Vertical profile of the clear-sky aerosol direct radiative effect in an Alpine valley, by the synergy of ground-based measurements and radiative transfer simulations

Abstract

Atmospheric aerosols play an important role in Earth’s radiative balance, directly interacting with solar radiation or influencing cloud formation and properties. In order to assess their radiative impact, it is necessary to accurately characterise their optical properties, together with their spatial and vertical distribution. The information on aerosol vertical profile is often scarce, in particular in mountainous, complex terrains. This study presents the first attempt to evaluate the shortwave aerosol direct radiative effect in the Aosta Valley, a mountainous region in the Northwestern Italian Alps. Ground-based, remote sensing instruments (a sky radiometer and an Automated Lidar Ceilometer) are used to derive two descriptions of the aerosol properties and vertical distribution: a first, more accurate description, which includes the whole spectral information about the aerosol extinction coefficient, phase function and single scattering albedo; a second, more approximate one, which only relies on spectrally constant values of aerosol single scattering albedo and asymmetry factor. This information is used as input for radiative transfer simulations, which allow to estimate, in cloudless conditions, the shortwave aerosol direct radiative effect and the vertical profile of the instantaneous heating rates in the lower layers of the atmosphere. The simulations obtained with the two descriptions do not differ significantly: they highlight a strong surface dimming (between − 25 and − 50 W m− 2) due to the presence of aerosol, with a considerable radiative absorption inside the atmospheric column (around + 30 W m− 2), and an overall small cooling effect for the Earth-atmospheric system. The absorption of solar radiation within the atmospheric column due to aerosol leads to instantaneous heating rates up to 1.5 K day− 1 in the tropospheric layers below 6 km a.s.l. These results show that, in some conditions, the shortwave aerosol direct radiative effect can be considerable even in this Alpine environment, usually considered as relatively pristine (yearly average PM10 concentration about 20 μg m− 3).

The Potential of Absorbing Aerosols to Enhance Extreme Precipitation

“…Thus, we use the same radiative calculations to represent the aerosol effect in the different SST scenarios. We note that the estimated RHR presented in Figure 1b aligns qualitatively in magnitude and vertical structure with observed RHR profiles from previous studies on absorbing aerosols (Z. Wang et al., 2018; Lu et al., 2020; Fasano et al., 2021; Cochrane et al., 2022).…”

Lidar Profiling of Aerosol Vertical Distribution in the Urbanized French Alpine Valley of Annecy and Impact of a Saharan Dust Transport Event

“…Previous lidar measurements in the Alps by a ground-based scanning system during VOTALP have allowed us to identify the two aerosol layers associated with inversions typically induced by local and regional orography, as well as horizontal inhomogeneities in aerosol concentrations at the opening of the Mesolcina valley near Grono, Switzerland [25]. More recently, a ground-based micropulse lidar was used in the Italian valley of Aosta to quantify, for the first time, the direct radiative effect of aerosols during two cases-one with local pollution, and one during a dust transport event in June 2019 [26]. Nonetheless, the three-dimensional distribution of pollutants was only previously investigated during POVA in the Chamonix and Maurienne valleys [17], coupling in situ airborne and lidar measurements to highlight their sources, along with urban heat island effects.…”

Effect of Aerosol Vertical Distribution on the Modeling of Solar Radiation

“…Microplastics can travel very long distances from their sources and have been detected at very high altitudes, on the order of 3 km, in the atmosphere [38,39]. Fasano et al [40] showed that even over a pristine alpine region, transported Saharan dust can increase atmospheric heating rates (HR) by more than 1 K/day (relative to aerosol-free atmosphere), and at the same time have a negative radiative effect at the top and bottom of the atmosphere. In such regions (i.e., alpine valleys) changes in atmospheric heating have an immediate impact on the climate of the slopes and are considered among the possible drivers of elevation-dependent warming (EDW) [41,42].…”