Environ Sci Pollut Res DOI 10.1007/s11356-015-4495-6
RESEARCH ARTICLE
Identification of aerosol types over Indo-Gangetic Basin: implications to optical properties and associated radiative forcing S. Tiwari 1 & A. K. Srivastava 2 & A. K. Singh 1 & Sachchidanand Singh 3
Received: 13 January 2015 / Accepted: 5 April 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract The aerosols in the Indo-Gangetic Basin (IGB) are a mixture of sulfate, dust, black carbon, and other soluble and insoluble components. It is a challenge not only to identify these various aerosol types, but also to assess the optical and radiative implications of these components. In the present study, appropriate thresholds for fine-mode fraction and single-scattering albedo have been used to first identify the aerosol types over IGB. Four major aerosol types may be identified as polluted dust (PD), polluted continental (PC), black carbon-enriched (BCE), and organic carbon-enriched (OCE). Further, the implications of these different types of aerosols on optical properties and radiative forcing have been studied. The aerosol products derived from CIMEL sun/sky radiometer measurements, deployed under Aerosol Robotic Network program of NASA, USA were used from four different sites Karachi, Lahore, Jaipur, and Kanpur, spread over Pakistan and Northern India. PD is the most dominant aerosol type at Karachi and Jaipur, contributing more than 50 % of all the aerosol types. OCE, on the other hand, contributes only about 12–15 % at all the stations except at Kanpur where its contribution is ∼38 %. The spectral dependence of AOD was relatively low for PD aerosol type, with the lowest AE values (1.0). SSA was found to be the highest for OCE (>0.9) and the lowest for BCE ( 0.44 μm) and less than ±0.02 for shorter wavelengths; however, it is less than ±5 % for the sky radiance measurements (Eck et al. 1999; Dubovik et al. 2000). The errors in the retrieved size distributions are not significant for the particles of radius (R) in the range 0.1–7 μm. The tendency for increasing
Instrumentation and methodology The present study involves CIMEL sun/sky radiometer measurements, deployed over the study locations in IGB region under AERONET program of NASA, USA. The sun/sky radiometer measures direct sun measurements made at eight spectral channels (0.34, 0.38, 0.44, 0.50, 0.67, 0.87, 0.94, and 1.02 μm) with triplet observations per wavelength and sky radiance measurements at four spectral channels (0.44, 0.675, 0.87, and 1.020 μm). Water vapor content in the atmosphere is retrieved from the direct measurements at 0.94 μm channel, and the columnar aerosol optical depth (AOD) data are retrieved at the remaining seven channels (Holben et al. 1998). On the other hand, the sky radiance measurements are
Environ Sci Pollut Res
errors in the retrieval of optical properties with the decrease in optical depth is higher in the case of refractive index and single-scattering albedo than in the case of volume size distribution (Dubovik et al. 2000). However, these errors do not significantly affect the important characteristic features of the size distribution (Dubovik et al. 2000). Identification of aerosol types The aerosol types were identified over the IGB following the approach given by Srivastava et al. (2012b), after Lee et al. (2010). The method is based on sun/sky radiometer-derived aerosol products associated with the size of aerosols such as fine-mode fraction (FMF at 0.50 μm) and radiation absorptivity such as SSA (at 0.44 μm). In the present study, appropriate thresholds for FMF and SSA are used, as suggested by Srivastava et al. (2012b) and Lee et al. (2010), to identify aerosols over IGB as polluted dust (PD, dominant dust with anthropogenic aerosols), polluted continental (PC, dominant anthropogenic aerosols with dust), black carbon-enriched (BCE, highly absorbing aerosols), and organic carbonenriched (OCE, low-absorbing aerosols). Based upon their studies, we have considered FMF0.95 for dominantly coarse mode, and hence, assigned it to PD aerosols, FMF>0.6 with SSA≤0.9 for dominantly fine mode, assigned to BCE along with SSA>0.9 is assigned to OCE aerosols; whereas, PC aerosols are considered for 0.4 ≤ FMF≤0.6 and for any SSA. Figure 2 shows the density plot of FMF (at 0.50 μm) versus SSA (at 0.44 μm) derived from the sun/sky radiometer measurements at the four different stations over IGB for the inferred aerosol types. A wide range of SSA for each aerosol type, except BCE and OCE, is noticed from the figure at all the stations. The mean values of SSA and the corresponding FMF for each aerosol type for different stations are given in Table 1. As expected, SSA was found to be the highest for OCE (>0.9) suggesting scattering in nature; however, it was the lowest for BCE (0.9) and the lowest for BCE (
