Heating Rate of Light Absorbing Aerosols: Time-Resolved Measurements, the Role of Clouds, and Source Identification

Light absorbing aerosols (LAA) absorb sunlight and heat the atmosphere. This work presents a novel methodology to experimentally quantify the heating rate (HR) induced by LAA into an atmospheric layer. Multiwavelength aerosol absorption measurements were coupled with spectral measurements of the direct, diffuse and surface reflected radiation to obtain highly time-resolved measurements of HR apportioned in the context of LAA species (black carbon, BC; brown carbon, BrC; dust), sources (fossil fuel, FF; biomass burning, BB), and as a function of cloudiness. One year of continuous and time-resolved measurements (5 min) of HR were performed in the Po Valley. We experimentally determined (1) the seasonal behavior of HR (winter 1.83 ± 0.02 K day-1; summer 1.04 ± 0.01 K day-1); (2) the daily cycle of HR (asymmetric, with higher values in the morning than in the afternoon); (3) the HR in different sky conditions (from 1.75 ± 0.03 K day-1 in clear sky to 0.43 ± 0.01 K day-1 in complete overcast); (4) the apportionment to different sources: HRFF (0.74 ± 0.01 K day-1) and HRBB (0.46 ± 0.01 K day-1); and (4) the HR of BrC (HRBrC: 0.15 ± 0.01 K day-1, 12.5 ± 0.6% of the total) and that of BC (HRBC: 1.05 ± 0.02 K day-1; 87.5 ± 0.6% of the total).