Evaluating the effects of columnar NO 2 on the accuracy of aerosol optical properties retrievals

• Main Authors: T. Drosoglou, I.-P. Raptis, M. Valeri, S. Casadio, F. Barnaba, M. Herreras-Giralda, A. Lopatin, O. Dubovik, G. Brizzi, F. Niro, M. Campanelli, S. Kazadzis
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2023
• Subjects: Environmental engineering; TA170-171; Earthwork. Foundations; TA715-787; Aerosol Robotic Network
• Online Access: https://doi.org/10.5194/amt-16-2989-2023; https://doaj.org/article/a5393fe60c7944c6b503ed277f6a683a

We aim to evaluate the NO 2 absorption effect in aerosol columnar properties, namely the aerosol optical depth (AOD), Ångström exponent (AE), and single scattering albedo (SSA), derived from sun–sky radiometers in addition to the possible retrieval algorithm improvements by using more accurate characterization of NO 2 optical depth from co-located or satellite-based real-time measurements. For this purpose, we employ multiannual (2017–2022) records of AOD, AE, and SSA collected by sun photometers at an urban and a suburban site in the Rome area (Italy) in the framework of both the Aerosol Robotic Network (AERONET) and SKYNET networks. The uncertainties introduced in the aerosol retrievals by the NO 2 absorption are investigated using high-frequency observations of total NO 2 derived from co-located Pandora spectroradiometer systems in addition to spaceborne NO 2 products from the Tropospheric Monitoring Instrument (TROPOMI). For both AERONET and SKYNET, the standard network products were found to systematically overestimate AOD and AE. The average AOD bias found for Rome is relatively low for AERONET ( ∼ 0.002 at 440 nm and ∼ 0.003 at 380 nm) compared to the retrieval uncertainties but quite a bit higher for SKYNET ( ∼ 0.007). On average, an AE bias of ∼ 0.02 and ∼ 0.05 was estimated for AERONET and SKYNET, respectively. In general, the correction seems to be low for areas with low columnar NO 2 concentrations, but it is still useful for low AODs ( < 0.3), where the majority of observations are found, especially under high NO 2 pollution events. For the cases of relatively high NO 2 levels ( > 0.7 DU), the mean AOD bias was found within the range 0.009–0.012 for AERONET, depending on wavelength and location, and about 0.018 for SKYNET. The analysis does not reveal any significant impact of the NO 2 correction on the derived aerosol temporal trends for the very limited data sets used in this study. However, the effect is expected to become more evident for trends derived from larger data sets and in the ...