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Heating rate of light absorbing aerosols: time-resolved measurements, the role of clouds and source-identification Luca Ferrero, Grisa Mocnik, Sergio Cogliati, Asta Gregoric, Roberto Colombo, and Ezio Bolzacchini Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04320 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018
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Environmental Science & Technology
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Heating rate of light absorbing aerosols: time-
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resolved measurements, the role of clouds and
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source-identification
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Luca Ferrero1,2*, Griša Močnik3,4, Sergio Cogliati2,5, Asta Gregorič3,6, Roberto Colombo2,5, and Ezio
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Bolzacchini1,2
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POLARIS Research Centre, Department of Earth and Environmental Sciences, University of Milano-
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Bicocca, 20126, Milano, Italy 2
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GEMMA Centre, Department of Earth and Environmental Sciences, University of Milano-Bicocca,
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20126, Milano, Italy 3
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Aerosol d.o.o., Kamniška 41, SI-1000 Ljubljana, Slovenia
Department of Condensed Matter Physics, Jozef Stefan Institute, Jamova 39
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SI-1000 Ljubljana, Slovenia 5
Remote Sensing of Environmental Dynamics Lab., Department of Earth and Environmental Sciences,
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University of Milano-Bicocca, 20126, Milano, Italy 6
Center for Atmospheric Research, University of Nova Gorica, SI-5000 Nova Gorica, Slovenia
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Luca Ferrero:
[email protected] 18
Luca Ferrero, POLARIS Research Centre, GEMMA Centre, Department of Earth and Environmental
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Sciences, University of Milano-Bicocca, 20126, Milano, Italy.
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Tel. ++39 02/64482814; Fax. ++39 02/64482839
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ABSTRACT
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Light Absorbing Aerosols (LAA) absorb sunlight and heat the atmosphere.
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This work presents a novel methodology to experimentally quantify the heating rate (HR) induced by
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LAA into an atmospheric layer. Multi-wavelength aerosol absorption measurements were coupled with ACS Paragon Plus Environment
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spectral measurements of the direct, diffuse and surface reflected radiation to obtain highly-time
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resolved measurements of HR apportioned in the context of LAA species (Black Carbon, BC; Brown
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Carbon, BrC; dust), sources (fossil fuel, FF; biomass burning, BB), and as a function of cloudiness.
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One year of continuous and time-resolved measurements (5 min) of HR were performed in the Po
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Valley. We experimentally determined: 1) the seasonal behavior of HR (winter: 1.83±0.02 K day-1;
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summer: 1.04±0.01 K day-1); 2) the daily cycle of HR (asymmetric, with higher values in the morning
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than in the afternoon); 3) the HR in different sky conditions (from 1.75±0.03 K day-1 in clear sky to
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0.43±0.01 K day-1 in complete overcast); 4) the apportionment to different sources: HRFF (0.74±0.01 K
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day-1) and HRBB (0.46±0.01 K day-1); 4) the HR of BrC (HRBrC: 0.15±0.01 K day-1, 12.5±0.6% of the
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total) and that of BC (HRBC: 1.05±0.02 K day-1; 87.5±0.6% of the total).
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KEYWORDS: Light Absorbing Aerosol, Radiation, Black Carbon, Brown Carbon, Sources, Heating
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Rate, Field spectroscopy, Po Valley.
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1.
