T. Chai

Changes in nitrogen oxides emissions in California during 2005–2010 indicated from top‐down and bottom‐up emission estimates

In California, emission control strategies have been implemented to reduce air pollutants. Here we estimate the changes in nitrogen oxides (NOxu2009=u2009NOu2009+u2009NO2) emissions in 2005–2010 using a state-of-the-art four-dimensional variational approach. We separately and jointly assimilate surface NO2 concentrations and tropospheric NO2 columns observed by Ozone Monitoring Instrument (OMI) into the regional-scale Sulfur Transport and dEposition Model (STEM) chemical transport model on a 12u2009×u200912u2009km2 horizontal resolution grid in May 2010. The assimilation generates grid-scale top-down emission estimates, and the updated chemistry fields are evaluated with independent aircraft measurements during the NOAA California Nexus (CalNex) field experiment. The emission estimates constrained only by NO2 columns, only by surface NO2, and by both indicate statewide reductions of 26%, 29%, and 30% from ~0.3 Tg N/yr in the base year of 2005, respectively. The spatial distributions of the emission changes differ in these cases, which can be attributed to many factors including the differences in the observation sampling strategies and their uncertainties, as well as those in the sensitivities of column and surface NO2 with respect to NOx emissions. The updates in Californias NOx emissions reduced the mean error in modeled surface ozone in the Western U.S., even though the uncertainties in some urban areas increased due to their NOx-saturated chemical regime. The statewide reductions in NOx emissions indicated from our observationally constrained emission estimates are also reflected in several independently developed inventories: ~30% in the California Air Resources Board bottom-up inventory, ~4% in the 2008 National Emission Inventory, and ~20% in the annual mean top-down estimates by Lamsal et al. using the global Goddard Earth Observing System (GEOS)-Chem model and OMI NO2 columns. Despite the grid-scale differences among all top-down and bottom-up inventories, they all indicate stronger emission reductions in the urban regions. This study shows the potential of using space-/ground-based monitoring data and advanced data assimilation approach to timely and independently update NOx emission estimates on a monthly scale and at a fine grid resolution. The well-evaluated results here suggest that these approaches can be applied more broadly.

Improved western U.S. background ozone estimates via constraining nonlocal and local source contributions using Aura TES and OMI observations

Western U.S. near-surface ozone (O3) concentrations are sensitive to transported background O3 from the eastern Pacific free troposphere, as well as U.S. anthropogenic and natural emissions. The current 75u2009ppbv U.S. O3 primary standard may be lowered soon, hence accurately estimating O3 source contributions, especially background O3 in this region has growing policy-relevant significance. In this study, we improve the modeled total and background O3, via repartitioning and redistributing the contributions from nonlocal and local anthropogenic/wildfires sources in a multi-scale satellite data assimilation system containing global Goddard Earth Observing System–Chemistry model (GEOS-Chem) and regional Sulfur Transport and dEposition Model (STEM). Focusing on NASAs ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) field campaign period in June–July 2008, we first demonstrate that the negative biases in GEOS-Chem free simulation in the eastern Pacific at 400–900u2009hPa are reduced via assimilating Aura-Tropospheric Emission Spectrometer (TES) O3 profiles. Using the TES-constrained boundary conditions, we then assimilated into STEM the tropospheric nitrogen dioxide (NO2) columns from Aura-Ozone Monitoring Instrument to indicate U.S. nitrogen oxides (NOxu2009=u2009NO2u2009+u2009NO) emissions at 12u2009×u200912u2009km2 grid scale. Improved model skills are indicated from cross validation against independent ARCTAS measurements. Leveraging Aura observations, we show anomalously high wildfire NOx emissions in this summer in Northern California and the Central Valley while lower anthropogenic emissions in multiple urban areas than those representing the year of 2005. We found strong spatial variability of the daily maximum 8u2009h average background O3 and its contribution to the modeled total O3, with the mean value of ~48u2009ppbv (~77% of the total).