Seasonal heterogeneity in aerosol types over Dibrugarh-North-Eastern India

Columnar aerosol properties retrieved from Multi-Wavelength solar Radiometer (MWR) measurements during the period 2001–2010 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 (March–May), while lowest AOD has been observed during the post-monsoon (Oct–Nov) season. The Ångström exponent (α) indicates predominance of fine aerosols during post-monsoon and winter (Dec–Feb) and dominance of coarse mode in pre-monsoon and monsoon (June–Sept). 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 AOD₅₀₀ and α, 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 background 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.