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https://doi.org/10.5194/acp-2020-264
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-264
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  18 May 2020

18 May 2020

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This preprint is currently under review for the journal ACP.

The impact of cloudiness and cloud type on the atmospheric heating rate of black and brown carbon

Luca Ferrero1, Asta Gregorič2,3, Griša Močnik3,4, Martin Rigler2, Sergio Cogliati1,5, Francesca Barnaba6, Luca Di Liberto6, Gian Paolo Gobbi6, Niccolò Losi1, and Ezio Bolzacchini1 Luca Ferrero et al.
  • 1GEMMA and POLARIS Research Centers, Department of Earth and Environmental Sciences, University of Milano-Bicocca, Piazza della Scienza 1, 20126, Milan, Italy
  • 2Aerosol d.o.o., Kamniška 39A, 1000 Ljubljana, Slovenia
  • 3Center for Atmospheric Research, University of Nova Gorica, Vipavska 11c, 5270 Ajdovščina, Slovenia
  • 4Department of Condensed Matter Physics, Jozef Stefan Institute, 1000 Ljubljana, Slovenia
  • 5Remote Sensing of Environmental Dynamics Lab., DISAT, University of Milano-Bicocca, P.zza della Scienza 1, 20126, Milano, Italy
  • 6ISAC -CNR, Roma -Tor Vergata, Via Fosso Del Cavaliere 100, 00133, Roma, Italy

Abstract. We experimentally quantified the impact of cloud fraction and cloud type on the heating rates (HRs) of black and brown carbon (HRBC and HRBrC).In particular, in this work, we examine in more detail the average cloud effect (Ferrero et al., 2018) using high time-resolution measurements of aerosol absorption at multiple-wavelengths coupled with spectral measurements of the direct, diffuse and surface reflected radiation and lidar data in the Po Valley. The experimental set-up allowed a direct determination of HRBC and HRBrC in any sky condition. The highest values of total HR were found in the middle of the winter (1.43 ± 0.05 K day−1) while the lowest in spring (0.54 ± 0.02 K day−1) Overall the HRBrC accounted for 13.7 ± 0.2 % of the total HR, the BrC being characterized by an AAE of 3.49 ± 0.01.

Simultaneously, sky conditions were classified (from clear-sky to cloudy) in terms of fraction of sky covered by clouds (oktas) and cloud types. Cloud types were grouped as a function of altitude into the following classes: 1) low level (< 2 km) stratus, cumulus and stratocumulus; 2) middle level (2–7 km) altostratus, altocumulus; 3) high level (> 7 km) cirrus, cirrocumulus-cirrostratus. Measurements carried out in different sky conditions at high-time resolution showed a constant decrease of HR with increasing cloudiness of the atmosphere enabling us to quantify for the first time the bias (in %) in the aerosol HR introduced by improperly assuming clear-sky conditions in radiative transfer calculations. In fact, during the campaign, clear sky conditions were only present 23 % of the time while the remaining time (77 %) was characterized by cloudy conditions. Our results show that, by incorrectly assuming clear-sky conditions, the HR of light absorbing aerosol can be largely overestimated (by 50 % in low cloudiness, oktas = 1–2), up to over 400 % (in complete overcast conditions, i.e., oktas = 7–8). The impact of different cloud types on the HR compared to a clear sky condition was also investigated. Cirrus were found to have a modest impact, decreasing the HRBC and HRBrC by −1– −5 %. Cumulus decreased the HRBC and HRBrC by −31 ± 12 and −26 ± 7 %, respectively, while cirrocumulus-cirrostratus by −60 ± 8 and −54 ± 4 %, which was comparable to the impact of altocumulus (−60 ± 6 and −46 ± 4 %). A high impact on HRBC and HRBrC was found for stratocumulus (−63 ± 6 and −58 ± 4 %, respectively) and altostratus (−78 ± 5 and −73 ± 4 %, respectively), although the highest impact was found to be associated to stratus that suppressed the HRBC and HRBrC by −85 ± 5 and −83 ± 3 %, respectively. Additionally, the cloud influence on the radiation spectrum that interacts with the absorbing aerosol was investigated. Black and brown carbon (BC and BrC) have different spectral responses (a different absorption Angstrom exponent, AAE) and our results show that the presence of clouds causes a greater decrease for the HRBC with respect to to HRBrC going clear sky to complete overcast conditions; the observed the difference is 12 ± 6 %. This means that, compared to BC, BrC is more efficient in heating the surrounding atmosphere in cloudy conditions than in clear sky. Overall, this study extends the results of a previous work (Ferrero et al., 2018), highlighting the need to take into account both the role of cloudiness and of different cloud types to better estimate the HR associated to both BC and BrC, and in turn decrease the uncertainties associated to the quantification of the impact of these species on radiation and climate.

Luca Ferrero et al.

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Luca Ferrero et al.

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Short summary
The work experimentally quantify the impact of cloudiness and cloud type on the atmospheric heating rate of black and brown carbon in the Po Valley. The most impacting clouds were: stratocumulus, altostratus and stratus. They caused a greater decrease on the heating rate of black carbon with respect to that of brown carbon. Moreover, the bias in the aerosol heating introduced by improperly assuming clear-sky conditions in radiative transfer calculations was quantified.
The work experimentally quantify the impact of cloudiness and cloud type on the atmospheric...
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