A study of tropospheric ozone column enhancements over North America using satellite data and a global chemical transport model

Abstract

Tropospheric ozone columns (TCOs) have been calculated from the differences between the Aura Ozone Monitoring Instrument (OMI) Total Ozone Mapping System (TOMS) total ozone (level 2 version 3) and the Aura Microwave Limb Sounding (MLS) measurements of stratospheric ozone (version 2.2). These OMI-MLS TCOs were compared against ozonesonde measurements from the Intercontinental Chemical Transport Experiment (INTEX) Ozonesonde Network Study (IONS) campaign over North America in spring and summer, 2006. The OMI-MLS potential vorticity mapped TCOs are smaller than IONS TCOs by 5.9 DU (9.9 ppb when expressed as volume mixing ratio) with a standard deviation of the differences of 8.4 DU (14.4 ppb) and a standard error of the mean differences of approximately 0.5 DU (0.7 ppb). Compared to previously published versions, these OMI-MLS TCOs are an additional 2 DU smaller relative to ozonesonde measurements. The extra 2 DU arises from changes in OMI (∼−3 to −6 DU) and MLS (−1 to 3 DU), giving a net change of −2 DU. OMI-MLS TCOs derived using OMI Differential Optical Absorption Spectroscopy (DOAS) show similar differences in summer, but these TCOs are smaller than the sondes by only 2 DU (5 ppb) in spring. OMI-MLS TCOs derived from TOMS total ozone retrievals lead to better results when validated against IONS data, with less noise and a better seasonal consistency. Tropospheric ozone columns were also compared to those from GEOS-Chem model simulations in main distribution features. In the spring and summer of 2005 and 2006, the most dominant enhancement features are a tongue of enhancement stretching from around Yellow Sea northeastward into the Pacific and an enhancement band over the North America centered over the eastern United States and the adjacent ocean. The OMI-MLS TCO enhancements over the western Pacific and over the eastern United States increased from March to June and then decreased. In the GEOS-Chem model simulations, the monthly variation tendency is similar to that of satellite data over the west Pacific but the decrease tendency from June into August over eastern United States is less dramatic. A springtime TCO enhancement event of a few days duration over coastal California was investigated to demonstrate the ability of OMI-MLS mapped TCO columns in capturing ozone enhancements associated with stratospheric intrusions and trans-Pacific transport. Tagged ozone model simulations support the stratospheric contributions to the high TCOs over coastal California and over the Baja peninsula, and meteorological fields indicate that the stratospheric intrusions are associated with Rossby wave breaking events. Furthermore, back trajectory studies and comparisons of GEOS-Chem standard simulations and sensitivity runs with Asia anthropogenic emissions turned off provide evidence that the high tropospheric ozone columns over coastal California near Santa Barbara, California, has been influenced by cross-Pacific transport. Two-day-average maps of tropospheric ozone columns from Aura OMI-MLS TCOs also indicate cross-Pacific propagating features.