Emission reductions and urban ozone responses under more stringent US standards

Abstract We use a photochemical grid model instrumented with the high-order Decoupled Direct Method (HDDM) to evaluate the response of ozone (O 3 ) to reductions in US-wide anthropogenic emissions, and to estimate emission reductions necessary to meet more stringent National Ambient Air Quality Standards (NAAQS) for O 3 . We simulate hourly O 3 response to nationwide reductions in nitrogen oxides (NOx) and volatile organic compound (VOC) emissions throughout 2006 and compare O 3 responses in 4 US cities: Los Angeles, Sacramento, St. Louis, and Philadelphia. We compare O 3 responses between NOx-rich, O 3 -inhibited urban core sites and NOx-sensitive, higher O 3 suburban sites and analyze projected O 3 frequency distributions, which can be used to drive health effect models. We find that 2006 anthropogenic NOx and VOC emissions must be reduced by 60–70% to reach annual 4th highest (H4) maximum daily 8-h (MDA8) O 3 of 75 ppb (the current US standard) in Sacramento, St. Louis, and Philadelphia, and by 80–85% to reach an H4 MDA8 of 60 ppb. Los Angeles requires larger emissions reductions and achieves an H4 MDA8 of 75 ppb with 92% reductions and 60 ppb with 97% reductions. As emissions are reduced, hourly and MDA8 frequency distributions tend toward mid-level background distributions. Mid-level O 3 exposure is an important driver of O 3 health impacts calculated by epidemiological models. A significant fraction (at least 48%) of summertime integrated MDA8 O 3 at all sites remains after complete elimination of US anthropogenic NOx and VOC emissions, implying that mid-level O 3 exposure due to background will become more important as domestic precursor emissions are controlled.