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Caution advised on use and interpretation of satellite-derived nitrogen dioxide tropospheric columns

With the recent global pandemic and resulting widespread changes in human behavior, there has been much interest in the use of satellite-derived nitrogen dioxide (NO2) tropospheric columns as a proxy for air quality and/or emissions from fossil fuel burning. For example, the NASA Earth Observatory (EO) issued a story showing how atmospheric levels of tropospheric NO2, which is emitted when fossil fuels (coal, gasoline) are burned, changed over China during the government's efforts to control the spread of the virus.  

Data Processing: Recently, the  European Union’s Copernicus Atmosphere Monitoring Service (CAMS) and program put out an advisory on the use of data from the Sentinel-5 Precursor (S5P) satellite TROPOspheric Monitoring Instrument (TROPOMI) that delivers data shown in many news stories worldwide (see “Flawed estimates of the effects of lockdown measures on air quality derived from satellite observations”). NASA similarly delivers NO2 tropospheric column products from the Ozone Monitoring Instrument (OMI), a Dutch Finnish instrument launched onboard NASA’s Aura satellite in 2004. While TROPOMI has surpassed OMI in several respects (e.g., spatial resolution – see images released by ESA for the impact of COVID-19 on European pollution), OMI has the longest record of any NO2-monitoring instrument and is one of the most radiometrically stable instruments of its type. In addition, the OMI long-term data record is critical for putting the recent reduction into context, especially given the complex variations in pollutant levels driven by meteorology and other events. 

Data Interpretation: Care must be taken when interpreting satellite NO2 data as the quantity observed by the satellite is not exactly the same as the NO2 abundance at ground level. NO2 levels are influenced by dynamical and chemical processes in the atmosphere. For instance, atmospheric NO2 levels can vary day-to-day due to changes in the weather, which influences both the lifetime of NO2 molecules as well as the dispersal of the molecules by the wind. It is also important to note that satellites that observe NO2 cannot see through clouds, so all data shown is for days with low amounts of cloudiness. If processed and interpreted carefully, NO2 levels observed from space serve as an effective proxy for NO2 levels measured at Earth's surface.