Reply on RC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> We aim to evaluate the NO₂ absorption effect in aerosol properties derived from sun-sky radiometers as well as the possible retrieval algorithm improvements by using more accurate characterization of NO₂ optical depth. For this purpose, we employ multiannual (2017&ndash;2022) records of Aerosol Optical Depth (AOD), &Aring;ngstr&ouml;m Exponent (AE) and Single Scattering Albedo (SSA) collected by sun photometers at an urban and a suburban site in the Rome area (Italy) in the framework of both the AERONET and SKYNET networks. The uncertainties introduced in the retrievals by the NO₂ absorption are investigated using high-frequency observations of total NO₂ derived from co-located Pandora spectroradiometer systems as well as space-borne NO₂ products from the Tropospheric Monitoring Instrument (TROPOMI). The correction is useful for lower AODs (&lt; 0.3), where the majority of observations is found, especially under high NO₂ pollution events. The analysis does not reveal any significant impact of the NO₂ 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 as well as in the case of an important NO₂ trend. In addition, the comparisons of the NO₂-modified ground-based AOD data with satellite retrievals from the Deep Blue (DB) algorithm of the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) resulted in a slight improvement in the agreement of about 0.003 and 0.006 for AERONET and SKYNET, respectively. Finally, the uncertainty in assumptions of NO₂ seem to have a non-negligible impact on the retrieved values of SSA at 440 nm leading to an average positive bias of 0.02 (2.5 %) in both locations for high NO₂ loadings (&gt; 0.9 DU).