Fire Smoke and Sahara Dust Take Turns Shaping Caribbean Air Pollution

Every year, the air over the Western Caribbean is governed by two very different invaders. From March to May, smoke from regional biomass burning drifts northward on the backs of cold-front intrusions, loading the atmosphere with fine combustion particles. Then, from June to August, the boreal summer wet season ushers in a different visitor altogether: vast plumes of mineral dust lifted from the Sahara and carried thousands of kilometers across the Atlantic by the trade winds. A new study published in Environmental Science and Pollution Research has now provided one of the most detailed portraits yet of how these two seasonal regimes alternately reshape the aerosol chemistry of the region, and what that means for the millions of people who breathe that air.

The research team, led by Daniel Rosas of the Universidad Autónoma de Yucatán together with colleagues from the National Autonomous University of Mexico and Droplet Measurement Technologies, conducted continuous observations in Mérida, Mexico, a near-coastal urban site that sits squarely within the Western Caribbean atmospheric domain. The city, home to more than a million people and one of the fastest-growing metropolitan areas in the country, offers an ideal vantage point for tracking the interplay between long-range transport and local emissions. The researchers combined ground-based measurements of fine and coarse particulate matter, known as PM2.5 and PM10, with continuous monitoring of particle-bound polycyclic aromatic hydrocarbons, a family of toxic organic compounds that cling to soot and other combustion particles and are among the most concerning carcinogens in urban air.

To untangle where the particles came from, the team paired their ground data with satellite-based fire detections from NASA’s FIRMS system and with HYSPLIT back-trajectory simulations, a modeling tool developed by NOAA that reconstructs the path an air parcel traveled before arriving at a measurement site. This combination of observations and trajectory analysis allowed the researchers to link specific pollution episodes to their origins, distinguishing between smoke plumes arriving from fires to the south and dust-laden air masses sweeping in from West Africa. The approach builds on earlier work in the region, including the ADABBOY campaign, which documented African dust and biomass burning over the Yucatán Peninsula, and a long history of studies showing that Saharan dust routinely crosses the entire Atlantic basin to reach the Caribbean and even the Amazon, where it delivers vital nutrients to rainforest soils.

The meteorological backdrop proved to be a decisive actor in the story. During the biomass burning season, the atmosphere over Mérida was dry and turbulent, dominated by southerly flow associated with cold-front intrusions and the dry-season circulation. When the African dust season arrived, conditions flipped: humid easterly trade winds took over, and the boundary layer dynamics changed accordingly. These shifts in wind direction, humidity, and turbulence did not merely accompany the changing aerosol regimes; they actively modulated particle composition at both seasonal and episodic timescales, determining which sources dominated the near-surface air on any given day.

The chemical fingerprints of the two regimes turned out to be strikingly different. During African dust intrusions, total particulate concentrations rose only modestly, but the ratio of PM2.5 to PM10 dropped noticeably. That decline is a classic signature of coarse mineral particles: desert dust grains are relatively large, often several micrometers across, so when they flood into a region they inflate the coarse fraction of the aerosol population while leaving the fine fraction comparatively untouched. Biomass burning, by contrast, produced the opposite pattern. The smoke season was characterized by enhanced fine-mode aerosol contributions, since combustion particles are typically submicron in size, and by episodic spikes in particle-bound PAHs that coincided with intense regional fire activity flagged by satellite sensors. In other words, the two regimes could be told apart not just by how much particulate matter was in the air, but by what kind of particles it contained and when they arrived.

Perhaps the most intriguing finding concerns the daily rhythm of pollution peaks. During the biomass burning season, the highest concentrations of particle-bound PAHs occurred predominantly in the afternoon hours, a timing consistent with regional plume transport, in which smoke generated by distant fires arrives overhead and mixes down to the surface as the day progresses. During the African dust season, by contrast, the peaks clustered in the morning rush-hour window, suggesting a tighter coupling between local traffic emissions and the accumulation of pollutants under early-morning stable conditions. The two regimes, in effect, deliver their pollution burdens at different times of day, through different mechanisms, and from different geographic origins, a distinction that could matter greatly for anyone trying to design effective public health warnings.

When the team translated their measurements into health-risk estimates using benzo[a]pyrene-equivalent concentrations, a standard toxicological approach that weights each PAH compound by its carcinogenic potency, the results were reassuring at first glance. Chronic carcinogenic risk under both seasonal regimes was assessed as low, and the two seasons were statistically indistinguishable in this respect. Benzo[a]pyrene is often used as the reference compound because it is one of the most potent carcinogens among the PAHs, and expressing the total PAH burden in its equivalent terms allows risk assessors to compare exposures across very different chemical mixtures. The finding that neither the smoke season nor the dust season elevates long-term cancer risk above low levels is an important baseline for a region where air quality monitoring has historically been sparse.

But the risk story is more nuanced than a single annual average can convey. The two regimes differed sharply in the magnitude and timing of short-term excursions, the brief episodes when concentrations spike well above their seasonal norms. Biomass burning peaks reached substantially higher concentrations than their dust-season counterparts, meaning that residents of Mérida and surrounding areas experience more intense, if intermittent, exposures to carcinogenic particles during the spring fire season. These acute excursions matter because epidemiological research increasingly suggests that short-term peaks in combustion-derived particulate matter can trigger cardiovascular and respiratory events even when long-term averages appear acceptable. The fact that the two regimes produce peaks at different times of day also implies that exposure mitigation strategies, from timing outdoor activities to scheduling hospital staffing, may need to be tailored to the season rather than applied uniformly year-round.

The broader significance of the study lies in its demonstration that the seasonal duality of biomass burning and African dust exerts a measurable, quantifiable influence on aerosol composition and exposure-relevant indicators across the entire Western Caribbean. Because the two regimes arise from fundamentally different processes, one from fires set and spreading across Central America and southern Mexico, the other from wind erosion in the Sahara followed by transatlantic transport, the appropriate policy responses differ as well. Managing fire-season exposure may require regional coordination on agricultural burning practices, early-warning systems based on satellite fire detection, and public advisories timed to afternoon plume arrivals. Managing dust-season exposure, meanwhile, is largely beyond local control, since the dust originates on another continent, but the morning clustering of peaks suggests that local traffic management and urban planning still have a role to play in limiting total exposure during dust episodes.

For a rapidly growing city like Mérida, perched on the edge of a karstic peninsula where the atmosphere connects the smoke of Central American fires with the dust of the Sahara, the study provides something previous work in the region had not: a season-by-season, hour-by-hour accounting of who is breathing what, and when. As climate change alters fire regimes in the tropics and as dust emissions from North Africa respond to shifting rainfall patterns, the delicate seasonal choreography documented in this research may itself begin to shift. Understanding the baseline, the two alternating regimes and their distinct chemical and temporal signatures, is the essential first step toward detecting those changes and protecting public health as the Western Caribbean’s atmospheric visitors continue their annual rotation.

Subject of Research: Seasonal influence of biomass burning and African dust on aerosol chemistry and particle-bound PAH health risk over the Western Caribbean

Article Title: 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

Article References: Rosas, D., Rodríguez-Gamboa, O. Y., Vega-De-Lille, M. I., Raga, G. B., Baumgardner, D., Ponce-Caballero, C., & Ladino, L. A. (2026). 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. Environmental Science and Pollution Research, 33(28), 14424-14445. https://doi.org/10.1007/s11356-026-38193-x

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38193-x

Keywords: biomass burning, African dust, particle-bound PAHs, PM2.5, PM10, Western Caribbean, Yucatán Peninsula, Mérida, HYSPLIT trajectories, air quality, carcinogenic risk, seasonal aerosol variability