Level-3 gridded column densities

The Sentinel-5P/TROPOMI Level-3 data are computed using a gridding algorithm developed by the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) and is based on the HARP atmospheric toolbox. The Level-3 binning algorithm is weighing individual pixels with the pixel overlap area in a given Level-3 grid cell size, see Figure 1 for a schematic representation.

Figure 1: Schematic representation of Level-2 TROPOMI pixels (blue) gridded into a pre-defined grid cell (red) for Level-3 averaging. Figure adapted from Copernicus Research and User Support (RUS).

The Level-2 retrievals at the nominal TROPOMI resolution are binned into 0.05^o by 0.05 ^o grid cells.
Weight and count vectors are used to apply this weighted average consistently, see here for more information. Note that the presented square cell shape in Figure 2 is only valid near the equator, going poleward the grid cells become increasingly stretched into rectangular shapes.

Further, it is important to note that the TROPOMI retrievals are the best possible integrated atmospheric column abundance estimate (troposphere+stratosphere), which means that the provided Level-3 values cannot straightforwardly be converted into surface concentrations. Such column-to-surface conversions are among others dependent on the vertical mixture of surface emissions, meteorological circumstances, TROPOMI signal-to-noise ratio and uncertainties, a priori model input quality, etc.

The tropospheric column densities are provided for the following atmospheric gases:

• Carbon Monoxide (CO) Carbon monoxide (CO) is an important atmospheric trace gas for our understanding of tropospheric chemistry. In certain urban areas, it is a major atmospheric pollutant. Its main sources are fossil fuel combustion, biomass burning, and atmospheric oxidation of methane (CH₄) and other hydrocarbons. While fossil fuel combustion is the main CO source at northern mid-latitudes, isoprene and biomass burning oxidation play an important role in the tropics. An example of monthly averaged CO is given in Figure 2.

Figure 2: Monthly average CO tropospheric column for December 2020.

• Nitrogen dioxide (NO₂)
  Nitrogen dioxide (NO₂) and nitrogen oxide (NO) together are usually referred to as nitrogen oxides (NO_x = NO + NO₂). They are important trace gases in the Earth’s atmosphere and are present in both troposphere and stratosphere. Nitrogen oxides enter the atmosphere as a result of anthropogenic activities (mostly fossil fuel combustion and biomass burning) and natural processes, such as microbiological processes in soils, wildfires, and lightning. An example of the global monthly averaged NO₂ concentration is given in Figure 3.

Figure 3: Monthly average NO₂ tropospheric column for December 2020.

• Methane (CH₄)
  Methane (CH₄) is a strong greenhouse gas that is emitted from both natural (e.g. wetlands, thawing permafrost, cattle) and anthropogenic sources (e.g. petrochemical industry, coal mining, landfill). Its heat capturing capacity is about 30 times more efficient than that of carbon dioxide (CO₂), but its atmospheric lifetime is only about 10 years, compared to hundreds of years for CO₂. Due to the human-induced global temperature rise, its effects (e.g. increased permafrost thawing) have led to an increased atmospheric methane growth rate over the last 2 years, as recently reported by the Copernicus Climate Change Service (C3S). Therefore monitoring the atmospheric methane increase, as well as any future reduction owing to policy measures, with TROPOMI observations is of paramount importance.

• Formaldehyde (HCHO) Formaldehyde (HCHO) is produced as an intermediate gas in most chemical formation chains that produce CO and eventually CO₂. Further, it is a proxy for the presence of certain components that are involved in the production of toxic tropospheric ozone (O₃). In tropical regions, formaldehyde is mainly present due to biomass burning processes, while in temperate regions it originates from anthropogenic activities (e.g. traffic and industry). Its detection is highly relevant in climate studies to monitor among others these anthropogenic emission changes.

More information on NO₂ and CO Level-3 data (global mosaics per day, month and year) can be found in the Sentinel-5P Level-3 atmospheric products ATBD, information on Level-2 retrievals is available from https://sentinels.copernicus.eu/web/sentinel/technical-guides/sentinel-5p/products-algorithms.

In the Virtual Machine environment, the Sentinel-5P data products can be found in /data/MTDA/TERRASCOPE_Sentinel5P.