Optically tweezed microscopic aerosol droplets are characterized using Raman spectroscopy to quantify chemical and physical transformations that occur due to change in environment. Droplets are used as micro-chemical reactors to probe polymorphic calcium carbonate formation during ion exchange and droplets are used as sensors for the localized environment. Analysis of Raman spectral peak shifts during aqueous CaCl₂ and aqueous Na₂CO₃ droplet coalescence events provides detailed insight into chemical composition, and measurement of morphology dependent resonances provides droplet size, refractive index, morphology and phase information. We explore the reversibility of meta-stable vaterite and amorphous states and stable calcite formation through Raman peak deconstruction. This work has scope for extension to droplet flow systems for controlled chemical reactions that can promote higher yields and higher concentrations due to the super-saturated states accessible to aerosol droplets.