Unveiling Urban Air Quality Seasonal Dynamics in San Miguel De Tucumán, Argentina, by Scanning Electron Microscopy

Abstract

Seasonal dynamics of atmospheric particulate matter (PM) deposition in San Miguel de Tucumán, Argentina, were investigated between August 2019 and August 2020 using passive sampling combined with scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM–EDS). More than 15,000 particles within the SEM-resolved size fraction (> 1 µm) were characterized through image-based analysis to evaluate temporal variations in particle abundance, size distribution, morphology, and composition under changing agricultural, urban, and meteorological conditions. The study period included sugarcane harvest and non-harvest seasons, as well as COVID-19 lockdown conditions, allowing the assessment of PM responses to both seasonal agricultural activity and reduced urban mobility. Based on gray scale intensity calibrated by EDS analysis, particles were classified into carbonaceous particulate matter (PMC) and terrigenous particulate matter (PMT). Particle number deposition rates (NDR) increased markedly during harvest periods, rising from 5,098 ± 1,658 particles·cm⁻2·day⁻1 under non-harvest conditions to 11,587 ± 6,312 and 14,499 ± 10,254 particles·cm⁻2·day⁻1 during harvest periods without and with lockdown, respectively. PMT numerically dominated throughout the annual cycle and exhibited recurrent size distributions mainly between 1 and 8 µm equivalent circular diameter (ECD), consistent with sustained soil re-suspension and agricultural disturbance processes. In contrast, PMC was concentrated predominantly within the 1–4 µm ECD fractions. The relative contribution of coarse carbonaceous particles (> 10 µm ECD) decreased by approximately 35% during the lockdown-affected harvest period, whereas fine PMC fractions remained comparatively stable across periods. These patterns suggest that coarse carbonaceous particles were more sensitive to reductions in anthropogenic activity, while fine carbonaceous particles likely reflected a combination of persistent environmental sources. Meteorological conditions further modulate deposition dynamics, with relative humidity negatively associated with particle deposition and wind speed positively associated with particle abundance. Satellite-derived burned area and NO₂ data provided additional contextual information supporting the influence of biomass burning and changes in urban activity during the study period. Our results demonstrate that SEM–EDS combined with passive particle collection and image analysis provides a valuable complementary approach for investigating seasonal particulate matter dynamics at the particle scale, particularly in regions where conventional air quality monitoring infrastructure is limited.