Abstract
Abstract. Retrievals of total column NO2 (TCNO2) 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. While OMI accurately provides the daily global distribution of retrieved TCNO2, OMI almost always underestimates the local amount of TCNO2 by 50 % to 100 % in polluted areas, while occasionally the daily OMI value exceeds that measured by PANDORA at very clean sites. Compared to local ground-based or aircraft measurements, OMI cannot resolve spatially variable TCNO2 pollution within a city or urban areas, which makes it less suitable for air quality assessments related to human health. In addition to systematic underestimates in polluted areas, OMI's selected 13:30 Equator crossing time polar orbit causes it to miss the frequently much higher values of TCNO2 that occur before or after the OMI overpass time. Six discussed Northern Hemisphere PANDORA sites have multi-year data records (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 TCNO2 (TCNO2 > 0.5 DU). Eight additional sites have shorter-term data records in the US and South Korea. One of these is a 1-year data record from a highly polluted site at City College in New York City with pollution levels comparable to Seoul, South Korea. OMI-estimated air mass factor, surface reflectivity, and the OMI 24 km × 13 km FOV (field of view) are three factors that can cause OMI to underestimate TCNO2. Because of the local inhomogeneity of NOx emissions, the large OMI FOV is the most likely factor for consistent underestimates when comparing OMI TCNO2 to retrievals from the small PANDORA effective FOV (measured in m2) calculated from the solar diameter of 0.5∘.
Highlights
Retrieval of total column NO2 (TCNO2) from the Ozone Monitoring Instrument (OMI) has been a scientific success story for the past 14 years
We show that the use of OMI TCNO2 for estimating local air quality and coastal nitrification on a global basis is misleading for most polluted locations, and especially on days when the morning or afternoon amounts are higher than those occurring at the OMI overpass time near 13:30 standard time
Examination of long-term TCNO2 monthly average time series from the OMI satellite and PANDORA ground-based observations shows that OMI systematically underestimates the amount of NO2 in the atmosphere by an average factor of 1.5 to 2 at the local OMI overpass time near the Equator crossing time of 13:30 ± 01:30
Summary
Retrieval of total column NO2 (TCNO2) from the Ozone Monitoring Instrument (OMI) has been a scientific success story for the past 14 years. There are other possible systematic retrieval errors with OMI TCNO2 The largest of these is determining the air mass factor (AMF) needed to convert slant column measurements into vertical column amounts followed by the surface reflectivity RS (Boersma et al, 2011; Lin et al, 2015; Nowlan et al, 2016; Lorente et al, 2018). We show that the use of OMI TCNO2 for estimating local air quality and coastal nitrification on a global basis is misleading for most polluted locations, and especially on days when the morning or afternoon amounts are higher than those occurring at the OMI overpass time near 13:30 standard time. We present a unique view of a year of fully time resolved diurnal variation of TCNO2 at two sites, Washington DC and New York City, which are similar to other polluted locations
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