Inter-annual Variability of Ice Nucleating Particles in Mexico City

The continuous and sustained population shift from rural to urban areas is driving most of the fast-growing megacities on a global scale. Considerable pollution, gases and particulate matter, is emitted yearly into the atmosphere of megacities, with unclear impacts on the local and regional hydrological cycle. The present work evaluated the inter-annual variability of super-micron (1.0-10 um) ice nucleating particles (INP), via immersion freezing, sampled during the dry-warm season in Mexico City on two consecutive years using the UNAM-MicroOrifice Uniform Deposit Impactor-Droplet Freezing Technique (UNAM-MOUDI-DFT). The aerosol particles emitted in Mexico City were found to act as INP via the immersion freezing mode at temperatures below -20 degrees C with INP concentrations ranging between 0.74 L-1 (-20 degrees C) and 20.42 L-1 (-30 degrees C). Although the INP concentration varied between the dry-warm seasons, the average concentrations between 2018 and 2019 were rather similar, with significant differences only observed at -20 degrees C. The large increase (>200%) in fine particulate matter -PM2.5 - observed during the 2019 severe air pollution episode caused by biomass burning (BB) emissions, did not impact the super-micron INP concentrations between -20 degrees C and -30 degrees C. However, INPs at temperatures above -15 degrees C were found to increase during the severe pollution episode. Given that such warm ice nucleation temperatures can be linked with the presence of biological particles, the present observations suggest that the BB plumes transported into Mexico City were most likely advecting co-emitted biological particles.