Measurement report: Ice nucleating particles variability across a megacity

Abstract. Megacities are a major source of urban aerosol particles, which can impact cloud formation and the local hydrological cycle. However, the aerosol-cloud interaction above megacities, especially in their different microclimates, is poorly understood. In the present study, the physicochemical and biological properties of urban aerosol particles, along with their ice nucleating particle (INP) concentration via immersion freezing and as a function of particle size (0.56–10 µm), were simultaneously characterized at two sites across the Mexico City Metropolitan Area (MCMA). We found differences in the chemical composition, criteria pollutants (PM2.5, O3, CO, NOx, and SO2), and biological content between northern and southern MCMA, separated by 16 km. The collected urban MCMA aerosol particles were found to act as INPs, with average concentrations ranging between 0.04±0.04L-1 (at −15 °C) and 24.9±18L-1 (at −30 °C). Although the measured INP concentrations were similar in both sites, the southern samples showed a higher INP concentration for larger aerosol particles (i.e., particles between 5.6–10 µm). Although the urban aerosol's physicochemical properties, biological content, and its sources were found to differ at both sites, it did not strongly impact the INP concentrations, with the exception of the largest measured particles. This highlights the importance of considering that aerosol-cloud interactions above a megacity may vary, especially when assessing the role of INPs in cloud formation.