Abstract
Abstract. A systematic study of black carbon (BC) vertical profiles measured at high-resolution over three Italian basin valleys (Terni Valley, Po Valley and Passiria Valley) is presented. BC vertical profiles are scarcely available in literature. The campaign lasted 45 days and resulted in 120 measured vertical profiles. Besides the BC mass concentration, measurements along the vertical profiles also included aerosol size distributions in the optical particle counter range, chemical analysis of filter samples and a full set of meteorological parameters. Using the collected experimental data, we performed calculations of aerosol optical properties along the vertical profiles. The results, validated with AERONET data, were used as inputs to a radiative transfer model (libRadtran). The latter allowed an estimation of vertical profiles of the aerosol direct radiative effect, the atmospheric absorption and the heating rate in the lower troposphere. The present measurements revealed some common behaviors over the studied basin valleys. Specifically, at the mixing height, marked concentration drops of both BC (range: from −48.4 ± 5.3 to −69.1 ± 5.5%) and aerosols (range: from −23.9 ± 4.3 to −46.5 ± 7.3%) were found. The measured percentage decrease of BC was higher than that of aerosols: therefore, the BC aerosol fraction decreased upwards. Correspondingly, both the absorption and scattering coefficients decreased strongly across the mixing layer (range: from −47.6 ± 2.5 to −71.3 ± 3.0% and from −23.5 ± 0.8 to −61.2 ± 3.1%, respectively) resulting in a single-scattering albedo increase along height (range: from +4.9 ± 2.2 to +7.4 ± 1.0%). This behavior influenced the vertical distribution of the aerosol direct radiative effect and of the heating rate. In this respect, the highest atmospheric absorption of radiation was predicted below the mixing height (~ 2–3 times larger than above it) resulting in a heating rate characterized by a vertical negative gradient (range: from −2.6 ± 0.2 to −8.3 ± 1.2 K day−1 km−1). In conclusion, the present results suggest that the BC below the mixing height has the potential to promote a negative feedback on the atmospheric stability over basin valleys, weakening the ground-based thermal inversions and increasing the dispersal conditions.
Highlights
Atmospheric aerosols influence the Earth’s climate through direct effects, indirect effects and semidirect effects (Ramanathan and Feng, 2009; Koren et al, 2008, 2004; IPCC, 2013; Kaufman et al, 2002)
We report a comparative study of black carbon (BC) and aerosol vertical profiles measured over three different Italian basin valleys (Terni Valley in the central Apennines; Po Valley in northern Italy; Passiria–Val Venosta valleys in the Alps)
The main features of BC and aerosol vertical distribution during the campaign can be highlighted considering, as a case study, the vertical profiles measured on 28 January 2010 (13:45–14:26 UTC) over TR; they are reported in Fig. 2 together with the corresponding potential temperature (T ) and RH profiles
Summary
Atmospheric aerosols influence the Earth’s climate through direct effects (sunlight absorption and scattering), indirect effects (i.e., modifying the lifetime of the clouds) and semidirect effects (i.e., affecting the thermal structure of the atmosphere) (Ramanathan and Feng, 2009; Koren et al, 2008, 2004; IPCC, 2013; Kaufman et al, 2002). The resulting overall degree of uncertainty attributable to the assumptions about the vertical distribution of BC was estimated to be in the range 20–50 % This is very important as the vertical heterogeneity of BC, and of its DRE, the thermal structure of the atmosphere; in particular, heating rates may vary as a function of height in a range from 0.5 to 2 K day−1 (Chakrabarty et al, 2012; Ramana et al, 2010; Tripathi et al, 2007). The Italian territory is characterized by a multitude of basin valleys surrounded by hills or mountains, where urban and industrial centers are usually settled These valleys represent areas where low wind speeds and conditions of atmospheric stability are common, promoting the formation of strong vertical aerosol (and BC) gradients in the lower troposphere (Moroni et al, 2012, 2013; Ferrero et al, 2011a; Carbone et al, 2010; Rodríguez et al, 2007).
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