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Light absorbing aerosol (LAA), such as black carbon (BC), brown carbon (BrC) or dust, directly
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interacts with the solar radiation (direct radiative effect; DRE) absorbing it and exerting a positive
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atmospheric forcing. This process releases energy (heat) to the surrounding air with a specific heating
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rate (HR).[1-9]
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Among the various LAA species, BC, generated from incomplete combustion of both fossil fuels (FF,
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i.e. diesel, coal) and biomass (BB)[2,10,11], represents the second most important anthropogenic climate-
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forcing agent after CO2.[2,12] Bond et al.[2] report a mean DRE at the top of atmosphere (TOA) in the
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range of 0.2-1.0 W m−2 (average: +0.71 W m-2) even if regionally should be larger (up ~10 W m-2 over
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Asia; 0.0-4.0 W m-2 over Europe).[7,13] The LAA do not only influence the DRE at TOA, but also its
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vertical distribution. In this respect, BC heats the atmosphere (average +2.6 W m-2, 10-20 W m-2 within
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regional plumes and instantaneously up to +50-80 W m-2) and dims the surface (mean value -1.7 W m-2;
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instantaneously down to -40 – -70 W m-2).[6,7,12,14] Thus, BC HR varies as a function of height (range:
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0.3 - 8 K day-1)[6,11,15,16]; over Asia, the LAA HR was found larger than that of CO2.[16]
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Considering the other LAA species, the BrC (the absorbing component of organic matter) is receiving a
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growing attention as it contributes ~10-30%[17,18] to the total LAA absorption on a global scale. It causes
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an atmospheric DRE of 2.6 W m-2 over the Indo-Gangetic Plain that can rise up to 3.6 W m-2 during
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high BB days.[18] Finally, also dust aerosol can exert a positive atmospheric DRE; instantaneously it was
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estimated up to ~80-100 W m-2,[3,19] and HRs of ~2-3 K day-1 were found in elevated dust layers over
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Korea.[20]
INTRODUCTION
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Unlike CO2, which is distributed quite homogeneously in the atmosphere, the LAA species are
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characterized by a significant spatial variation, due to their relatively short lifetime (i.e., weeks).[21,22]
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Thus, on a regional scale, the radiative effects of LAA can be larger than that of greenhouse gases[14,16]
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triggering different local/regional feedbacks: alterations of the atmospheric thermal structure, of the
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atmospheric stability, of the cloud distribution (cloud “burn-off”, and cloud cover) and of synoptic
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winds such as the monsoon.[2,3,6,10,12,14,15,23-27] Furthermore, the retreat of important glaciers (i.e.
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Himalayan) can be also influenced by the LAA induced HR at elevated altitudes.[14,28] Due to the
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importance of the aforementioned feedbacks on climate, an experimental determination of the HR
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induced by LAA is called for.
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Actually, most of the forcing values and atmospheric feedbacks reported in literature rely on the use of
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several theoretical models.[2,10] Model outputs are continuous in space (horizontally and vertically) and
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time, allowing the description of the impact of LAA on climate at the global scale. However,
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uncertainties in the modelling chain (i.e. LAA mass concentrations, optical properties and mixing
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state)[13,24] result in large discrepancies among model outputs. This amplifies the uncertainty of the LAA
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influence on the global and regional climate.[1] As a conclusion, Andreae and Ramanathan[1] showed the
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need of better observational constraints at all scales (local/regional/global), while Chung et al.[17] called
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for model independent and observationally based estimates of the LAA direct radiative forcing. One
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step further in this direction is given by hybrid approaches, based on experimental measurements of
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LAA properties (mass/number concentrations) coupled with optical/radiative transfer calculations.[6,15,29-
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31]
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be affected by the same uncertainties of the modelling approach. Moreover, radiative transfer
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calculations are usually carried out under clear-sky approximation, which represents a strong limitation
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of the technique (it is useful only if the experimental campaign is conducted during such sky
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conditions).[6,16] Hybrid approaches based on vertical profile data describe both the atmospheric
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behavior and the related feedbacks, but they are usually limited to short/intense campaigns.[6,15,29,32-37]
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On the contrary, hybrid approaches based on ground observations only are usually continuous in time,
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but they are limited in the vertical direction.[30,31] A complete experimental determination of the
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radiative power absorbed into the atmosphere (and the related HR) requires the measurement of the
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vertical divergence of the net radiative flux with altitude.[9,16] With this approach, the obtained HR are
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usually limited to short/intense campaigns and to specific locations; moreover they are comprehensive
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of the whole atmospheric composition (greenhouse gases and LAA) thus again theoretical models need
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be used (coupled with concentrations measured in situ) to unravel the contribution of each single
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atmospheric component.[11,16]
They have the advantage that are based on experimental measurements but, at the same time they can
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Thus, a method to provide continuous, LAA specific, experimental determination of atmospheric
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forcing and HR is required. Theoretically, the method should be simple and accessible enough to be
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used in reference stations belonging to widely distributed monitoring networks (i.e. the European
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Monitoring and Evaluation Programme, EMEP network) and should provide an estimation of the
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contribution of each aerosol source to the atmospheric HR.
