h i g h l i g h t sDerived multi-year urban NOx trend from satellite (OMI) and ground observations (AQS). Revealed NOx responses to the 2008 Economic Recession by OMI and AQS. The trend not well captured by emissions used for national air quality forecasting. Demonstrated how to use space and ground observations to evaluate emission updates. Emission Trend Air quality forecast Recession OMI NO2 Ozone AQS NAQFC a b s t r a c t National emission inventories (NEIs) take years to assemble, but they can become outdated quickly, especially for time-sensitive applications such as air quality forecasting. This study compares multi-year NO x trends derived from satellite and ground observations and uses these data to evaluate the updates of NO x emission data by the US National Air Quality Forecast Capability (NAQFC) for next-day ozone prediction during the 2008 Global Economic Recession. Over the eight large US cities examined here, both the Ozone Monitoring Instrument (OMI) and the Air Quality System (AQS) detect substantial downward trends from 2005 to 2012, with a seven-year total of À35% according to OMI and À38% according to AQS. The NO x emission projection adopted by NAQFC tends to be in the right direction, but at a slower reduction rate (À25% from 2005 to 2012), due likely to the unaccounted effects of the 2008 economic recession. Both OMI and AQS datasets display distinct emission reduction rates before, during, and after the 2008 global recession in some cities, but the detailed changing rates are not consistent across the OMI and AQS data. Our findings demonstrate the feasibility of using space and ground observations to evaluate major updates of emission inventories objectively. The combination of satellite, ground observations, and in-situ measurements (such as emission monitoring in power plants) is likely to provide more reliable estimates of NO x emission and its trend, which is an issue of increasing importance as many urban areas in the US are transitioning to NO x -sensitive chemical regimes by continuous emission reductions.

[1] Retrievals from the Microwave Limb Sounder (MLS) and the Ozone Monitoring Instrument (OMI) on EOS-Aura were included in the Goddard Earth Observing System version 4 (GEOS-4) ozone data assimilation system. The distribution and daily to seasonal evolution of ozone in the stratosphere and troposphere during 2005 are investigated. In the lower stratosphere, where dynamical processes dominate, comparisons with independent ozonesonde and Measurement of Ozone and Water Vapour by Airbus In-Service Aircraft (MOZAIC) data indicate mean agreement within 10%. In the troposphere, OMI and MLS provide constraints on the ozone column, but the ozone profile shape results from the parameterized ozone chemistry and the resolved and parameterized transport. Assimilation of OMI and MLS data improves tropospheric column estimates in the Atlantic region but leads to an overestimation in the tropical Pacific and an underestimation in the northern high and middle latitudes in winter and spring. Transport and data biases are considered in order to understand these discrepancies. Comparisons of assimilated tropospheric ozone columns with ozonesonde data reveal root-mean-square (RMS) differences of 2.9-7.2 Dobson units (DU), which are smaller than the model-sonde RMS differences of 3.2-8.7 DU. Four different definitions of the tropopause using temperature lapse rate, potential vorticity (PV), and isentropic surfaces or ozone isosurfaces are compared with respect to their global impact on the estimated tropospheric ozone column. The largest sensitivity in the tropospheric ozone column is found near the subtropical jet, where the ozone-or PV-determined tropopause typically lies below the lapse rate tropopause. Citation: Stajner, I., et al. (2008), Assimilated ozone from EOS-Aura: Evaluation of the tropopause region and tropospheric columns,

Since 1997, baseline ozone monitoring from the surface to the tropopause along the U.S. west coast has been limited to the weekly ozonesondes from Trinidad Head, California. To explore baseline ozone at other latitudes, an ozonesonde network was implemented during spring 2010, including four launch sites along the California coast. Modeling indicated that North American pollution plumes impacted the California coast primarily below 3 km, but had no measurable impact on the average coastal ozone profiles. Vertical and latitudinal variation in free tropospheric baseline ozone appears to be partly explained by polluted and stratospheric air masses that descend isentropically along the west coast. Above 3 km, the dominant sources of ozone precursors were China and international shipping, while international shipping was the greatest source below 2 km. Approximately 8–10% of the baseline ozone that enters California in the 0–6 km range impacts the surface of the USA, but very little reaches the eastern USA. Within California, the major impact of baseline ozone above 2 km is on the high elevation terrain of eastern California. Baseline ozone below 2 km has its strongest impact on the low elevation sites throughout the state. To quantify ozone production within California we compared inland ozone measurements to baseline measurements. For average daytime conditions, we found no enhancements of lower tropospheric ozone in the northern Central Valley, but enhancements of 12–23% were found in the southern Central Valley. Enhancements above Joshua Tree were greater, 33–41%, while the greatest enhancements occurred over the LA Basin, 32–63%.