A two-year study based on chemical characteristics of ambient aerosols, from an urban site of a semi-arid region in western India, provides an unequivocal evidence for the quantitative neutralization of acidic constituents by mineral aerosols. The mineral dust component varied in the range 16--220 ¿g m-3 (Av: 88, sd: 42); and contributed to 69 ¿ 5% of the TSP (range: 43--273 ¿g m-3, Av: 125, sd: 49). A characteristic feature of the in-situ chemical reactions is well reflected by the relative dominance of Ca2+ and HCO3 - ions in the water-soluble composition of aerosols and anti-correlation among HCO3 -/Ca2+ and "Excess Acid"/Ca2+. A ubiquitous alkaline-nature of rainfall events over this high dust region further attests to our chemical data. Such regional-scale atmospheric transformation processes suggest the over estimation of negative radiative forcing (climate cooling) due to sulphate aerosols in the global climate models.