Climatology of columnar aerosol properties and the influence of synoptic conditions: First‐time results from the northeastern region of India

Abstract

[1] Six years of spectral aerosol optical depths (AODs), from the northeastern part of India (Dibrugarh), are used to evolve a climatology for this region. The results indicate that the seasonal mean AODs at 500 nm go as high as 0.45 ± 0.05 during premonsoon season (March to May), decrease gradually through the monsoon (June to September) to reach the lowest value of 0.19 ± 0.06 during the retreating-monsoon season (October and November), and increase to 0.31 ± 0.04 in winter (December to February). The AOD spectra are generally flatter than those seen typically over continental sites of India (and elsewhere in the neighboring regions) with Å ngström exponent a remaining below 1.0 during February through August, indicating a relatively low abundance of fine and accumulation mode aerosols. The columnar size distributions (CSD) retrieved from spectral AODs are, in general, bimodal with primary mode at $ 0.1 mm and secondary mode at $ 1.0 mm. High mass loading ($309.5 ± 65.9 mg m À2 ) and effective radius ($0.40 ± 0.09 mm) occur during premonsoon and are attributed to significant abundance of coarse (natural) aerosols. Cluster analysis of air mass back trajectories indicate significant transport of mineral dust from the arid regions of west Asia and northwest India across the Indo-Gangetic plains and marine aerosols advected from the Bay of Bengal contributing largely to the coarse mode aerosols during this season. On the other hand, the peculiar topography combined with the local conditions and the widespread rainfall lead to a more pristine environment during retreating-monsoon season with quite low AODs and columnar loading.

Contrasting aerosol characteristics and radiative forcing over Hyderabad, India due to seasonal mesoscale and synoptic‐scale processes

“…The results reveal a rather significant monthly variation with high values during the period March-June and decreasing towards the post-monsoon (October-November) and winter (December-February) seasons. The annual mean AOD 500 over the two-year period is found to be 0.52 ± 0.23, slightly lower than that reported by Kaskaoutis et al (2009) for the urban region of Hyderabad (0.57 ± 0.22) but significantly higher than those at other Indian cities not so industrialized (Suresh and Desa, 2005;Moorthy et al, 2007;Pantithurai et al, 2007;Gogoi et al, 2009). An interesting feature is the different monthly variation in the spectral AODs suggesting difference in the aerosol load and particle size.…”

“…More details about instrumentation, method of analysis and uncertainties are discussed elsewhere (e.g. Moorthy et al, 1997;Dumka et al, 2008;Gogoi et al, 2008Gogoi et al, , 2009) and the AODs retrieved using the MWR are compared with estimates made using several commercial sun-photometers with very good agreement (Kompalli et al, 2010).…”

“…The presence of clouds, mainly in the monsoon, limits the radiation measurements; however, the present study focuses on the monthly averages and seasonal patterns of aerosol properties, and thus a sufficient number of spectral AODs are available for each month, except for the month of July that was omitted from the analysis. The 935 nm channel was used for estimating the columnar water vapour (CWV), along with the window channel of 1025 nm (Nair and Moorthy, 1998); these records were also used to correct for water vapour absorption at 850 nm, which is ∼0.005 (Gogoi et al, 2009). The typical error in the retrieved AOD is ∼0.01 excluding the variance of the Langley fit.…”

“…(4), spherical aerosols are assumed; thus, the presence of non-spherical dust aerosols would modify the results leading to underestimation of the mean and effective radius. More details about the application of the inversion algorithm to MWR data, errors and uncertainties are presented elsewhere (Moorthy et al, 1991;Saha and Moorthy, 2004;Dumka et al, 2009;Gogoi et al, 2009). For describing the radiative transfer properties of aerosols, the area-weighted mean or effective radius (R eff ) constitutes a key parameter.…”

“…The wavelengthdependent complex refractive index by Lubin et al (2002) was used for the present study. The seasonal variation in refractive index is not considered in the inversion since it is based on the measurement of spectral solar radiation rather than the scattered sky radiances, and under such conditions the retrieved columnar size distribution shows a weak dependency on changes in the aerosol refractive index (Gogoi et al, 2009). The spectral AODs are then re-estimated using the direct Mie equation and are compared with the measured ones.…”