supplemental control measures to complement Greenhouse Gases (GHG) reduction strategies in California. Two base scenarios were analyzed: (i) a 2050 “business-as-usual” (BAU) scenario with O3 violations on 43% of simulated days, and (ii) a 2050 Greenhouse Gases reduction (GHGAi) scenario with O3 violations on 32% of simulated days. The source apportionment results of the GHGAi scenario identified offroad equipment & rail, marine vessels & aircraft, and industrial & agricultural emissions as the major NOx sources that contributed to the O3 violations, while boundary conditions & initial conditions (BCIC) and biogenic emissions were identified as the major volatile organic compounds (VOCs) sources. Despite a HCHO/NO2 ratio (FNR) suggesting that the SoCAB is in a VOC-limited chemical regime, the major VOC sources that contribute to O3 formation are not controllable. Therefore, complementary O3 control strategies are proposed that focus on NOx emissions within the GHGAi scenario. Three cumulative Supplemental Control Steps were evaluated. Control Step I reduced emissions from off-road equipment & rail, and marine vessels & aircraft (65% NOx emissions reduction relative to GHGAi). Control Step II expanded the measures in Control Step I by reducing emissions from industrial and agricultural sources (cumulative 73.6% NOx emissions reduction relative to GHGAi), while Control Step III extended the amount of control applied to all sources covered in Control Steps I and II (cumulative 85.4% NOx emissions reduction relative to GHGAi). All three Control Steps lowered O3 concentrations significantly in the SoCAB, with residual O3 violations mainly predicted in non-populated wilderness areas. The significant reductions in O3 produced by the Supplemental Control Steps decreased the predicted incidence of hay fever/rhinitis, asthma, and all-cause mortality by an order of magnitude compared to the transformation from the BAU to GHGAi scenario. The predicted public health savings associated with the Supplemental Control Steps are valued at $19.70–30.36B/yr.