Diagnostic evaluation of aerosol classification over the Indo-Gangetic Plain: insights from AE-FMF-SSA relationships

Aerosol classification over the Indo-Gangetic Plain (IGP) commonly relies on fixed threshold combinations of fine mode fraction (FMF) and single scattering albedo (SSA), with the Ångström exponent (AE) often treated descriptively despite its sensitivity to humidity-driven particle modification. As a result, a substantial fraction of observations remains unrepresented within such classification frameworks, particularly when AE and FMF do not vary coherently. Using multi-year AERONET observations from Kanpur, Jaipur, and Lahore, this study provides a regional assessment of aerosol classification incompleteness associated with AE-FMF mismatch conditions. Classification schemes that do not include residual categories leave approximately 20% of observations unassigned, indicating a substantial AE-FMF mismatch fraction (M%) across all three sites, whereas the AE-FMF-SSA framework of Logothetis et al. (2020) assigns these cases to the Other Absorbing (OA) and Other Non-Absorbing (ON) types. These observations form a compact cluster in AE, FMF, and SSA space, characterised by intermediate AE, high FMF, and moderately absorbing SSA. Comparison with global AERONET applications of the same framework shows that the IGP exhibits higher OA + ON frequencies, and therefore higher M%, than marine, desert-dominated, and most European sites. The results demonstrate that AE-FMF mismatch conditions correspond to a coherent fine-mode aerosol regime that fixed AE-FMF-SSA threshold systems do not capture, and that explicit AE-based diagnostics provide additional capability for aerosol classification in complex, humidity-influenced environments.