Experimental determination of the effect of clouds on the atmospheric heating rate of black and brown carbon in the Po Valley

<p>The impact of cloud fraction and cloud type on the heating rate (HR) of black and brown carbon (HR_BC and HR_BrC) 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).</p><p>The set-up allowed the experimental determination of the total HR (and its speciation: HR_BC and HR_BrC) in all sky conditions (from clear-sky to cloudy) with the highest total HR values found in the middle of winter (1.43&#177;0.05 K day^-1). The HR_BrC accounted for 13.7&#177;0.2% of the total HR (BrC absorption Angstrom exponent: 3.49&#177;0.01).</p><p>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&#177;0.04 oktas (highest in February: 4.56&#177;0.07 oktas, lowest in November: 2.91&#177;0.06 oktas). St were mostly responsible of overcast conditions (oktas=7-8, frequency: 87 and 96%).</p><p>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_BC and HR_BrC by -5% at most. Cumulus decreased the HR_BC and HR_BrC by -31&#177;12 and -26&#177;7%, respectively; cirrocumulus-cirrostratus decreased the HR_BC and HR_BrC by -60&#177;8 and -54&#177;4%, which was comparable to the impact of altocumulus (-60&#177;6 and -46&#177;4%). A higher impact on HR_BC and HR_BrC suppression was found for stratocumulus (-63&#177;6 and -58&#177;4%, respectively) and altostratus (-78&#177;5 and -73&#177;4%, respectively). The highest impact was associated to stratus, suppressing the HR_BC and HR_BrC by -85&#177;5 and -83&#177;3%, respectively. The presence of clouds caused a decrease of both HR_BC and HR_BrC (normalized to the absorption coefficient of the respective species) of -11.8&#177;1.2% and -12.6&#177;1.4% per okta&#160; (Ferrero et al., 2021b) allowing to parametrize the BC and BrC radiative impact in non clear sky conditions around the world.</p><p>References:</p><p>Ferrero L., et al., 2018. Environ. Sci. Tech., 52, 3546&#8722;3555, DOI: 10.1021/acs.est.7b04320, 2018.</p><p>Ferrero, L., et al., 2021a. Science of the Total Environment 791. doi:10.1016/j.scitotenv.2021.148277.</p><p>Ferrero, L., et al. 2021b. Atmospheric Chemistry and Physics 21, 4869&#8211;4897. doi:10.5194/acp-21-4869-2021.</p>