Assessment of NOx and O3 forecasting performances in the US National Air Quality Forecasting Capability before and after the 2012 major emissions updates

In this study, we address outdated emissions inventory problems in air quality forecasting systems. The National Emissions Inventory for NOx from area and mobile sources is projected from 2005 to 2012 and NOx from point sources is projected from 2010 to 2012, in which we find that NOx emissions from area, mobile and point sources reduce by 8.1%, 37.8% and 4.1%, respectively. The majority of the NOx emissions reduction occurs in megacities over the CONtiguous U.S. (CONUS), in which the spatial distribution pattern is generally supported by the NO2 column result retrieved from the GOME-2 satellite data. The CMAQ-predicted NOx and O-3 concentrations using updated NOx emissions were then compared to Air Quality System (AQS) ground observations in order to evaluate the updated NOx emissions inventory. The comparison showed an improvement in NOx and O-3 predictions over the CONUS. The NOx bias, in July 2011, for urban, suburban and rural land-use types was reduced by 234 ppb, 2.09 ppb and 0.57 ppb, respectively. Meanwhile, the O-3 bias is reduced by 0.92 ppb, 1.26 ppb and 1.87 ppb, respectively. However, problems remain in CMAQ for NOx and O-3 simulations despite undertaking this emissions adjustment. For example, the O-3 overestimation in CMAQ during the daytime over the CONUS decreases when the NOx underestimation increases, suggesting that in addition to the NOx emissions inventory, further study of VOC emissions, NOx chemical and physical mechanisms as well as meteorology parameters in the NAQFC is necessary. (C) 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).