Ultrafine aerosol particles in the western Caribbean: A first case study in Merida

Aerosol particles have been shown to trigger cardiovascular and respiratory diseases in populations living in polluted regions, depending on their size and composition. Although fine aerosol particles have been found to reach the lungs and the blood stream, their acute effects are related to the fraction of ultrafine aerosol particles (i.e., UFPs, d < 0.1 μm). In the present study, the concentration of UFPs (d = 0.02–0.1 μm) was monitored in Merida (Yucatan) between April 2017 and July 2018. Additionally, the total aerosol particle concentration (d > 0.03 μm), PM₁₀, PM_2.5, criteria gases, and meteorological variables, were also monitored. The average UFPs concentration over the full sampling period was found to be 2,070 ± 1,831 cm^-3, with events of peak concentrations as high as 55,117 cm⁻³. The average daily UFP profile is bimodal, with peak concentrations observed around 07:00 and 20:00 h LT. The correlation of the UFP with other measured variables suggests that the UFPs peaks observed in the morning and at night are likely caused by primary vehicular combustion particles. In contrast, atypical high concentrations of UFPs were occasionally observed around noon LT, which are likely of secondary origin and the result of new particle formation promoted by photochemistry. Overall, UFPs are the major contributor to the total particle number concentration, and they are likely an important contributor to PM_2.5 in Merida during the morning hours, with clear anthropogenic sources.