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  • View in gallery

    Sampling sites in the Yucatan Peninsula: Merida (urban; 20.9754°N, 89.6170°W) and Sisal (remote seaside; 21.1653°N, 90.0311°W), Mexico. EI-UNAM and SC-UADY refer to the Engineering Institute of the National University of México and the School of Chemistry of the University of Yucatán, respectively (map source: Google Maps).

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    Mean monthly PM10 concentration associated with dust derived from MERRA-2 over 2003–19, for June–September. The two sampling sites are represented by a single black dot due to their proximity.

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    Particle (a) number and (b) volume concentrations as a function of equivalent optical diameter (EOD) for particles larger than 50 nm stratified by air masses, normalized by the logarithm of the channel size intervals. The distributions are the result of combining measurements from the PMS LasAir with those from the TSI CPC. The volume distributions are computed assuming particle sphericity at the EOD. The sizing uncertainty is on the order of ±20%. The AD distributions (yellow) and the BB distributions (red) were obtained from selected individual plumes that affected Merida. The average background distribution (blue) was obtained from samples not affected by AD or BB in Merida. Vertical error bars indicate one standard deviation and are only shown in the postive direction for clarity.

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    Percentage elemental mass concentration for background samples in (a) Merida and (b) Sisal. (c) Enrichment factors calculated for samples obtained under the influence of BB (red bars) and AD (yellow bars) plumes with respect to the background conditions sampled in Merida. The dashed horizontal line corresponds to enrichment factor equal to 1.

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    Boxplots of the concentration [CFU m−3] of (a) bacteria and (b) fungal propagules. BB, AD, and MA refer to biomass burning, African dust, and marine aerosol IOPs during 2017 and 2018. Samples are exposed for 5 min and results are counts after 48 h (modified from Rodriguez-Gomez et al. 2020).

  • View in gallery

    INP concentration as a function of temperature for three distinct air masses: marine aerosol (MA, blue), biomass burning (BB, red), and African dust (AD, yellow) for particles with sizes ranging between 0.32 and 10 µm. The vertical black lines indicate the standard deviation (modified from Córdoba et al. 2021).

  • View in gallery

    Frozen fraction curves for the sea surface microlayer (SML, red line) and bulk surface water (BSW, blue line) samples collected at the Gulf of Mexico. The solid black line denotes the homogeneous freezing curve and the shaded areas denote the standard deviation for each sample type (modified from Ladino et al. 2021).

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ADABBOY: African Dust And Biomass Burning Over Yucatan

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  • 1 Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico
  • | 2 Droplet Measurement Technologies, LLC, Longmont, Colorado
  • | 3 Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico
  • | 4 Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, Mexico
  • | 5 Facultad de Química, Universidad Autónoma de Yucatán, Mérida, Yucatán, Mexico
  • | 6 Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico
  • | 7 Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico
  • | 8 Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico
  • | 9 Dalhousie University, Halifax, Nova Scotia, Canada
  • | 10 Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico
  • | 11 Instituto de Ingeniería, Unidad Académica de Sisal, Universidad Nacional Autónoma de México, Sisal, Yucatán, Mexico
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Abstract

Biomass burning (BB) emissions and African dust (AD) are often associated with poor regional air quality, particularly in the tropics. The Yucatan Peninsula is a fairly pristine site due to predominant trade winds, but occasionally BB and AD plumes severely degrade its air quality. The African Dust And Biomass Burning Over Yucatan (ADABBOY) project (January 2017–August 2018) was conducted in the Yucatan Peninsula to characterize physical and biological properties of particulate pollution at remote seaside and urban sites. The 18-month-long project quantified the large interannual variability in frequency and spatial extent of BB and AD plumes. Remote and urban sites experienced air quality degradation under the influence of these plumes, with up to 200% and 300% increases in coarse particle mass under BB and AD influence, respectively. ADABBOY is the first project to systematically characterize elemental composition of airborne particles as a function of these sources and identify bioaerosol over Yucatan. Bacteria, actinobacteria (both continental and marine), and fungi propagules vary seasonally and interannually and revealed the presence of very different species and genera associated with different sources. A novel contribution of ADABBOY was the determination of the ice-nucleating abilities of particles emitted by different sources within an undersampled region of the world. BB particles were found to be inefficient ice-nucleating particles at temperatures warmer than −20°C, whereas both AD and background marine aerosol activated ice-nucleating particles below −10°C.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Darrel Baumgardner, darrel.baumgardner@gmail.com

