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The impact of emission and climate change on ozone in the United States under representative concentration pathways (RCPs)

by Y. Gao, J. S. Fu, J. B. Drake, J. F. Lamarque, Y. Liu
Atmospheric Chemistry and Physics ()
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Dynamical downscaling was applied in this study to link the global climate-chemistry model Community At- mosphere Model (CAM-Chem) with the regional models Weather Research and Forecasting (WRF) Model and Com- munity Multi-scale Air Quality (CMAQ). Two representative concentration pathway (RCP) scenarios (RCP 4.5 and RCP 8.5) were used to evaluate the climate impact on ozone con- centrations in the 2050s. From the CAM-Chem global simulation results, ozone concentrations in the lower to mid-troposphere (surface to ∼300 hPa), from mid- to high latitudes in the Northern Hemisphere, decreases by the end of the 2050s (2057–2059) in RCP 4.5 compared to present (2001–2004), with the largest decrease of 4–10 ppbv occurring in the summer and the fall; and an increase as high as 10 ppbv in RCP 8.5 re- sulting from the increased methane emissions. From the regional model CMAQ simulation results, un- der the RCP 4.5 scenario (2057–2059), in the summer when photochemical reactions are the most active, the large ozone precursor emissions reduction leads to the greatest decrease of downscaled surface ozone concentrations compared to present (2001–2004), ranging from 6 to 10 ppbv. However, a few major cities show ozone increases of 3 to 7 ppbv due to weakened NO titration. Under the RCP 8.5 scenario, in winter, downscaled ozone concentrations increase across nearly the entire continental US in winter, ranging from 3 to 10 ppbv due to increased methane emissions. More in- tense heat waves are projected to occur by the end of the 2050s in RCP 8.5, leading to a 0.3 ppbv to 2.0 ppbv increase (statistically significant except in the Southeast) of the mean maximum daily 8 h daily average (MDA8) ozone in nine cli- mate regions in the US. Moreover, the upper 95% limit of MDA8 increase reaches 0.4 ppbv to 1.5 ppbv in RCP 4.5 and 0.6 ppbv to 3.2 ppbv in RCP 8.5. The magnitude differences of increase between RCP 4.5 and 8.5 also reflect that the in- crease of methane emissions may favor or strengthen the effect.

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