Multi-year investigations of near surface and columnar aerosols over Dibrugarh, Northeastern location of India: heterogeneity in source impacts

Abstract

The total and size segregated near surface aerosol mass concentrations obtained using a Quartz Crystal Microbalance (QCM) impactor over Dibrugarh (27.3°N, 94.6°E, 111 m amsl), in the extreme northeastern part of India, during the period 2007 to 2009 are analyzed with concurrent measurements of columnar spectral aerosol optical depths (AOD) to infer on the seasonal changes in the contribution of near surface aerosols to columnar aerosol properties. Mass concentrations within the well-mixed atmospheric boundary layer (ABL) showed systematic seasonal dependence, with annual high (∼75.6 ± 17.7 μg m⁻³) in February and low (∼24.8 ± 6.5 μg m⁻³) in June; with both the total (Mt) and accumulation mode (Ma) aerosol mass concentrations depicted similar variations. The accumulation mode aerosols contributed more than 50% to the total aerosol mass concentration throughout the year; accumulation mass fraction (A_f = M_a/M_t) being highest during Dec–Feb (mean value of Af ∼ 0.87 ± 0.03) and lowest (Af ∼ 0.54 ± 0.01) in July. Examination of the AOD properties, estimated using Multiwavelength solar radiometer (MWR) observations, showed moderate to high (∼33–72%) contributions of near surface aerosols to columnar extinction during retreating-monsoon (Oct–Nov) and winter (Dec–Feb) seasons, while it was lowest (<26%) during pre-monsoon (Mar–May) season, implying large contribution by aerosols above the ABL and in the free troposphere. Trajectory clustering and concentration weighted trajectory (CWT) analysis indicated that the potential sources over the west Asian locations and that over the Indo-Gangetic Plains (IGP) significantly contribute to the total aerosol abundance over Dibrugarh during winter and pre-monsoon seasons.