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This paper fills this gap, describing a new methodology to experimentally determine both the radiative
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power density absorbed into a near-surface atmospheric layer, and the consequent HR induced by LAA.
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The main goal of this method is to provide continuous experimental observations of LAA HR by means
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of a simple and cost-effective technique (see supporting information) applicable in established aerosol
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monitoring networks.
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Other very important goals of the new method are: 1) the apportionment of the HR as a function of the
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LAA species (i.e. BC, BrC, dust) and the LAA sources (i.e. FF, BB); 2) the experimental determination
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of HR in any sky condition; 3) the apportionment of different radiation components (i.e., downward
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direct/diffuse fluxes and the reflected) on the HR, giving the possibility to determine the influence of
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clouds on the HR.
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2.
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This section describes both the theoretical considerations (section 2.1) and the experimental design
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(section 2.2) of the novel method for the experimental determination of the LAA induced HR.
METHODOLOGY
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2.1
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The instantaneous aerosol direct radiative effect (DRE; W m-2) at a z altitude can be quantified as the
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change in the net radiative flux between the atmospheric conditions with and without the aerosols (Qaer,z
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and Qw/o-aer,z, respectively) across the surface at z altitude:[6,9]
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= , − / ,
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Considering an atmospheric layer of thickness ∆z, the difference between the DRE at the top and the
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bottom of this atmospheric layer represents the instantaneous radiative power density absorbed by the
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aerosol (∆DRE; W m-2):[6-9]
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= −
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From ∆DRE, the instantaneous heating rate (HR; K day-1) of the same atmospheric layer can be
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computed as follows:[6-9]
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=
THEORETICAL CONSIDERATIONS
(1)
(2)
= −
(3)
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where ∂T/∂t represents the instantaneous HR, g is the gravitational acceleration constant, Cp (1005 J kg-1
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K-1) is the isobaric specific heat of dry air, ∆P is the pressure difference between the top and the bottom
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of the considered layer.
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A more useful definition of the HR is based on the thickness of the atmospheric layer (∆z), and can be
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obtained introducing the hydrostatic equation (dp = -ρgdz) in eq. 3:[9]
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=
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where ρ represents the air density (kg m-3). The last term of eq. 4 (∆DRE/∆z) represents the radiative
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power absorbed by the aerosol for unit volume of the atmosphere (W m-3) and was previously defined in
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Ferrero et al.6 as the absorptive direct radiative effect (ADRE) of LAA; the ADRE is the vertical
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derivative of the DRE (dDRE/dz; the implications of the derivative are detailed and discussed in section
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4). Hence, the HR becomes:[6]
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=
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Thus, any method able to determine the ADRE at high time resolution will produce continuous time
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series of HR. This possibility is given by the simple observation that ADRE is expressed per unit volume
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of air, coherent with the concentrations of LAA routinely measured in monitoring networks. Below the
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rigorous methodology used for the experimental determination of the ADRE is reported.
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Let us consider a near-surface atmospheric layer (but can be applied to any atmospheric layer; see
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section 4 for further details) of thickness ∆z on which a n-th kind (direct or diffuse or reflected) of
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monochromatic radiation ray Fn(λ,θ) of wavelength λ strikes with a zenith angle θ (Figure S1a, supporting
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information). The amount of radiation absorbed by the aerosol within the present layer is as follows:[9,38]
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#(%,&) = )#(%,&) (1 − +% ),1 − - .//0 1
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where (1- ωλ)(1-e-τλ/µ) represents the fraction of light absorbed within the layer[9,38] and is function of: ωλ
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which is the single scattering albedo of the aerosol within the atmospheric layer, τλ the aerosol optical
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depth and µ, the cosine of θ (cosθ). The ωλ and τλ terms can be computed from the aerosol extinction,
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scattering and absorption coefficients (bext(λ), bsca(λ) and babs(λ)):
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+% =
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7% = 8 9:(%) ;
