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.

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

“…An improved V2.0 DOMINO retrieval (Boersma et al, 2011) algorithm reduced the retrieval errors while increasing the estimated air mass factor, which reduces the retrieved TCNO 2 by up to 20 % in winter and 10 % in summer. The current versions of OMNO2-NASA (Krotkov et al, 2017) and v2.0 DOMINO (Boersma et al, 2011) are generally in good agreement (Marchenko et al, 2015;Zara et al, 2018). However, the OMNO2-NASA TCNO 2 retrievals are 10 % to 15 % lower than the v2.0 DOMINO retrievals and with Quality Assurance for Essential Climate Variables (QA4ECV) retrievals.…”

“…TCNO 2 amounts (data used: OMNO2-NASA v3.1) retrieved from OMI over various specified land locations show a strong local underestimate compared to co-located PANDORA spectrometer in-struments (the abbreviation PAN is used for graph and table labels). The underestimate of OMI TCNO 2 at the overpass time compared to ground-based measurements has previously been reported at a few specific locations (Bechle, 2013;Lamsal et al, 2014;Ialongo et al, 2016;Kollonige et al, 2018;Goldberg et al, 2019;Herman et al, 2018). The accuracy and precision of PANDORA TCNO 2 measurements have been previously discussed (Herman et al, 2009(Herman et al, , 2018.…”

“…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 R S (Boersma et al, 2011;Lin et al, 2015;Nowlan et al, 2016;Lorente et al, 2018). Accurately determining the AMF for TCNO 2 requires a priori knowledge of the NO 2 profile shape (Krotkov et al, 2017), which is estimated from coarse-resolution model calculations (Boersma et al, 2011) and using the correct R S . Currently R S is found using a statistical process of sorting through years of data to find relatively clear-sky scenes for each location (Kleipool et al, 2008;O'Byrne et al, 2010).…”

Comparison of Near‐Surface NO2 Pollution With Pandora Total Column NO2 During the Korea‐United States Ocean Color (KORUS OC) Campaign

“…We averaged all observations to 5 min to match the 5 min PSI averages. Uncertainties for these instruments specified in Martins et al (2012) and Kollonige et al (2018) are 5% (NO 2 ), 1.3% (O 3 ), and 10 ppbv (CO), respectively. Observations influenced by ship exhaust (identified from coincident upward spikes in surface NO 2 and downward spikes in surface O 3 ) were omitted from the 5-min averages and comparisons.…”

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

“…As we consider the ability of satellite data to detect hourly change in column NO 2 , the gold standard for comparison is the Pandora instrument. 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.…”