RESEARCH ARTICLE
Identification of aerosol types over Indo-Gangetic Basin: implications to optical properties and associated radiative forcing S. Tiwari 1 & A. K. Srivastava 2 & A. K. Singh 1 & Sachchidanand Singh 3
Received: 13 January 2015 / Accepted: 5 April 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract The aerosols in the Indo-Gangetic Basin (IGB) are a mixture of sulfate, dust, black carbon, and other soluble and insoluble components. It is a challenge not only to identify these various aerosol types, but also to assess the optical and radiative implications of these components. In the present study, appropriate thresholds for fine-mode fraction and single-scattering albedo have been used to first identify the aerosol types over IGB. Four major aerosol types may be identified as polluted dust (PD), polluted continental (PC), black carbon-enriched (BCE), and organic carbon-enriched (OCE). Further, the implications of these different types of aerosols on optical properties and radiative forcing have been studied. The aerosol products derived from CIMEL sun/sky radiometer measurements, deployed under Aerosol Robotic Network program of NASA, USA were used from four different sites Karachi, Lahore, Jaipur, and Kanpur, spread over Pakistan and Northern India. PD is the most dominant aerosol type at Karachi and Jaipur, contributing more than 50 % of all the aerosol types. OCE, on the other hand, contributes only about 12–15 % at all the stations except at Kanpur where its contribution is ∼38 %. The spectral dependence of AOD was relatively low for PD aerosol type, with the lowest AE values (1.0). SSA was found to be the highest for OCE (>0.9) and the lowest for BCE ( 0.44 μm) and less than ±0.02 for shorter wavelengths; however, it is less than ±5 % for the sky radiance measurements (Eck et al. 1999; Dubovik et al. 2000). The errors in the retrieved size distributions are not significant for the particles of radius (R) in the range 0.1–7 μm. The tendency for increasing
Instrumentation and methodology The present study involves CIMEL sun/sky radiometer measurements, deployed over the study locations in IGB region under AERONET program of NASA, USA. The sun/sky radiometer measures direct sun measurements made at eight spectral channels (0.34, 0.38, 0.44, 0.50, 0.67, 0.87, 0.94, and 1.02 μm) with triplet observations per wavelength and sky radiance measurements at four spectral channels (0.44, 0.675, 0.87, and 1.020 μm). Water vapor content in the atmosphere is retrieved from the direct measurements at 0.94 μm channel, and the columnar aerosol optical depth (AOD) data are retrieved at the remaining seven channels (Holben et al. 1998). On the other hand, the sky radiance measurements are
Environ Sci Pollut Res
errors in the retrieval of optical properties with the decrease in optical depth is higher in the case of refractive index and single-scattering albedo than in the case of volume size distribution (Dubovik et al. 2000). However, these errors do not significantly affect the important characteristic features of the size distribution (Dubovik et al. 2000). Identification of aerosol types The aerosol types were identified over the IGB following the approach given by Srivastava et al. (2012b), after Lee et al. (2010). The method is based on sun/sky radiometer-derived aerosol products associated with the size of aerosols such as fine-mode fraction (FMF at 0.50 μm) and radiation absorptivity such as SSA (at 0.44 μm). In the present study, appropriate thresholds for FMF and SSA are used, as suggested by Srivastava et al. (2012b) and Lee et al. (2010), to identify aerosols over IGB as polluted dust (PD, dominant dust with anthropogenic aerosols), polluted continental (PC, dominant anthropogenic aerosols with dust), black carbon-enriched (BCE, highly absorbing aerosols), and organic carbonenriched (OCE, low-absorbing aerosols). Based upon their studies, we have considered FMF0.95 for dominantly coarse mode, and hence, assigned it to PD aerosols, FMF>0.6 with SSA≤0.9 for dominantly fine mode, assigned to BCE along with SSA>0.9 is assigned to OCE aerosols; whereas, PC aerosols are considered for 0.4 ≤ FMF≤0.6 and for any SSA. Figure 2 shows the density plot of FMF (at 0.50 μm) versus SSA (at 0.44 μm) derived from the sun/sky radiometer measurements at the four different stations over IGB for the inferred aerosol types. A wide range of SSA for each aerosol type, except BCE and OCE, is noticed from the figure at all the stations. The mean values of SSA and the corresponding FMF for each aerosol type for different stations are given in Table 1. As expected, SSA was found to be the highest for OCE (>0.9) suggesting scattering in nature; however, it was the lowest for BCE (0.9) and the lowest for BCE (
