Biomass burning and African dust influence on seasonal aerosol chemistry over the Western Caribbean (Part I): Atmospheric variability and human health risk from particle-bound PAHs.

Seasonal aerosol variability over the Western Caribbean is strongly influenced by two recurrent atmospheric regimes: regional biomass burning (BB, March-May) associated with cold-front intrusions and dry-season circulation, and long-range transport of African dust (AD, June-August) during the boreal summer wet season. Despite their relevance for air quality and public health across the region, the combined influence of these regimes on near-surface aerosol composition and toxicological exposure remains insufficiently characterized. Here, we present an integrated assessment of meteorology, aerosol dynamics, fire activity, and combustion tracers based on continuous observations conducted in Mérida, Mexico, a representative near-coastal urban site within the Western Caribbean atmospheric domain. The analysis combines measurements of PM2.5, PM10, and particle-bound polycyclic aromatic hydrocarbons (pPAH) with satellite-based fire detections (FIRMS) and HYSPLIT back-trajectory simulations. Meteorological conditions exhibited a marked transition from dry, turbulent regimes dominated by southerly flow during BB to humid, easterly trade-wind conditions during AD. These shifts strongly modulated aerosol composition at both seasonal and episodic timescales: while PM2.5 and PM10 concentrations increased modestly during AD, the PM2.5/PM10 ratio decreased, reflecting an increased role of coarse mineral particles during dust intrusions. In contrast, BB was characterized by enhanced fine-mode aerosol contributions and episodic pPAH peaks linked to intense regional fire activity, consistent with satellite observations and trajectory analysis. Health-risk estimates based on benzo[a]pyrene-equivalent concentrations indicated low chronic carcinogenic risk under both seasonal regimes, which were statistically indistinguishable. The two regimes differed instead in the magnitude and diurnal timing of short-term excursions: peaks during BB reached substantially higher concentrations and occurred predominantly during afternoon hours, consistent with regional plume transport, whereas AD peaks clustered in the morning rush-hour window. Overall, this study demonstrates that the seasonal duality of BB and AD exerts a measurable influence on aerosol composition and exposure-relevant indicators across the Western Caribbean, with the distinct regional and local origins of BB and AD exposure peaks pointing toward correspondingly differentiated air-quality management strategies.