Inter annual and Seasonal Variability in ARF at a Location in the Sub Himalayan Region of North-East India
Pathak, Binita; Bhuyan, Pradip Kumar; Biswas, Jhuma
Dibrugarh University, INDIA

Regional characterization of atmospheric aerosols is essential from the viewpoint of reducing the current uncertainties in the understanding of their climate implications at regional and global scale. The north-eastern part of India owing to its unique topography and geography located at sub Himalayan range and the middle of Indian Subcontinent and South-East Asian region as well as with scattered local hilly regions persevere complex aerosol environment. Collocated measurements of parameters corresponding to aerosol optical and physical properties i.e., spectral aerosol optical depths (AODs) by a 10 channel Multi-Wavelength solar Radiometer (MWR), near surface aerosol mass concentration of composite aerosols by a Quartz Crystal Microbalance Impactor (QCM) and Black Carbon (BC) mass concentration by an Aethalometer have been used in the Optical Properties of Aerosols and Clouds (OPAC) model (Hess et al., 1998) to estimate the optical properties of composite aerosols over Dibrugarh (27.3°N, 94.6°E, 111 m amsl) for the short wavelength range. The OPAC outputs are then used as inputs to the Rdiative Transfer model 'Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART)', developed by the University of California, Santa Barbara (Ricchiazzi et. al., 1998), to derive the shortwave aerosol radiative properties. The aerosol radiative forcing (ARF) estimated for the period pre-monsoon 2008- pre-monsoon 2012 shows maximum value in the pre-monsoon season at the surface as well as in the atmosphere corresponding to highest columnar aerosol loading (Figure 1). The surface forcing varies between ~ -37 Wm-2 in Pre-monsoon 2009 and 2011 to ~ -13Wm-2 in Post-monsoon 2008 while forcing in the Atmosphere varies between ~36 Wm-2 in Pre-monsoon 2009 and 13 Wm-2 in Post-monsoon 2008. The atmospheric forcing shows distinct annual variation influenced by the total amount of precipitation. The annual average forcing in the atmosphere varied from a high of ~24 Wm-2 in 2009 (drought year) to the lowest value ~16 Wm-2 in 2010 (heavy rainfall year). The TOA forcing is higher either in pre-monsoon or in winter. Figure 1: Seasonal variation of aerosol radiative forcing with and without Black Carbon (BC) conditions at Top of the atmosphere (TOA), in the atmosphere (ATM) and at the surface (SUR) for the four seasons (pre-monsoon (PM1), monsoon, post-monsoon and Winter ( W) for the period May 2008 - March 2012 Presence of BC determines the variability in ARF at TOA and at the surface, because without BC both TOA and Surface ARFs are almost equal while the magnitude of TOA ARF decreases when BC is considered in the ARF calculations. The TOA is negative in all the seasons without BC but is positive occasionally with BC. The climatic impact of the aerosols is determined from the heating of the atmosphere due to presence of aerosols. Over Dibrugarh this can be mainly attributed to the BC. Over the continental location Dibrugarh BC alone contributes ~60-80% of forcing in the Atmosphere, while 30-50 % at the surface in the shortwave region. Seasonally maximum heating rate has been observed during winter or pre-monsoon seasons corresponding to maximum aerosol loading near the surface and in the column. The magnitude of Forcing estimated over the location is less than those observed over the polluted regions of India. Figure 2: Seasonal variation of atmospheric heating rate with, without and only Black Carbon (BC) conditions at Top of the atmosphere (TOA), in the atmosphere (ATM) and at the surface (SUR) for the four seasons (pre-monsoon (PM1), monsoon, post-monsoon and Winter ( W) for the period May 2008 - March 2012.