|Paper title||Assessing the aerosol impact on satellite retrievals of CO2: a city-level and global perspective|
|Form of presentation||Poster|
Satellite observations of carbon dioxide have recently matured to the level where they can be used to estimate anthropogenic CO2 emissions of large power plants and other point sources. The true added value of these observations is gained specifically over regions that are otherwise not measured or where the reported emission inventories may be defective. However, satellite observations of CO2 are sensitive to other atmospheric pollutants, specifically aerosol particles that affect the path length of radiation through scattering and absorption. For further complication, these particles are often co-emitted with anthropogenic CO2 emissions. The impact of aerosols on CO2 retrievals can be considered to some extent in the retrieval process and post-processing bias correction. Still, little attention has been dedicated to the evaluation of CO2 retrievals in high aerosol loadings that are characteristic to megacity environments and other regions with persistently poor air quality and high aerosol optical depth (AOD).
In this work we present two approaches to investigate potential aerosol effects on OCO-2 XCO2 observations. To obtain global statistics, a co-located database for OCO-2 XCO2 (OCO-2 v10r) and MODIS Aqua AOD (L2, 10km Dark Target) is created. For each OCO-2 pixel the corresponding MODIS AOD value was defined from the nearest good quality MODIS observation that was found within 0.2 deg. lat., lon. distance from the XCO2 observation. The dataset consists of 5 years of observations between 2015 and 2019. This unique global dataset enables the investigation of large scale variation patterns, regional dependencies and allow also to identify potentially interesting areas for more detailed study. In the local scale approach, the aerosol effects are studied in the vicinity of urban TCCON stations, that also have an operating AERONET or other sunphotometer station close by. To investigate the spatial patterns, L2 MODIS Aqua 3km Dark Target AOD is analysed together with L2 OCO-2 XCO2. Hence, in this approach for both XCO2 and AOD, a ground-based reference measurement can be obtained in addition to the satellite observations. Also, aerosol vertical profiles from Calipso will be analysed if an overpass over the study area is obtained. With this combination of observations the potential risks for aerosol induced biases in a city scale can be assessed in detail.
This research will lay important ground work to the planned Copernicus Anthropogenic CO2 Monitoring Mission where the ultimate purpose is to support the goals of the Paris Agreement with independent emission estimates derived from satellite observations. For this purpose, it is crucial to investigate the validation of CO2 observations in urban, high AOD environments and establish the current state of the art and gaps in both retrievals and validation.