Planetary scale modulations in aerosol properties at Delhi, in the Indo‐Gangetic Plain: a quantitative analysis from intra‐seasonal to inter‐annual timescales

Abstract

ABSTRACTAnalysis 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.