4. Absorption enhancement by black carbon (BC) cored polydisperse aerosols under hygroscopic conditions
Of the different aerosol species present in atmosphere, Black Carbon (BC) has great importance due to its highly absorbing nature. The estimation of radiative impact of BC strongly depends on the accurate measurement of its absorption coefficient, mass concentration and its mixing state. Mixing of BC with other inorganic species induces change in optical properties. Internal mixing of BC (homogeneous or a core-shell structure) shows more realistic absorption estimates as compared to external mixing models in which BC particles co-exist with other particles in a physically separated manner (Bondet al., 2006). Absorption by BC increases when BC particles are mixed and/or coated with other less absorbing materials, which are hygroscopic in nature. This enhanced absorption in a core-shell structure is because of the focusing effect of coated materials (shell) which act as a lens (Fuller 1995; Fuller et al., 1999).
In a recent study, the hygroscopic growth of aerosols during winter season over an urban site (Kanpur) in the Indo-Gangetic Plane (IGP) was estimated and the enhancement in BC absorption coefficient observed for the same period was explained based on this. From the calculated hygroscopic growth factor (g_exp) , a model has been derived to predict the chemical composition of particles. Absorption and scattering coefficients are derived using a core-shell assumption based on Mie theory. These derived optical parameters are compared with experimental values and the closure is found to be very close. The estimated optical properties agree within 5% for absorption coefficient and 30% for scattering coefficient with that of measured values. The enhancement of absorption is found to vary according to the thickness of the shell and BC mass, with a maximum of 2.3 for a shell thickness of 18 nm for the particles (Shamjad et al., 2012).
To quantify the hygroscopic growth of particles, a laboratory experiments were setup using two Scanning Mobility Particle Sizers (SMPSs) operating in parallel for 5 days in winter season of February 2011. One SMPS (TSI-model 3696) measures the ambient size distribution while other SMPS (Grimm-model no: 5.403) has a dryer attached to its inlet to remove the water content from the atmospheric aerosols thereby, measuring dry size distribution. The ratio of mode diameters of the ambient distribution to that of dry distribution is reported as gexp. Using this hygroscopic growth factor (i.e. Zdanovskii-Stokes-Robinson (ZSR) approach), a model has been developed to predict the chemical composition of particles. An internal mixing of BC with a core-shell structure is assumed to be present during the winter season over Kanpur and the refractive index of core and shell is calculated using volume mixing rule. The core is assumed as a mixture of all species (BC, WISOC, WSOC, (NH4)2 SO4 and NH4 NO3) in dry state and shell as a mixture of soluble species (WSOC, (NH4 )2 SO4 and NH4 NO3) with water content. Observed parameters such as size distribution, radii of core and shell, and their refractive indices are used to calculate the optical parameters of aerosol using Mie theory. These optical parameters showed good agreement with measured values. Enhancement in absorption (γ) due to hygroscopic growth is also calculated from Mie Theory. Figure 1 shows the variation in γ as a function of BC mass fraction and shell thickness. This figure shows a clear trend of increase in γ values as shell thickness increases for a constant BC mass fraction. While a maximum γ of 2.3 is observed for the shell thickness of 18 nm, the lowest γ of 1.035 corresponds to very thin coating (2 nm) (Shamjad et al., 2012)
Figure 1: Absorption enhancement as a function of BC mass fraction and shell thickness. Each color shows range of absorption enhancement(Figure 4, Shamjad et al., 2012).
The study showed that a coating of soluble material over black carbon can significantly increase the absorption depending upon the thickness of the coating and type of coating material. High RH conditions and the presence of hygroscopic materials are very much favorable for forming such coatings. Knowledge of easily measurable hygroscopic growth factors can be effectively used to identify the volume fractions of different species present in the ambient aerosol. Using this information in conjunction with the size distribution data, an effective optical closure can be performed to match the experimental optical parameters.
References
- Bond, T. C.; Habib, G.; Bergstrom, R. W., Limitations in the enhancement of visible light absorption due to mixing state. J. Geophys. Res., 2006,111, (D20).
- Fuller, K. A., Scattering and absorption cross sections of compounded spheres. III. Spheres containing arbitrarily located spherical inhomogeneities. , J. Opt. Soc. Am. A 12, 893-904 (1995) 1995.
- Fuller, K. A.; Malm, W. C.; Kreidenweis, S. M., Effects of mixing on extinction by carbonaceous particles. Journal of Geophysical Research-Atmospheres 1999,104, (D13), 15941-15954.
- P. M, Shamjad.; Tripathi, S. N.; Aggarwal, S. K.; Mishra, S. K.; Joshi, M.; Khan, A.; Sapra, B. K.; Ram, K., Comparison of experimental and modeled absorption enhancement by Black Carbon (BC) cored polydisperse aerosols under hygroscopic conditions. Environmental Science & Technology 2012.
Mr. P. M. Shamjad
IIT Kanpur;
E-Mail: shamjad@iitk.ac.in
