Abstract
Abstract. The impact of the aerosol optical properties on the ultraviolet index (UVI) in the urban area of Rome is investigated in this study. In particular, the influence of aerosol optical depth (AOD) and single scattering albedo (SSA), estimated at the wavelength of 340 nm, and of the Ångström exponent, calculated in the range 340–500 nm, over a period of 11 years (2010–2020) in the months from March to September are analyzed. The UVI is monitored by a Brewer spectrophotometer, whereas measurements of the direct Sun and diffuse sky irradiances are performed by a co-located PREDE-POM sun–sky radiometer of the ESR/SKYNET network; the aerosol optical properties are obtained by the Skyrad MRIv2 retrieval. A novel method, based on physical principles and easily adaptable to other contexts, is developed to extrapolate the aerosol properties to the UV range during periods when only visible to near-infrared measurements are available. The retrievals from the sun–sky radiometer are consistent with the chemical characterization of urban PM10 (particulate matter 10 mm or less in diameter) samples collected during an intensive field campaign held in summer 2011 at the same site (URBan Sustainability Related to Observed and Monitored Aerosol – URBS ROMA). The PM macro components identified during the campaign are grouped in order to evaluate the contribution of the main macro sources (soil, sea, secondary inorganic, organics, and traffic) whose relative role is indeed expected to strongly affect the aerosol absorption capability. The surface forcing efficiency, calculated as the change in the UV index for a unit AOD variation, shows that AOD is the primary parameter affecting the surface irradiance under clear-sky conditions in Rome. SSA and the Ångström exponent are also identified as secondary influencing factors, i.e., the surface forcing efficiency is found to be greater for smaller zenith angles and for larger and more absorbent particles in the UV range (such as, e.g., mineral dust).
Highlights
The aerosol influence on the incoming and outgoing solar radiation is a widely studied topic because of its relation with the Earth’s radiative balance and climate
The aerosol optical depth (AOD) and single scattering albedo (SSA), that is the ratio of the aerosol scattering to extinction coefficient, representing an index of the aerosol absorption capability, are important radiative parameters to determine the aerosol effect on the UV irradiance at the surface
4.1 Validation of the method to extrapolate the aerosol properties to 340 nm Figure 1 shows a comparison of the AOD and SSA extrapolated at 340 nm by aerosol optical properties (AOPs) and retrieved by Skyrad MRIv2, using all wavelengths for the period 2017–2020
Summary
The aerosol influence on the incoming and outgoing solar radiation is a widely studied topic because of its relation with the Earth’s radiative balance and climate. The aerosol capability of attenuating the UV radiation is partly responsible for its short and long-term variations, and it has important implications for tropospheric photochemistry, human health, and for the properties of organic materials, such. M. Campanelli et al.: Aerosol optical characteristics and their impact on the UV index in Rome, Italy as plastics and wood routinely exposed to solar radiation, as well as aquatic systems (Dickerson et al, 1997; He and Carmichael, 1999; Castro et al, 2001; Casasanta et al, 2011; Mok et al 2018). The aerosol optical depth (AOD) and single scattering albedo (SSA), that is the ratio of the aerosol scattering to extinction coefficient, representing an index of the aerosol absorption capability, are important radiative parameters to determine the aerosol effect on the UV irradiance at the surface
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