Abstract

Biomass burning (BB) emissions and African dust (AD) are often associated with poor regional air quality, particularly in the tropics. The Yucatan Peninsula is a fairly pristine site due to predominant trade winds, but occasionally BB and AD plumes severely degrade its air quality. The African Dust And Biomass Burning Over Yucatan (ADABBOY) project (January 2017–August 2018) was conducted in the Yucatan Peninsula to characterize physical and biological properties of particulate pollution at remote seaside and urban sites. The 18-month-long project quantified the large interannual variability in frequency and spatial extent of BB and AD plumes. Remote and urban sites experienced air quality degradation under the influence of these plumes, with up to 200% and 300% increases in coarse particle mass under BB and AD influence, respectively. ADABBOY is the first project to systematically characterize elemental composition of airborne particles as a function of these sources and identify bioaerosol over Yucatan. Bacteria, actinobacteria (both continental and marine), and fungi propagules vary seasonally and interannually and revealed the presence of very different species and genera associated with different sources. A novel contribution of ADABBOY was the determination of the ice-nucleating abilities of particles emitted by different sources within an undersampled region of the world. BB particles were found to be inefficient ice-nucleating particles at temperatures warmer than −20°C, whereas both AD and background marine aerosol activated ice-nucleating particles below −10°C.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Darrel Baumgardner, darrel.baumgardner@gmail.com

Mineral dust particles and marine salts globally constitute the most abundant natural aerosols in the atmosphere (Andreae 1995). Other natural sources, e.g., vegetation and microorganisms, emit substantial quantities of primary biological aerosol (Despreìs et al. 2012) and precursor gaseous species resulting in secondary aerosol formation (Chen et al. 2009). Particles affect the radiation budget of the Earth (Forster et al. 2007) and impact the local and regional hydrological cycle as cloud condensation nuclei (CCN) (Lau and Wu 2003; Zhang et al. 2007; Raga et al. 2016; Liu et al. 2018) and ice-nucleating particles (INPs) (Creamean et al. 2013; Yakobi-Hancock et al. 2014; Wilson et al. 2015; Kanji et al. 2017).

Biomass burning (BB) encompasses wildfires, forest/savannah clearance for agriculture, removal of agricultural residue, and domestic cooking/heating. BB emissions constitute a health threat (Wiedinmyer et al. 2006). Anthropogenic BB is prevalent in tropical regions and exhibits a clear peak during the dry season (Giglio et al. 2006; Giglio 2007). BB emissions affect the regional radiative budget, increasing atmospheric stability and inhibited convection (Koren et al. 2004; Feingold et al. 2005). While most of the aerosol emitted from BB impacts local and regional areas, under particular weather patterns BB plumes are advected over thousands of kilometers. Aerosol plumes emitted by BB in southern Mexico and Central America have impacted urban air quality in the United States (Peppler et al. 1999; van der Werf et al. 2006; Dominguez-Martinez and Rodriguez, 2008; Yokelson et al. 2009, 2011). Some BB particles are active as CCN (Rogers et al. 1991), but it is still unclear under which conditions BB particles are efficient INPs (Kanji et al. 2017).

