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.

Determining effective strategies for mitigating surface ozone (O₃) pollution requires knowledge of the relative ambient concentrations of its precursors, NO_x, and VOCs. The space‐based tropospheric column ratio of formaldehyde to NO₂ (FNR) has been used as an indicator to identify NO_x‐limited versus NO_x‐saturated O₃ formation regimes. Quantitative use of this indicator ratio is subject to three major uncertainties: (1) the split between NO_x‐limited and NO_x‐saturated conditions may shift in space and time, (2) the ratio of the vertically integrated column may not represent the near‐surface environment, and (3) satellite products contain errors. We use the GEOS‐Chem global chemical transport model to evaluate the quantitative utility of FNR observed from the Ozone Monitoring Instrument over three northern midlatitude source regions. We find that FNR in the model surface layer is a robust predictor of the simulated near‐surface O₃ production regime. Extending this surface‐based predictor to a column‐based FNR requires accounting for differences in the HCHO and NO₂ vertical profiles. We compare four combinations of two OMI HCHO and NO₂ retrievals with modeled FNR. The spatial and temporal correlations between the modeled and satellite‐derived FNR vary with the choice of NO₂ product, while the mean offset depends on the choice of HCHO product. Space‐based FNR indicates that the spring transition to NO_x‐limited regimes has shifted at least a month earlier over major cities (e.g., New York, London, and Seoul) between 2005 and 2015. This increase in NO_x sensitivity implies that NO_x emission controls will improve O₃ air quality more now than it would have a decade ago.

We assess the standard operational nitrogen dioxide (NO₂) data product (OMNO2, version 2.1) retrieved from the Ozone Monitoring Instrument (OMI) onboard NASA's Aura satellite using a combination of aircraft and surface in~situ measurements as well as ground-based column measurements at several locations and a bottom-up NO_x emission inventory over the continental US. Despite considerable sampling differences, NO₂ vertical column densities from OMI are modestly correlated (r = 0.3–0.8) with in situ measurements of tropospheric NO₂ from aircraft, ground-based observations of NO₂ columns from MAX-DOAS and Pandora instruments, in situ surface NO₂ measurements from photolytic converter instruments, and a bottom-up NO_x emission inventory. Overall, OMI retrievals tend to be lower in urban regions and higher in remote areas, but generally agree with other measurements to within ± 20%. No consistent seasonal bias is evident. Contrasting results between different data sets reveal complexities behind NO₂ validation. Since validation data sets are scarce and are limited in space and time, validation of the global product is still limited in scope by spatial and temporal coverage and retrieval conditions. Monthly mean vertical NO₂ profile shapes from the Global Modeling Initiative (GMI) chemistry-transport model (CTM) used in the OMI retrievals are highly consistent with in situ aircraft measurements, but these measured profiles exhibit considerable day-to-day variation, affecting the retrieved daily NO₂ columns by up to 40%. This assessment of OMI tropospheric NO₂ columns, together with the comparison of OMI-retrieved and model-simulated NO₂ columns, could offer diagnostic evaluation of the model.

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.

NASA satellite measurements show that ozone reductions throughout the Northern Hemisphere (NH) free troposphere reported for spring‐summer 2020 during the COronaVIrus Disease 2019 pandemic have occurred again in spring‐summer 2021. The satellite measurements show that tropospheric column ozone (TCO) (mostly representative of the free troposphere) for 20°N–60°N during spring‐summer for both 2020 and 2021 averaged ∼3 Dobson Units (DU) (or ∼7%–8%) below normal. These ozone reductions in 2020 and 2021 were the lowest in the 2005–2021 record. We also include satellite measurements of tropospheric NO₂ that exhibit reductions of ∼10%–20% in the NH in early spring‐to‐summer 2020 and 2021, suggesting that reduced pollution was the main cause for the low anomalies in NH TCO in 2020 and 2021. Reductions of TCO ∼2 DU (7%) are also measured in the Southern Hemisphere in austral summer but are not associated with reduced NO₂.