Experimental determination of the effect of clouds on the atmospheric heating rate of black and brown carbon in the Po Valley
Authors: Ferrero Luca , Gregorič Asta , Močnik Grisa , Rigler Martin , Cogliati Sergio , Barnaba Francesca , Di Liberto Luca , Gobbi Gian Paolo , Losi Niccolò , Bolzacchini Ezio 
Published: March 2022

&lt;p&gt;The impact of cloud fraction and cloud type on the heating rate (HR) of black and brown carbon (HR&lt;sub&gt;BC&lt;/sub&gt; and HR&lt;sub&gt;BrC&lt;/sub&gt;) was experimentally determined using a methodology developed in a previous study (Ferrero et al., 2018). High time-resolution measurements of the aerosol absorption coefficient at multiple-wavelengths (Aethalometer AE33 calibrated in COLOSSAL Campaign, Ferrero et al., 2021a) were coupled with spectral measurements of the direct, diffuse and surface reflected irradiance (Multiplexer-Radiometer-Irradiometer coupled with LSI-Lastem DPA154 and C201R, class 1 radiometers), and with lidar-ceilometer (Jenoptik Nimbus 15k biaxial lidar-ceilometer) during a one year field campaign in Milan, Po Valley (Italy).&lt;/p&gt;&lt;p&gt;The set-up allowed the experimental determination of the total HR (and its speciation: HR&lt;sub&gt;BC&lt;/sub&gt; and HR&lt;sub&gt;BrC&lt;/sub&gt;) in all sky conditions (from clear-sky to cloudy) with the highest total HR values found in the middle of winter (1.43&amp;#177;0.05 K day&lt;sup&gt;-1&lt;/sup&gt;). The HR&lt;sub&gt;BrC&lt;/sub&gt; accounted for 13.7&amp;#177;0.2% of the total HR (BrC absorption Angstrom exponent: 3.49&amp;#177;0.01).&lt;/p&gt;&lt;p&gt;Sky conditions were classified in terms of cloudiness (fraction of sky covered by clouds: oktas) and cloud types: stratus (St), cumulus (Cu), stratocumulus (Sc), altostratus (As), altocumulus (Ac), cirrus (Ci) and cirrocumulus-cirrostratus (Cc-Cs). During the campaign, clear sky conditions were present 23% of the time, the remaining time (77%) being characterized by cloudy conditions. The average cloudiness was 3.58&amp;#177;0.04 oktas (highest in February: 4.56&amp;#177;0.07 oktas, lowest in November: 2.91&amp;#177;0.06 oktas). St were mostly responsible of overcast conditions (oktas=7-8, frequency: 87 and 96%).&lt;/p&gt;&lt;p&gt;HR measurements showed a constant decrease with increasing cloudiness allowing to quantify the bias (in %) of the aerosol HR introduced by the simplified assumption of clear-sky conditions in radiative transfer model calculations. Results showed that the HR of light absorbing aerosol was ~20-30% lower in low cloudiness (oktas=1-2) up to 80% lower in complete overcast conditions (i.e., oktas=7-8), compared to clear sky ones. The impact of different cloud types on the HR was also investigated. Cirrus were found to have a modest impact, decreasing the HR&lt;sub&gt;BC&lt;/sub&gt; and HR&lt;sub&gt;BrC&lt;/sub&gt; by -5% at most. Cumulus decreased the HR&lt;sub&gt;BC&lt;/sub&gt; and HR&lt;sub&gt;BrC&lt;/sub&gt; by -31&amp;#177;12 and -26&amp;#177;7%, respectively; cirrocumulus-cirrostratus decreased the HR&lt;sub&gt;BC&lt;/sub&gt; and HR&lt;sub&gt;BrC&lt;/sub&gt; by -60&amp;#177;8 and -54&amp;#177;4%, which was comparable to the impact of altocumulus (-60&amp;#177;6 and -46&amp;#177;4%). A higher impact on HR&lt;sub&gt;BC&lt;/sub&gt; and HR&lt;sub&gt;BrC&lt;/sub&gt; suppression was found for stratocumulus (-63&amp;#177;6 and -58&amp;#177;4%, respectively) and altostratus (-78&amp;#177;5 and -73&amp;#177;4%, respectively). The highest impact was associated to stratus, suppressing the HR&lt;sub&gt;BC&lt;/sub&gt; and HR&lt;sub&gt;BrC&lt;/sub&gt; by -85&amp;#177;5 and -83&amp;#177;3%, respectively. The presence of clouds caused a decrease of both HR&lt;sub&gt;BC&lt;/sub&gt; and HR&lt;sub&gt;BrC&lt;/sub&gt; (normalized to the absorption coefficient of the respective species) of -11.8&amp;#177;1.2% and -12.6&amp;#177;1.4% per okta&amp;#160; (Ferrero et al., 2021b) allowing to parametrize the BC and BrC radiative impact in non clear sky conditions around the world.&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;p&gt;Ferrero L., et al., 2018. Environ. Sci. Tech., 52, 3546&amp;#8722;3555, DOI: 10.1021/acs.est.7b04320, 2018.&lt;/p&gt;&lt;p&gt;Ferrero, L., et al., 2021a. Science of the Total Environment 791. doi:10.1016/j.scitotenv.2021.148277.&lt;/p&gt;&lt;p&gt;Ferrero, L., et al. 2021b. Atmospheric Chemistry and Physics 21, 4869&amp;#8211;4897. doi:10.5194/acp-21-4869-2021.&lt;/p&gt;