Abstract
Abstract. We present a new and improved version (V4.0) of the NASA standard nitrogen dioxide (NO2) product from the Ozone Monitoring Instrument (OMI) on the Aura satellite. This version incorporates the most salient improvements for OMI NO2 products suggested by expert users and enhances the NO2 data quality in several ways through improvements to the air mass factors (AMFs) used in the retrieval algorithm. The algorithm is based on the geometry-dependent surface Lambertian equivalent reflectivity (GLER) operational product that is available on an OMI pixel basis. GLER is calculated using the vector linearized discrete ordinate radiative transfer (VLIDORT) model, which uses as input high-resolution bidirectional reflectance distribution function (BRDF) information from NASA's Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) instruments over land and the wind-dependent Cox–Munk wave-facet slope distribution over water, the latter with a contribution from the water-leaving radiance. The GLER combined with consistently retrieved oxygen dimer (O2–O2) absorption-based effective cloud fraction (ECF) and optical centroid pressure (OCP) provide improved information to the new NO2 AMF calculations. The new AMFs increase the retrieved tropospheric NO2 by up to 50 % in highly polluted areas; these differences arise from both cloud and surface BRDF effects as well as biases between the new MODIS-based and previously used OMI-based climatological surface reflectance data sets. We quantitatively evaluate the new NO2 product using independent observations from ground-based and airborne instruments. The new V4.0 data and relevant explanatory documentation are publicly available from the NASA Goddard Earth Sciences Data and Information Services Center (https://disc.gsfc.nasa.gov/datasets/OMNO2_V003/summary/, last access: 8 November 2020), and we encourage their use over previous versions of OMI NO2 products.
Highlights
The Dutch–Finnish-built Ozone Monitoring Instrument (OMI) has been operating onboard the NASA EOS Aura spacecraft since July 2004 (Levelt et al, 2006, 2018)
Since the transmittance and the water-leaving contribution are coupled, we develop a simple coupling scheme in vector linearized discrete ordinate radiative transfer (VLIDORT) to ensure that the value of water-leaving radiance used as an input at the ocean surface will correspond to the correct value of the downwelling flux reaching the surface interface (Fasnacht et al, 2019)
The corresponding terrain pressure for each OMI pixel (Ps) is calculated from the terrain pressure–height relationship established based on MERRA-2 monthly terrain pressure (Ps_GMI) at a spatial resolution of 1◦ latitude × 1.25◦ longitude used in the Global Modeling Initiative (GMI) model discussed above: Ps where z (= z −zGMI) represents the difference between the average terrain height for an OMI pixel (z) and the terrain height at kT Mg represents the scale height, where k is the Boltzmann constant, T is the temperature at the surface, M is the mean molecular weight of air, and g is the acceleration due to gravity
Summary
The Dutch–Finnish-built Ozone Monitoring Instrument (OMI) has been operating onboard the NASA EOS Aura spacecraft since July 2004 (Levelt et al, 2006, 2018). Taking advantage of the MODIS highresolution data, albeit neglecting the BRDF and atmospheric effects, Russell et al (2011) and McLinden et al (2014) created improved NO2 products from the NASA standard product (Bucsela et al, 2013; Lamsal et al, 2014) over the continental US and Canada, respectively While these and subsequent studies (e.g., Kuhlmann et al, 2015; Laughner et al, 2019) addressed the limitation of climatological LER data for NO2 retrievals, they did not account for the surface BRDF effect on the OMI cloud products (cloud pressure and fraction), which are inputs to the NO2 algorithm.
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