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 (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 NOxrich, 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) 03 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. (C) 2014 The Authors. Published by Elsevier Ltd.