Abstract: Aerosol particles are of great importance to both human health and the climate system. Due to the large size of Mexico City (MC), the large number of inhabitants, as well as the diversity of ecosystems and anthropogenic activities throughout the city, previous studies have found differences in the characteristics of the aerosol particles present in different parts of MC. However, most of these studies were carried out at a single sampling point, which has prevented a more in-depth evaluation of the observed differences. With the aim to improve the current understanding of the spatial variability of particulate matter along the MC, aerosol particles were simultaneously sampled/monitored in the north and south of MC during the warm dry season (May 2022) and the cold dry season (Jan-Feb 2023). The present study mostly focuses on the evaluation of i) PM2.5 variability and sources, ii) their chemical composition, and iii) the presence and concentration of culturable bacteria and fungi. Most of the time, the PM2.5 concentrations were found to be higher in the sampling site located in the north of the City. When looking at the chemical composition of the aerosol particles collected in both sites, it was found that the anions (Cl – , NO3 – , and SO4 2- ) and cations (Na + , Ca 2+ , K + , Mg 2+ , and NH4 + ) concentrations were always higher in the north. Similarly, the concentration of most of the analyzed elements (i.e., Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Cu, and Zn) was higher in the north of MC. Additionally, the overall concentration of bacteria and fungi was found to be higher in the north of MC. This indicates that despite the fact that the two sampling sites are clearly under urban influence and 16 km away, the pollution levels (and likely their sources) are rather different. This is somehow expected as most of the industrial activity in MC is located in the northern part. However, the wind patterns and the correlations of the ionic and the elemental composition between the two sites suggest that in some cases the pollution emitted in the north of MC is transported towards the south, impacting a larger zone. A similar behavior was observed for the biological particles as the concentration of some bacteria (Kocuria rosea, Pantoea agglomerans, and Pantoea vagans) and fungi (Cladosporium) showed a correlation above 0.5 between the north and south.

Keywords: Aerosol particles, Mexico City, Air pollution, bioparticles, PM2.5