Climate Change and the Extension of the Ozone Season in the United States: Extreme Case Studies

Abstract:

Summer (June-September) is usually considered as the season for high ozone. Owing to the emission reduction, long-term EPA surface ozone records show a decreasing trend during the summer over the US. However, the records also reveal increasing trends of concentration and variation of ozone during the spring and the fall in many regions of the US, indicating an extension of the ozone season. Here we analyze two cases of high monthly mean extremes over the Southeast: May 2007 and October 2011.We conduct a series of model simulation using the Regional chEmical trAnsport Model (REAM). Although doing a reasonably good job in general, the regional chemical transport model tends to underestimate the ozone by ~ 10 ppbv when relative humidity is low, indicating that a mechanism linking ozone and relative humidity is not represented in the model. The correlation between ozone and relative humidity is verified using 30-year ozone and meteorological data. Previous phytological studies in a controlled environment suggest that the stress under low humidity can stimulate trees to release more biogenic isoprene and this mechanism is not yet included in current biogenic emission algorithms such as MEGAN. Inclusion of this mechanism in the REAM model improves the model performance in the extreme years. We suggest that a drier condition in the future may be a key factor for the extension of the ozone season through the feedback of relative humidity on isoprene emissions. This feedback will also affect the production of secondary organic aerosols from isoprene oxidation.