Underestimation of Column NO 2 Amounts from the OMI Satellite Compared to Diurnally Varying Ground-Based Retrievals from Multiple Pandora Spectrometer Instruments

Abstract. Retrievals of Total Column NO 2 (TCNO 2 ) are compared for 14 sites from the Ozone Measuring Instrument (OMI using OMNO2-NASA v3.1) on the AURA satellite and from multiple ground-based PANDORA spectrometer instruments making direct-sun measurements. Six of these sites with multi-year PANDORA data records are in the Northern Hemisphere (Busan, Seoul, Washington DC, Waterflow New Mexico, Boulder Colorado, and Mauna Loa) and one site in the Southern Hemisphere (Buenos Aires Argentina). The first four of these sites and Buenos Aires frequently have high TCNO 2 (TCNO 2  > 0.5 DU) and are likely have significant air quality problems that can affect human health. Eight additional sites have shorter term data records in the US and South Korea. One of these is a one-year data record from a highly polluted site at City College in New York City with pollution levels comparable to Seoul, South Korea. The result is that on a weekly or monthly average basis, OMI almost always underestimates the amount TCNO 2 by 50 to 100 %, while occasionally the daily OMI value exceeds that measured by PANDORA at very clean sites. OMI estimated air mass factor, surface reflectivity, and the OMI 24 x 13 km 2 FOV (field of view) are three factors that can cause OMI to underestimate TCNO 2 . Because of the local inhomogeneity of NO 2 emissions, the large OMI FOV is the most likely factor when comparing OMI TCNO 2 to retrievals from the small PANDORA effective FOV calculated from the solar diameter of 0.5°. As part of air quality assessments, OMI always misses the frequently much higher values of TCNO 2 that occur after the OMI overpass time.