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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]

The solar global ultraviolet (GUV) and broadband global (G) radiation flux obtained on a horizontal plane in Delhi, during April 2010 to March 2011 have been used to investigate the temporal variability of these radiations and their ratio, fraction of UV (FUV). For the first time the clearness index ( KT) has been estimated over Delhi and its variability during different months of the year and season has been studied in detail. The impact of atmospheric aerosols on KT has also been studied. It has been found that for every unit increase in aerosol optical depth (AOD) at 340 nm, KT decreases by 0.06. A strong anti-correlation with correlation coefficient − 0.75 is observed between AOD and KT. On the basis of our field experience and observations at Delhi it is found that for highly cloudy and overcast conditions 0⩽ KT⩽0.15, for partial cloudy or hazy conditions 0.15⩽ KT⩽0.21 and KT > 0.21 for clear sky conditions. In addition, during foggy days in winter we have found KT values lying in the range 0.12-0.18 at Delhi. The day-time daily-averaged fluxes GUV and G varied in the range 0.15-1.23 MJ m−2 and 3.36-27.02 MJ m−2, respectively. The GUV and G showed similar pattern during the year except for the wet season when the FUV increased possibly due to an increase in water vapour concentration. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]