Sensitivity of summertime surface ozone to surface temperature over Southeastern U.S.: interannual variability during 1989-2010 as a diagnostic for model chemical mechanism

Predictions by climate-chemistry models disagree as to whether the summertime surface ozone levels will increase or decrease over Southeastern U.S. under future climate conditions. While such disagreement may be due in part to the differences in the simulated regional climate, previous studies have pointed to the formation and the NOx-recycling efficiency of isoprene nitrate as major sources of uncertainty. In this study, we used the GEOS-Chem chemical transport model to conduct multiple simulations of summertime surface ozone over Southeastern U.S. during 1989-2010 with different assumptions for isoprene nitrate chemistry. The model was driven by assimilated meteorology to remove the uncertainty associated with climate conditions. The simulated sensitivities of monthly surface ozone anomalies to monthly surface temperature anomalies (dΔO3/dΔT) were compared against those observed by CASTNet. We focused on the interannual variability of dΔO3/dΔT, which is driven by regional scale climate variability and relevant to predictions for the future. For July, the observed dΔO3/dΔT averaged 5.8 ppb/K during the early 1990s and decreased to 4.0 ppb/K during the late 2000s due to reduced anthropogenic NOx emissions. Model simulation with no recycling of isoprene nitrate significantly underestimated dΔO3/dΔT. Better agreement with observed dΔO3/dΔT was obtained by assuming a 4-12% yield of isoprene nitrate and a 40-50% NOx recycling when isoprene nitrates reacts with OH and ozone. For August, the observed dΔO3/dΔT averaged 4.5 ppb/K during the early 1990s, increased to 7 ppb/K during 1998-2001, then decreased to 3 ppb/K during the late 2000s. All model simulations, with various assumptions for isoprene nitrate chemistry, failed to reproduce the observed high dΔO3/dΔT during 1998-2001. This suggests that, in addition to isoprene nitrate chemistry, there may be other important chemical pathways driving the climate sensitivity of surface ozone over the Southeastern U.S. in summer.