Vertical distributions of the microscopic morphological characteristics and elemental composition of aerosols over India

Particle morphology and elemental compositions are among the crucial parameters of aerosols required for accurate understanding of the climatic effect of aerosols in the earth-atmosphere system; yet their vertical distributions and region specific properties are poorly characterised due to sparse in-situ measurements. This is the first study to classify and quantify the vertical distributions of the morphological characteristics and elemental composition of aerosols based on single particle as well as bulk chemical analysis over seven geographically diverse regions of northern and central parts of India during spring (April–May, 2013), carried out as a part of Regional Aerosol Warming Experiment (RAWEX). Significant regional distinctiveness in shapes (non-sphericity), sizes and elemental compositions of the airborne particles were conspicuous, having dominance of highly irregular granular aggregates over the north Indian sites. The non-spherical coarse mode particles dominated the lower free tropospheric regions (> 2?km) of the Indo-Gangetic Plains (IGP). These particles could be responsible for enhanced spring time aerosol absorption in the elevated region of the atmosphere. Elemental compositions of the single particle analysis indicate that the free tropospheric layer over the IGP and central India is enriched with Na and Ca compounds mixed with Fe or Al (soil particles), indicating long range transport of crustal aerosols. This finding is very well supported by the bulk particle analysis indicating abundance of Ca²⁺ in the free troposphere with low contribution of ssNa⁺. Particles with irregular rough surfaces having dominance of SiO₂ were observed over all the study sites. The percentage share of spherical (either smooth or rough) particles to the total morphological characteristics of the particles was found to be highly subdued (< 10%). The present study thus critically assesses the relevant knowledge pertaining to the morphological features of aerosols over the IGP during spring for the accurate estimation of aerosol radiative properties. More such efforts are required in future to study the connections and dependencies between morphological and radiative properties of aerosols in different seasons.