Ice nucleating particles (INPs) play an important role in cloud microphysics by initiating ice formation, influencing the hydrological cycle and the Earth’s radiative budget. Despite their importance, measurements of INPs remain limited in tropical regions and in urban environments where complex aerosol dynamics may influence their concentrations and activity. High-altitude mountain sites located near urban areas are useful to study the influence of urban emissions in the region. In this study we analyzed the INP concentrations for rainwater, cloud water, and airborne aerosol samples collected at two sites in the central Mexico basin: an urban location and a high-altitude mountain observatory (4000 m a.s.l.). Samples were collected during the rainy season and sampling took place within and above the planetary boundary layer -e.g. the free troposphere- for airborne aerosol at the mountain site.

INP concentrations in rainwater were significantly higher at the mountain site at −15 °C and − 20 °C. Two mountain rainwater samples exhibited unusually efficient freezing spectra with onset freezing temperatures close to −6 °C and strong sensitivity to heat treatment, suggesting the presence of active biological INPs. In contrast, airborne aerosol concentrations were significantly higher at the urban site at −25 °C. Mountain site aerosol samples influenced by the planetary boundary layer showed slightly higher concentrations than free tropospheric samples, though this remains a qualitative observation. INP concentrations at both sites were lower than those reported in mid-latitude precipitation studies, which could be an important contribution to the understanding of INP distributions at tropical latitudes. These results indicate that the INP populations could differ between precipitation and airborne aerosol and suggest that biological particles may play an important role in the INP population in the development of precipitation at the mountain site.