OMI Satellite and Ground-Based Pandora Observations and Their Application to Surface NO₂ Estimations at Terrestrial and Marine Sites

Abstract

The Pandora spectrometer that uses direct-Sun measurements to derive total column amounts of gases provides an approach for (1) validation of satellite instruments and (2) monitoring of total column (TC) ozone (O₃) and nitrogen dioxide (NO₂). We use for the first time Pandora and Ozone Monitoring Instrument (OMI) observations to estimate surface NO₂ over marine and terrestrial sites downwind of urban pollution and compared with in situ measurements during campaigns in contrasting regions: (1) the South African Highveld (at Welgegund, 26°34′10″S, 26°56′21″E, 1,480 m asl, ~120 km southwest of the Johannesburg-Pretoria megacity) and (2) shipboard U.S. mid-Atlantic coast during the 2014 Deposition of Atmospheric Nitrogen to Coastal Ecosystems (DANCE) cruise. In both cases, there were no local NOx sources but intermittent regional pollution influences. For TC NO₂, OMI and Pandora difference is 20%, with Pandora higher most times. Surface NO₂ values estimated from OMI and Pandora columns are compared to in situ NO₂ for both locations. For Welgegund, the planetary boundary layer (PBL) height, used in converting column to surface NO₂ value, has been estimated by three methods: co-located Atmospheric Infrared Sounder (AIRS) observations; a model simulation; and radiosonde data from Irene, 150 km northeast of the site. AIRS PBL heights agree within 10% of radiosonde-derived values. Absolute differences between Pandora- and OMI-estimated surface NO₂ and the in situ data are better at the terrestrial site ( 0.5 ppbv and 1 ppbv or greater, respectively) than under clean marine air conditions, with differences usually >3 ppbv. Cloud cover and PBL variability influence these estimations.