Assessment of NO2 uncertainty impact on aerosol optical depth retrievals at a global scale

• Main Authors: Masoom, Akriti, Kazadzis, Stelios, Valeri, Masimo, Raptis, Ioannis-Panagiotis, Brizzi, Gabrielle, Papachristopoulou, Kyriakoula, Barnaba, Francesca, Casadio, Stefano, Kreuter, Axel, Niro, Fabrizio
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2024
• Subjects: article; Verlagsveröffentlichung; Aerosol Robotic Network
• Online Access: https://doi.org/10.5194/egusphere-2024-682; https://noa.gwlb.de/receive/cop_mods_00072403; https://noa.gwlb.de/servlets/MCRFileNodeServlet/cop_derivate_00070618/egusphere-2024-682.pdf; https://egusphere.copernicus.org/preprints/2024/egusphere-2024-682/egusphere-2024-682.pdf

This work aims at investigating the effect of NO2 absorption on aerosol optical depth (AOD) and Ångström exponent (AE) retrievals of sun photometers by synergistic use of the accurate NO2 characterization for optical depth estimation from co-located ground-based measurements. The analysis was performed for ~7 years (2017–2023) at a global scale for the AOD and AE retrievals by Aerosol Robotic Network (AERONET) sun photometers which uses OMI (Ozone Monitoring Instrument) climatology for NO2 representation. The deviations in AOD and AE retrievals by NO2 absorption is accounted for using high-frequency columnar NO2 measurements by co-located Pandora spectroradiometer belonging to Pandonia Global Network (PGN). The AERONET retrieved AOD was found to be overestimated in half of the cases while also underestimated in other cases as an impact of the NO2 deviation from “real” (PGN NO2) values. Over or underestimations are relatively low. About one-third of these stations showed a mean deviation in NO2 and AOD (at 380 nm and 440 nm) above 0.5x10-4 mol-m-2 and 0.002, respectively, which can be considered as a systematic contribution to the uncertainties of AOD retrievals that are reported to be in the order of 0.01. However, under extreme NO2 loading scenarios (i.e., 10 % highest deviations), even higher AOD deviations were observed that were at the limit or higher than the reported 0.01 uncertainty of the AOD retrieval. The PGN NO2 based sensitivity analysis of AOD deviation suggested that for PGN NO2 varying between 2x10-4 and 8x10-4 mol-m-2, the median AOD differences were found to rise above 0.01 (even above 0.02) with the increase in NO2 threshold (i.e., the lower limit from 2 x10-4 mol-m-2 to 8 x10-4 mol-m-2). The AOD-derivative product, AE, was also affected by the NO2 correction (discrepancies between the AERONET OMI climatological representation of NO2 values and the real PGN NO2 measurements) on the spectral AOD. The normalized frequency distribution of AE (at 440–870 nm and 380–675 nm wavelength pair) was found ...