Underestimation of column NO<sub>2</sub> amounts from the OMI satellite compared to diurnally varying ground-based retrievals from multiple PANDORA spectrometer instruments

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∘.

Validation of tropospheric NO<sub>2</sub> column measurements of GOME-2A and OMI using MAX-DOAS and direct sun network observations

“…Several studies have discussed in detail the impact of algorithmic differences on the NO2 column uncertainty, which can reach 42%, mainly due to tropospheric AMF uncertainties (Lorente et al, 2017). The underestimation of the NO2 satellite products identified here at a large number of stations, confirms what was 5 obtained in previous validation exercises using fewer sites and different satellite products (Celarier et al, 2008;Brinksma et al, 2008;Vlemmix et al, 2010;Irie et al, 2008;Lin et al, 2014;Halla et al, 2011;Irie et al 2012;Shaiganfar et al, 2011;Ma et al, 2013;Kanaya et al, 2014;Wang et al 2017b;Mendolia et al 2013;Tzortziou et al, 2014;Lamsal et al, 2014;Drosoglou et al, 2017;Herman et al, 2019;Judd et al, 2019;Compernolle et al 2020). These studies generally reported small negative or positive biases over rural (unpolluted) measurement sites 10 and stronger (systematic) negative biases over urban polluted sites.…”

“…BIRA-IASB MAX-DOAS stations have been regularly used for the validation of GOME-2 GDP products from MetOp-A and MetOp-B (Valks et al, 2011;Pinardi et al, 2011;Liu et al, 2019b) as part of the AC SAF activities (Hassinnen et al, 2016; see also www.cdop.aeronomie.be/validation/validresults). Pandora datasets have also been used in satellite validation of total and tropospheric NO2 columns (Herman et al 2009;Tzortziou et al, 2014;Judd et al, 2019) and a recent study of Herman et al (2019) 35 presented an overview at 14 Pandora sites showing that NASA OMI NO2 overpass data consistently underestimate the Pandora derived NO2 amounts. One general conclusion of these exercises was to find a low bias of the satellites tropospheric NO2 columns in urban conditions and, in contrast, a better agreement with ground-based data in background and pristine locations (Celarier et al, 2008;Halla et al, 2011;Kanaya et al, 2014).…”

Assessment of the quality of TROPOMI high-spatial-resolution NO<sub>2</sub> data products in the Greater Toronto Area

“…One of its primary data products is NO 2 total vertical column density (VCD total ) from the directsun viewing mode, where VCD total represents the vertically integrated number of molecules per unit area and is reported in units of molec cm −2 or Dobson units (1 DU = 2.6870 × 10 16 molec cm −2 ). The Pandora direct-sun NO 2 VCD total products have been validated through many field campaigns (Flynn et al, 2014;Lamsal et al, 2017;Martins et al, 2016;Piters et al, 2012;Reed et al, 2015) and ground-based comparisons (Herman et al, 2009;Wang et al, 2010) and used in satellite validations (Griffin et al, 2019;Herman et al, 2019;Ialongo et al, 2016Ialongo et al, , 2020Lamsal et al, 2014). Since their introduction in 2006, Pandora spectrometers have been deployed at more than 50 sites globally.…”

Evaluating current satellite capability to observe diurnal change in nitrogen oxides in preparation for geostationary satellite missions

“…Several papers have related Pandora measurements to in situ, surface measurements (Flynn et al 2014, Knepp et al 2015, Kollonige et al 2018, and a few have explicitly examined diurnal variability. The most thorough examination of diurnal variability to date has been presented by Herman et al (2019), who find significant time-of-day structure in the NO 2 columns, with patterns differing by location, season, and individual days.…”