Seasonal heterogeneity in aerosol types over Dibrugarh-North-Eastern India Binita Pathak a , Pradip Kumar Bhuyan a, * , Mukunda Gogoi b , Kalyan Bhuyan a a Centre for Atmospheric Studies, Dibrugarh University, Dibrugarh 786 004, India b Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram 695022, India article info Article history: Received 11 July 2011 Received in revised form 20 October 2011 Accepted 29 October 2011 Keywords: Aerosol type Multi-wavelength solar Radiometer AOD Ångström exponent HYSPLIT abstract Columnar aerosol properties retrieved from Multi-Wavelength solar Radiometer (MWR) measurements during the period 2001e2010 over Dibrugarh (27.3  N, 94.6  E, 111 m amsl), North-Eastern India are analyzed to identify the types of aerosols in the atmospheric column. Highest Aerosol optical depth (AOD) characterizes the pre-monsoon (MarcheMay), while lowest AOD has been observed during the post-monsoon (OcteNov) season. The Ångström exponent (a) indicates predominance of fine aerosols during post-monsoon and winter (DeceFeb) and dominance of coarse mode in pre-monsoon and monsoon (JuneeSept). NOAA HYSPLIT back trajectory analysis suggests that the seasonal heterogeneity in aerosol characteristics can be attributed to the varying contribution from different source regions. Using the relationship between AOD 500 and a, the aerosols can be classified into five main types viz. continental average (CA), marine continental average (MCA), urban/industrial and biomass burning (UB) and desert dust (DD) while the remaining cases are considered as unidentified or mixed type (MT). These aerosol types exhibit seasonal heterogeneity in their contribution depending upon variability in sources. In winter, local production contributes to observed appreciable CA aerosol type, while highest percentage of UB type is attributed to both local and transported aerosols. On the other hand, transported UB and DD types play a significant role in the pre-monsoon season. Post-monsoon season is indicative of back- ground continental average aerosol condition with a significant contribution from CA and MCA aerosols. Monsoon aerosols couldn’t be distinguished properly due to different particle growth processes like humidification, hygroscopic growth etc. and hence MT aerosol type is predominant in this season. This is the first ever attempt to classify aerosols over this environment. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The atmospheric aerosols along with green house gases are one of the principal internal agents of climate change (Kaufman et al., 2002). However, the regional and global impact of atmospheric aerosols on climate is still uncertain owing to the large heterogeneity in their spatial and temporal distribution throughout the globe. This heterogeneity results from the variability of sources or origin of different aerosols as well as their short residence time in the atmosphere (Textor et al., 2006; Kinne et al., 2006). Consequently aerosols present the largest source of uncertainties in the model simulations of climate change (Ferrare et al., 2005). This is because describing the aerosol characteristics as input to these models is vital for determining the uncertainties in simulations of aerosol radiative forcing and hence climate change. The heterogeneity in spatial and temporal distribution also leads to presence of different aerosol types in both spatial and temporal domain. Based on various measurements over the globe, aerosols can generally be classified into four main types namely biomass-burning aerosols produced by forest and grassland fires, urban/industrial aerosols from fossil fuel combustion in populated urban/industrial regions, desert dust blown into the atmosphere by wind and aerosol of maritime origin (Kaskaoutis et al., 2007a). Each of these types can be further divided into others depending on aerosol absorbing or scattering capabil- ities, sphericity, chemical composition, mineralogy, etc. Due to strong dependence of both AOD and Ångström exponent on wave- length, a realistic characterization of aerosol properties can be attempted using these two parameters (Holben et al., 2001) and therefore, are widely used to identify different aerosol types over the globe (Kaskaoutis et al., 2007b and references therein). The geographical diversity and meteorological pattern driven primarily by regional monsoon over the Indian subcontinent implicitly ensures a very diverse and complex aerosol environment, which needs to be investigated for the aerosol types present in each zone and their impact on both regional and global climate. * Corresponding author. E-mail addresses: pathak.binita8@gmail.com (B. Pathak), pkbhuyan@gmail.com (P.K. Bhuyan), dr_mukunda@vssc.gov.in (M. Gogoi), kalyanbhuyan@gmail.com (K. Bhuyan). Contents lists available at SciVerse ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.10.061 Atmospheric Environment 47 (2012) 307e315