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ABSTRACT Analysis of the long-term time-series data on aerosol parameters at Delhi, an urban city in the western part of Indo-Gangetic Plain (IGP), brought out certain interesting findings. The time-series data are observed to be a complex function of different periodicities varying from intra-seasonal to inter-annual timescales. Detailed investigations of the periodicities in conjunction with the meteorological data demonstrated the prevalence of four periodicities such as intra-seasonal (periodicities of ∼2-3 months, significant at shorter wavelengths), semi-annual oscillation ( SAO, ∼6 months), annual oscillation ( AO, ∼12 months) and quasi-biennial oscillation ( QBO, ∼2 years) and quasi-triennial oscillation ( QTO, ∼3 years) at the measurement site in varying amplitudes. The AO and SAO are the dominant components in the aerosol optical depth ( AOD) as well as in the Angstrom parameters with a combined contribution of ∼40% to the climatological mean and are attributed to the changes in the prevailing meteorological conditions, which are conducive for either production, transport and/or size-transformation processes. Among the inter-annual variability in aerosol parameters, the most significant mode observed is the QTO and is found to be associated with the ENSO variability in the tropical pacific region. While the intra-seasonal fluctuations (∼2-3 months) observed in AOD (at shorter wavelengths) are associated with the eastward propagating Madden-Julian oscillations (MJO) in the lower troposphere, the biennial variability is related with variability in the meteorology of the upper and lower atmospheric regions. Periodicity analysis on aerosol parameters is useful to determine its predictability in the atmosphere. [ABSTRACT FROM AUTHOR]

Abstract: Indo-Gangetic (IG) alluvial plains, one of the largest river basins in the world, suffers from the long range transport of mineral dust from the western arid and desert regions of Africa, Arabia and Rajasthan during the summer (pre-monsoon season, April–June). These dust storms influence the aerosol optical depth (AOD) across the IG plains. The Kanpur AERONET (Aerosol Robotic Network) station and Moderate Resolution Imaging Spectro-radiometer (MODIS) data show pronounced effect on the aerosol optical properties and aerosol size distribution during major dust storm events over the IG plains that have significant effect on the aerosol radiative forcing (ARF). The multi-band AOD, from AERONET and MODIS, show contrasting changes in wavelength dependency over dust affected regions. A time collocated (±30min) validation of AERONET AOD with MODIS Terra (level 2 swath product) over Kanpur, at a common wavelength of 550nm for the period 2001–2005 show moderate correlation (R 2∼0.6) during the summer season. The average surface forcing is found to change by −23Wm−2 during dust events and the top of the atmosphere (TOA) forcing change by −11Wm−2 as compared to the non-dusty clear-sky days. A strong correlation is found between AOD at 500nm and the ARF. At surface, the correlation coefficient between AOD and ARF is found to be high (R 2=0.925) and is found to be moderate (R 2=0.628) at the TOA. The slope of the regression line gives the aerosol forcing efficiency at 500nm of about −46±2.6Wm−2 and −17±2.5Wm−2 at the surface and the TOA, respectively. The ARF is found to increase with the advance of the dry season in conjunction with the gradual rise in AOD (at 500nm) from April (0.4–0.5) to June (0.6–0.7) over the IG plains. [Copyright &y& Elsevier]

Abstract: Aerosol measurements over an urban site at Delhi in the western Ganga basin, northern India, were carried out during 2009 using a ground-based automatic sun/sky radiometer to identify their different types and to understand their possible radiative implications. Differentiation of aerosol types over the station was made using the appropriate thresholds for size-distribution of aerosols (i.e. fine-mode fraction, FMF at 500nm) and radiation absorptivity (i.e. single scattering albedo, SSA at 440nm). Four different aerosol types were identified, viz., polluted dust (PD), polluted continent (PC), mostly black carbon (MBC) and mostly organic carbon (MOC), which contributed ~48%, 32%, 11% and 9%, respectively to the total aerosols. Interestingly, the optical properties for these aerosol types differed considerably, which were further used, for the first time, to quantify their radiative implications over this station. The highest atmospheric forcing was observed for PC aerosol type (about +40Wm−2, along with the corresponding atmospheric heating rate of 1.10Kday−1); whereas the lowest was for MBC aerosol type (about +25Wm−2, along with the corresponding atmospheric heating rate of 0.69Kday−1). [Copyright &y& Elsevier]