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

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).

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±0.05 K day^-1). The HR_BrC accounted for 13.7±0.2% of the total HR (BrC absorption Angstrom exponent: 3.49±0.01).

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±0.04 oktas (highest in February: 4.56±0.07 oktas, lowest in November: 2.91±0.06 oktas). St were mostly responsible of overcast conditions (oktas=7-8, frequency: 87 and 96%).

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±12 and -26±7%, respectively; cirrocumulus-cirrostratus decreased the HR_BC and HR_BrC by -60±8 and -54±4%, which was comparable to the impact of altocumulus (-60±6 and -46±4%). A higher impact on HR_BC and HR_BrC suppression was found for stratocumulus (-63±6 and -58±4%, respectively) and altostratus (-78±5 and -73±4%, respectively). The highest impact was associated to stratus, suppressing the HR_BC and HR_BrC by -85±5 and -83±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±1.2% and -12.6±1.4% per okta  (Ferrero et al., 2021b) allowing to parametrize the BC and BrC radiative impact in non clear sky conditions around the world.

References:

Ferrero L., et al., 2018. Environ. Sci. Tech., 52, 3546−3555, DOI: 10.1021/acs.est.7b04320, 2018.

Ferrero, L., et al., 2021a. Science of the Total Environment 791. doi:10.1016/j.scitotenv.2021.148277.

Ferrero, L., et al. 2021b. Atmospheric Chemistry and Physics 21, 4869–4897. doi:10.5194/acp-21-4869-2021.