Characterization of ice nucleating particles in rainwater, cloud water, and aerosol samples at two different tropical latitudes

Abstract

Ice nucleating particles (INPs) induce ice crystal formation and therefore, they are able to influence precipitation development. INP sources remain highly uncertain, with most of the observational studies performed in mid- and high-latitudes, bypassing the Tropics. In the present study, rainwater, cloud water, and aerosol samples were collected during the rainy seasons in 2018 and 2019 in two major tropical capital cities: Quito and Mexico City, and at the high-altitude rural site Altzomoni (Mexico). The ice nucleating abilities of the rainwater samples from the urban sites (INP concentrations varying between 1.1 × 102 and 105 L−1 water at −9 °C and −24 °C) were not influenced by the pollution levels, consistent with the literature for other cities (e.g., Beijing and New Delhi), suggesting similarities in the behavior of INPs in densely populated and polluted cities. On the other hand, although the INP concentrations of the rural site samples were similar to those found in Quito and Mexico City (i.e., 7.1 × 101 and 1.1 × 105), their onset freezing temperatures (T0) were found to be higher (−7.5 °C as the highest). In terms of T50 (the temperature at which 50% of the droplets freeze), the rainwater and the cloud water samples were found to be more efficient than the aerosol samples. The ice nucleating abilities of the rural site samples were reduced when applying the heating test, resulting in lower T0 values, by more than 5 °C. Moreover, the presence of bacteria and fungal propagules on the rainwater and cloud water samples was also confirmed. Therefore, the high ice nucleating abilities observed on the samples from the rural site are likely related to biological material. Although K, Ca, S, Zn, and NO3− were found to be enriched in the cloud water samples in comparison to the aerosol particles, only Cu, Mn, and Zn were found to moderately correlate with the INP concentrations at −15 °C and −17 °C out of all the elements detected. Finally, the rainwater samples collected at those tropical sites were found to contain INPs concentrations lower than those found in other mid- and high-latitude sites. The information herein will improve the knowledge of INPs' contained in precipitation, their influence in tropical latitudes, and the development of new parameterizations.