African dust (AD) increases atmospheric particulate mass (PM) significantly affecting most of the countries surrounding the Mediterranean Sea (Ganor and Mamane 1982; Guerzoni et al. 1997) and the tropical Atlantic Ocean (Mukhopadhyay and Kreycik 2008; Evan and Mukhopadhyay 2010). Annual AD episodes affect the tropical Atlantic that degrades air quality in Barbados and Miami during boreal summers (Prospero and Mayol-Bracero 2013). AD originating in deserts located between 15° and 25°N travels over the Atlantic in an elevated layer (the Saharan aerosol layer, SAL), reported originally by Carlson and Prospero (1972) and Prospero and Carson (1972). This warm and dry air mass laden with dust particles travels westward at a speed of about 1,000 km day−1, reaching Barbados in the Lesser Antilles in ∼5 days. The SAL exhibits large seasonal variability (Huang et al. 2010), and is transported primarily into the Caribbean and Florida in June and July (Weinzierl et al. 2017). Aircraft and shipborne measurements during the Saharan Aerosol Long-Range Transport and Aerosol–Cloud–Interaction Experiment (SALTRACE) (Weinzierl et al. 2017) characterized the plume as it leaves Africa (Cape Verde) and upon reaching Barbados. Airborne lidar observations show that the SAL over Cape Verde extends from a base at 1.5 km above the cool and dust-free trade winds to a top around 6–7 km. As the SAL progresses westward, its depth decreases to ∼4 km above Barbados as it reaches the surface. The trade wind inversion weakens westward, favoring entrainment of the overlaying dust and mixing throughout the boundary layer, where turbulence and aerosol–cloud interactions affect dust concentration. A similarly detailed picture has yet to emerge to describe the vertical structure and evolution of the SAL as it continues westward over the Caribbean Sea. Prospero and Mayol-Bracero (2013) recommended the expansion of AD monitoring throughout the Caribbean; thus, our research group implemented a field project to document the arrival of dust plumes in Mexico.

The Yucatan Peninsula, in southeastern Mexico surrounded by the Caribbean Sea and the Gulf of Mexico, is home to sensitive ecosystems and invaluable archeological sites (Fig. 1). Agricultural lands dominate with only a few small- and medium-sized cities, such as Merida and Cancun with ∼890,000 and 740,000 inhabitants (INEGI 2015). Yucatan was selected for this study because its relatively clean air, due to predominant trade winds, is significantly perturbed during periods of BB in southern Mexico and Central America and during occasional incursions of AD.

Fig. 1.
Fig. 1.

Sampling sites in the Yucatan Peninsula: Merida (urban; 20.9754°N, 89.6170°W) and Sisal (remote seaside; 21.1653°N, 90.0311°W), Mexico. EI-UNAM and SC-UADY refer to the Engineering Institute of the National University of México and the School of Chemistry of the University of Yucatán, respectively (map source: Google Maps).

Citation: Bulletin of the American Meteorological Society 102, 8; 10.1175/BAMS-D-20-0172.1

Scientists from the National University of Mexico (UNAM), the University of Yucatan (UADY), and Dalhousie University in Canada conducted the African Dust And Biomass Burning Over Yucatan (ADABBOY) project from January 2017 to August 2018, with continuous measurements supplemented by six intensive observation periods (IOPs).

ADABBOY’s primary goal was to identify main sources of particles affecting Yucatan, focusing on BB, AD, marine, and biological particles, and establishing a baseline of background conditions for future studies. Specific objectives were to

  • identify and characterize background, BB and AD particles;

  • quantify the influence of BB and AD particles on urban air quality;

  • document biological microorganisms in BB and AD plumes; and

  • locate the main sources of INPs and quantify the ice-nucleating abilities of marine, BB, and AD particles.

ADABBOY campaigns

The Yucatan Peninsula (Fig. 1) is a flat limestone terrace with a warm, semidry climate on the Gulf of Mexico coast and a warm, subhumid climate throughout the rest of the region. In situ measurements were made in Merida (20.9754°N, 89.6170°W), located ∼23 km from the coastline, and in the coastal village of Sisal (21.1653°N, 90.0311°W). Merida is rapidly growing due to tourism and the relocation of industries from within Mexico. Sisal, a fishing village (population 1,837; SEDESOL 2013), enjoys clean air, with the closest source of industrial emissions located ