10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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The Cloud Feedback on the Heating Rate of Black Carbon and Brown Carbon
FERRERO LUCA, Grisa Mocnik, Gregorič Asta, Cogliati Sergio, Colombo Roberto, Rizzi Cristiana, Di Liberto Luca, Barnaba Francesca, Gobbi Gian Paolo, Bolzacchini Ezio, GEMMA Centre, DISAT, University of Milano-Bicocca
Abstract Number: 1343 Working Group: Clouds and Climate
Abstract Black carbon (BC) and Brown Carbon (BrC) absorb sunlight and heat the atmosphere[1]. The heating rate (HR) can be determined from the divergence of the net radiative flux with altitude (vertical profiles) or from the modelling activity; however, its determination is, up to now, too sparse. Moreover, it does not account for light-absorbing-aerosol (LAA) speciation and for the influence of different sky conditions in the atmospheric layer below clouds[2,3]. This work applies a new method[4] to experimentally determine (at high time resolution) the HR induced by LAA in a atmospheric layer. Highly-time resolved measurements of HR were apportioned in the context of LAA species (BC, BrC), sources (fossil fuel, FF; biomass burning, BB)[5], and as a function of cloudiness.
Multi-wavelength aerosol absorption measurements were coupled with spectral measurements of the direct, diffuse and surface reflected radiation using: 1) Aethalometer (AE-31, Magee Scientific, 7-λ), 2) Multiplexer-Radiometer-Irradiometer (diffuse, direct and reflected radiance: 350-1000 nm in 3648 spectral bands), 3) a meteorological station (LSI-Lastem) including a full set of radiometers, 4) Lidar Ceilometer (CHM 15k „NIMBUS“ was used for the determination of cloud base height and its vertical depth; www.alice-net.eu), 5) condensation and optical particle counters (TSI 3775 and Grimm 1.107), 6) low volume sampler (FAI Hydra, PM2.5 and PM10).
To understand the importance of sky condition on direct, diffuse and reflected component of HR (HRdir, HRdif, HRref) the fraction of sky covered by clouds (oktas)[6] and the cloud type were considered. Cloud types were classified based on radiometric, spectroradiometric and lidar measurements[7] into several classes in function of cloud base height: 1) low level (<2 km) stratus, cumulus and stratocumulus; 2) mid level (2-7 km) altostratus, altocumulus; 3) high level (> 7 km) cirrus, cirrocumulus-cirrostratus; 4) clear sky conditions.
More than one year of 5 min data (March 2015 - November 2016) of HR were collected in Milan (Po Valley). The HR showed a clear seasonal behavior (winter: 1.83±0.02 K day-1; summer: 1.04±0.01 K day-1) with a daily cycle characterized by higher values in the morning (than in the afternoon) following the BC-BrC concentrations. On average (one year) the BC accounted for 1.05±0.02 K day-1 while the remaining 0.15±0.01 K day-1 was due to BrC.
Most important, the HR varied 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 (annual averages). Also the kind of radiation involved in the HR varied with sky conditions. For example, in clear sky conditions HRdir was higher than HRdif and HRref accounting on average for the 55±5% of total HR and leading alone to a HRdir of 0.49±0.01 K day-1 in summer and of 2.04±0.03 K day-1 in winter. In cloudy conditions the only efficient component was HRdif (82±1% of total HR; from 0.28±0.01 K day-1 in autumn to 0.49±0.01 K day-1 in winter). Thus, clouds have a very important feedback also on the aerosol direct radiative effect.
Cloud types were investigated also considering their effect on radiation spectrum striking on LAA characterized by different absorption Angstrom Exponent (AAE of BrC and BC). Particularly, both clear sky and cirrus conditions showed an impinging spectrum of diffuse radiation peaking in the blue region exerting a highly efficient interaction with BrC (with an average AAE of 3.66±0.03).
Thus, in clear sky and cirrus conditions the HRBrC accounted for 11.4±0.6% of the total HR while this percentage decreased just to 2.8±0.8% in cloudy cases, as stratus, cumulus, stratocumulus, altostratus, altocumulus. To investigate the role of clouds, the variability of HR due to the radiation was decoupled from that due to LAA concentrations by normalizing it for unit mass of BC. Results showed the strongest impact on HR by low level cumulus clouds, which (according to lidar data) form on the top of the mixing layer in late spring.
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