Emission reductions and urban ozone responses under more stringent US standards

We use a photochemical grid model instrumented with the high-order Decoupled Direct Method (HDDM) to evaluate the response of ozone (O3) to reductions in US-wide anthropogenic emissions, and to estimate emission reductions necessary to meet more stringent National Ambient Air Quality Standards (NAAQS) for O3. We simulate hourly O3 response to nationwide reductions in nitrogen oxides (NOx) and volatile organic compound (VOC) emissions throughout 2006 and compare O3 responses in 4 US cities: Los Angeles, Sacramento, St. Louis, and Philadelphia. We compare O3 responses between NOx-rich, O3-inhibited urban core sites and NOx-sensitive, higher O3 suburban sites and analyze projected O3 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) O3 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 O3 exposure is an important driver of O3 health impacts calculated by epidemiological models. A significant fraction (at least 48%) of summertime integrated MDA8 O3 at all sites remains after complete elimination of US anthropogenic NOx and VOC emissions, implying that mid-level O3 exposure due to background will become more important as domestic precursor emissions are controlled.