Abstract. We present an improved tropospheric nitrogen dioxide column retrieval algorithm (DOMINO v2.0) for OMI based on better air mass factors (AMFs) and a correction for across-track stripes resulting from calibration errors in the OMI backscattered reflectances. Since October 2004, NO 2 retrievals from the Ozone Monitoring Instrument (OMI), a UV/Vis nadir spectrometer onboard NASA's EOSAura satellite, have been used with success in several scientific studies focusing on air quality monitoring, detection of trends, and NO x emission estimates. Dedicated evaluations of previous DOMINO tropospheric NO 2 retrievals indicated their good quality, but also suggested that the tropospheric columns were susceptible to high biases (by 0-40 %), probably because of errors in the air mass factor calculations. Here we update the DOMINO air mass factor approach. We calculate a new look-up table (LUT) for altitude-dependent AMFs based on more realistic atmospheric profile parameters, and include more surface albedo and surface pressure reference points than before. We improve the sampling of the TM4 model, resulting in a priori NO 2 profiles that are better mixed throughout the boundary layer. We evaluate the NO 2 profiles simulated with the improved TM4 sampling as used in the AMF calculations and show that they are highly consistent with in situ NO 2 measurements from aircraft during the INTEX-A and INTEX-B campaigns in 2004 and 2006. Our air mass factor calculations are furCorrespondence to: Folkert Boersma (boersma@knmi.nl) ther updated by the implementation of a high-resolution terrain height and a high-resolution surface albedo climatology based on OMI measurements. Together with a correction for across-track stripes, the overall impact of the improved terrain height and albedo descriptions is modest (<5 %) on average over large polluted areas, but still causes significant changes locally. The main changes in the DOMINO v2.0 algorithm follow from the new LUT and the improved TM4 sampling that results in more NO 2 simulated aloft, where sensitivity to NO 2 is higher, and amount to reductions in tropospheric NO 2 columns of up to 20 % in winter, and 10 % in summer over extended polluted areas. We investigate the impact of aerosols on the NO 2 retrieval, and based on a comparison of concurrent retrievals of clouds from OMI and aerosols from MODIS Aqua, we find empirical evidence that OMI cloud retrievals are sensitive to the presence of scattering aerosols. It follows that an implicit correction for the effects of aerosols occurs through the aerosol-induced cloud parameters in DOMINO, and we show that such an empirical correction amounts to a 20 % AMF reduction in summer and ±10 % changes in winter over the eastern United States.

Abstract. We present a new algorithm for the near-real time retrieval -within 3 h of the actual satellite measurement -of tropospheric NO 2 columns from the Ozone Monitoring Instrument (OMI). The retrieval is based on the combined retrieval-assimilation-modelling approach developed at KNMI for off-line tropospheric NO 2 from the GOME and SCIAMACHY satellite instruments. We have adapted the off-line system such that the required a priori informationprofile shapes and stratospheric background NO 2 -is now immediately available upon arrival (within 80 min of observation) of the OMI NO 2 slant columns and cloud data at KNMI. Slant columns for NO 2 are retrieved using differential optical absorption spectroscopy (DOAS) in the 405-465 nm range. Cloud fraction and cloud pressure are provided by a new cloud retrieval algorithm that uses the absorption of the O 2 -O 2 collision complex near 477 nm. Online availability of stratospheric slant columns and NO 2 profiles is achieved by running the TM4 chemistry transport model (CTM) forward in time based on forecast ECMWF meteo and assimilated NO 2 information from all previously observed orbits. OMI NO 2 slant columns, after correction for spurious across-track variability, show a random error for individual pixels of approximately 0.7×10 15

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TROPOspheric Monitoring Instrument (TROPOMI), on‐board the Sentinel‐5 Precurser satellite, is a nadir‐viewing spectrometer measuring reflected sunlight in the ultraviolet, visible, near‐infrared, and shortwave infrared. From these spectra several important air quality and climate‐related atmospheric constituents are retrieved, including nitrogen dioxide (NO2) at unprecedented spatial resolution from a satellite platform. We present the first retrievals of TROPOMI NO2 over the Canadian Oil Sands, contrasting them with observations from the Ozone Monitoring Instrument satellite instrument, and demonstrate TROPOMI's ability to resolve individual plumes and highlight its potential for deriving emissions from individual mining facilities. Further, the first TROPOMI NO2 validation is presented, consisting of aircraft and surface in situ NO2 observations, and ground‐based remote‐sensing measurements between March and May 2018. Our comparisons show that the TROPOMI NO2 vertical column densities are highly correlated with the aircraft and surface in situ NO2 observations, and the ground‐based remote‐sensing measurements with a low bias (15–30 %); this bias can be reduced by improved air mass factors.

Abstract. Retrievals of tropospheric nitrogen dioxide (NO 2 ) from the Ozone Monitoring Instrument (OMI) are subject to errors in the treatments of aerosols, surface reflectance anisotropy, and vertical profile of NO 2 . Here we quantify the influences over China via an improved retrieval process. We explicitly account for aerosol optical effects (simulated by nested GEOS-Chem at 0.667 • long. × 0.5 • lat. and constrained by aerosol measurements), surface reflectance anisotropy, and high-resolution vertical profiles of NO 2 (simulated by GEOS-Chem). Prior to the NO 2 retrieval, we derive the cloud information using consistent ancillary assumptions.We compare our retrieval to the widely used DOMINO v2 product, using MAX-DOAS measurements at three urban/suburban sites in East China as reference and focusing the analysis on the 127 OMI pixels (in 30 days) closest to the MAX-DOAS sites. We find that our retrieval reduces the interference of aerosols on the retrieved cloud properties, thus enhancing the number of valid OMI pixels by about 25 %. Compared to DOMINO v2, our retrieval better captures the day-to-day variability in MAX-DOAS NO 2 data (R 2 = 0.96 versus 0.72), due to pixel-specific radiative transfer calculations rather than the use of a look-up table, explicit inclusion of aerosols, and consideration of surface reflectance anisotropy. Our retrieved NO 2 columns are 54 % of the MAX-DOAS data on average, reflecting the inevitable spatial inconsistency between the two types of measurement, errors in MAX-DOAS data, and uncertainties in our OMI retrieval related to aerosols and vertical profile of NO 2 .Sensitivity tests show that excluding aerosol optical effects can either increase or decrease the retrieved NO 2 for individual OMI pixels with an average increase by 14 %. Excluding aerosols also complexly affects the retrievals of cloud fraction and particularly cloud pressure. Employing various surface albedo data sets slightly affects the retrieved NO 2 on average (within 10 %). The retrieved NO 2 columns increase when the NO 2 profiles are taken from MAX-DOAS retrievals (by 19 % on average) or TM4 simulations (by 13 %) instead of GEOS-Chem simulations. Our findings are also relevant to retrievals of other pollutants (e.g., sulfur dioxide, Published by Copernicus Publications on behalf of the European Geosciences Union. 1442J.-T. Lin et al.: Improved retrieval of tropospheric nitrogen dioxide from OMI formaldehyde, glyoxal) from UV-visible backscatter satellite instruments.