Frequently Asked Questions (FAQs) on OMI NO2 time series and maps
- What does “vertical column” mean, and what are the units of the NO2 data?
Satellite instruments that measure NO2 are sensitive to the number of molecules between the instrument and the Earth’s surface (i.e., a Vertical Column Density or VCD). For NO2 typically the amount high in the atmosphere (in the stratosphere) is removed in order to provide an estimate of the column in the lower part of the atmosphere (the troposphere), a Tropospheric Vertical Column Density or TVCD. The units of VCD or TVCD are molecules per square centimeter or molec./cm2. Because these numbers are large, they may be divided by 1015. Units may also be given as moles per unit area.
- Is the NO2 vertical column the same as NO2 emissions?
No. NO2 vertical columns are observed from space and are the results of emissions of nitrogen oxides (NOx = NO + NO2) that are formed by fossil fuel combustion as well as fires, lightning, and in soil. With a lifetime of the order of hours near the surface, NO2 is transported away from its sources and this transport depends on winds. Its lifetime varies with a number of factors including the amount of sunlight and other chemical species in the atmosphere. Therefore, NO2 column amounts in the atmosphere can be highly variable and dependent on local emissions, chemistry and transport. NO2 columns can serve as a proxy of NOx emissions if averaged appropriately in time and space. However, if appropriate averaging is not undertaken and/or effective quality control (for example, to remove pixels substantially affected by clouds), a flaw interpretation may result (see the Copernicus article “Flawed estimates of the effects of lockdown measures on air quality derived from satellite observations“).
- Why do the 2020 data sometimes fall outside the baseline before the COVID-19 outbreak?
In the time series shown, estimates of the uncertainties are provided. These estimates include contributions from natural variability (e.g., weather variations) and random errors in satellite data. The values shown are based on several assumption such as normal distributions (which may or may not be the case) and are standard errors (based on one standard deviation) divided by square root of the number of points being analyzed. Therefore, the greater number of points, the better the estimate (assuming that errors are not correlated; note that the uncertainties do NOT account for any systematic errors). Statistically, we should expect that a small fraction of the time, values will naturally fall outside of this range. However, when we see values consistently falling outside the expected range for an extended period of time, we would consider this to be unusual and perhaps caused by an unexpected event such as traffic reductions that result from measures taken to reduce exposure to COVID-19.
- For the general public, the most prominent sources of NO2 are internal combustion engines burning fossil fuels. Outdoors, NO2 can be a result of traffic from motor vehicles.
- The TROPOMI instrument, single payload onboard Sentinel-5 Precursor, retrieves operationally tropospheric and stratospheric NO2 column products. The TROPOMI NO2 data products pose an improvement over previous NO2 data sets, particularly in their unprecedented spatial resolution (7×3.5 km2), but also in the separation of the stratospheric and tropospheric contributions of the retrieved slant columns, and in the calculation of the air-mass factors used to convert slant to total columns.
NDVI (Normalized difference vegetation index)
- General description:
- The well known and widely used NDVI is a simple, but effective index for quantifying green vegetation. It normalizes green leaf scattering in Near Infra-red wavelengths with chlorophyll absorption in red wavelengths.
- The value range of the NDVI is -1 to 1. Negative values of NDVI (values approaching -1) correspond to water. Values close to zero (-0.1 to 0.1) generally correspond to barren areas of rock, sand, or snow. Low, positive values represent shrub and grassland (approximately 0.2 to 0.4), while high values indicate temperate and tropical rain forests (values approaching 1). It is a good proxy for live green vegetation.
- The normalized difference vegetation index, abbreviated NDVI, is defined as:
- For Sentinel-2, the index looks like this:
- Color legend: