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Absorbing Refractive Index and Direct Radiative Forcing of Atmospheric Brown Carbon Over Gangetic Plain Puthukkadan Moosakutty Shamjad, Rangu Venkata Satish, Navaneeth Meena Thamban, Neeraj Rastogi, and Sachchida Nand N. Tripathi ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00074 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017
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ACS Earth and Space Chemistry
Absorbing Refractive Index and Direct Radiative Forcing of Atmospheric Brown Carbon Over Gangetic Plain
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Department of Civil Engineering, Indian Institute of Technology-Kanpur, Kanpur, India
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Geosciences Division, Physical Research Laboratory, Ahmedabad, India
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Centre for Environmental Science and Engineering, Indian Institute of Technology-Kanpur,
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P. M. Shamjad1, R.V. Satish2, Navaneeth M. Thamban1, N. Rastogi2, S.N. Tripathi1,3,*
Kanpur, India Corresponding author: Phone: +91-512 2597845; email:
[email protected] (S.N.T) Keywords: Carbonaceous aerosols, mixing state, absorbing refractive index, radiative forcing, biomass burning Abstract Atmospheric carbonaceous aerosols consisting of black carbon and organic carbon influence
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Earth’s radiative balance by interacting with solar radiation. A subset of organic aerosols known
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as brown carbon is absorbing in nature and poorly characterized in terms of optical properties.
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Brown carbon can warm the local and regional atmosphere depending on its absorbing capacity,
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mixing state and meteorological conditions. We report diurnal spectral absorbing refractive index
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of brown carbon over North India and its influence on regional radiative forcing. Measurements
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show the presence of highly absorbing brown carbon consisting of soluble and non-soluble
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fractions having distinct spectral absorption. Brown carbon refractive index at 365 nm shows a
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50% reduction during daytime when compared to nighttime due to combined effects of reduced
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primary emissions and photo-bleaching/volatilization. Brown carbon and the lensing effect due
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to a thin absorbing coating exert a forcing of -0.93 ± 0.27 and 0.13 ± 0.06 W m-2, respectively, at
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top of atmosphere. Considering externally mixed absorbing organic carbon in radiative forcing
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calculations produces 48% less cooling when compared to the forcing induced by scattering
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organic carbon. The presence of internally mixed absorbing organic carbon as a shell over black
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carbon induces 31% more warming compared to a similar shell made of scattering organic
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carbon. Overall results suggest that brown carbon and the lensing effect need to be included in
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global climate models while calculating radiative forcing parameters.
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Introduction
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climate studies due to their capacity to warm the atmosphere.1 Unlike black carbon (BC),
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absorption due to brown carbon (BrC) is not well-characterized due to the large heterogeneity in
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its optical properties arising from its diverse sources and involvement in secondary processes
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such as photochemical ageing.2 Primary emissions, for example, fossil fuel combustion and
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biomass burning, contribute mainly to the atmospheric abundance of carbonaceous aerosols.
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Global emission inventory studies report annual emissions of 8 and 33.9 Terra gram (Tg) for BC
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and organic carbon (OC), respectively, in which open biomass burning contributes up to 42% for
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BC and 74% for OC.3 In regions such as Central Africa, China, and Northern India, open
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biomass burning is a dominant source for OC.4 The perturbation to the Earth-atmosphere
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radiative balance due to aerosols is generally expressed in terms of radiative forcing (W m-2),
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which is a measure of the change in net radiative flux at the top of atmosphere (TOA).5 The
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radiative forcing due to BrC aerosols is subject to large uncertainty due to a lack of accurate
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knowledge of (a) the optical and physical properties of BrC and (b) the mixing state of BC and
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BrC.6 Since BC and OC are usually co-emitted, their mixing state can significantly influence the
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overall absorption thus further influencing forcing estimates. The term ‘mixing state’ is used to
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indicate the physical state of aerosols in the atmosphere and is divided into external and internal
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mixing. In external mixing, aerosols co-exist as individual particles. On the other hand, in
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internal mixing, aerosols are homogeneously mixed with each other in various configurations
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(core-shell, spheroids etc.).7 In core-shell internal mixing, an insoluble aerosol species (BC) gets
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coated with a soluble species (organics and/or inorganics). Core-shell type mixing leads to
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enhancement in absorption due to the shell acting as a lens (lensing effect).8, 9
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Absorption due to BrC can reduce the net cooling effect caused by scattering aerosols or even
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induce warming in areas with large-scale biomass burning.4 A number of global modeling
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studies6,
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compared to purely scattering OC. Two other studies12, 13 report higher warming values (0.24 to
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0.4 W m-2) for direct radiative forcing by absorbing OC. Another study, conducted over the
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central USA using aircraft sampling, reports a 20% reduction in TOA cooling when absorbing
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OC is considered.14 Water-soluble OC (WSOC) measured from New Delhi15 and Gangetic plain
Carbonaceous aerosols having diverse optical and physical properties are of particular interest in
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considering absorbing OC aerosols report 0.11 W m-2 warming at TOA when
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ACS Earth and Space Chemistry
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outflow16, 17 were reported to contribute 6% and 35% respectively to solar absorption relative to
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that of BC. The lensing effect also induces a radiative effect in the atmosphere depending upon
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the type of material coated and its thickness. A study18 shows that externally mixed absorbing
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OC and the lensing effect increase the global mean direct forcing from −0.46 W m-2 to +0.05 W
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m-2. However, global climate model-based OC forcing estimates have large uncertainties due to
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the uncertainty associated with OC refractive index and its assumed mixing state.2, 4 A number of
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studies reported (measured or modeled or both) absorbing refractive index for atmospheric and
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laboratory generated OC.19-23 These studies used various methods such as closure between
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measured and modeled absorption20,
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quantify the refractive index of OC. The large variation (0.000921 to 0.2722 at ~550 nm) in the
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reported refractive index values shows the heterogeneous nature of OC in the atmosphere.
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Accurate measurement of the BrC refractive index on a regional scale is necessary to calculate
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its contribution to total aerosol absorption.
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The overall goal of this study is to quantify the absorbing refractive indices of WSOC and total
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OC and identify their spectral dependence. Using this refractive index and simultaneously
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measured physical properties as input, species-specific radiative forcing is calculated to
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understand the effects of absorbing OC and the lensing effect on the radiative balance. The
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influence of varying shell thickness on net radiative forcing is also calculated.
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, liquid extraction21 and electron microscopy22 to
Experimental section Sampling details Data presented in this study was collected during a high aerosol loading winter season (28
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December 2015 to 31 January 2016) in Kanpur, an urban India city located in the Gangetic plain
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(GP). The GP is known for its high aerosol loading due to open biomass burning especially
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during the winter season.25 The mean ± standard deviation of sub-micron aerosol mass
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concentration during the campaign was found to be 184 ± 108 µg m-3, which indicates high
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loading conditions in terms of air quality standards.26
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Instrumentation
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Figure S1 shows a schematic of the instrument setup used to collect aerosols in this study. An
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Aerodyne high-resolution time of flight aerosol mass spectrometer (HR-ToF-AMS) and TSI
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scanning mobility particle sizer (SMPS) were operated in atmospheric and thermally denuded (~
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300 °C) conditions with a 10-minute switching interval to collect sub-micron chemical
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composition and particle size distributions. Aerosols were pre-dried to Relative Humidity (RH)
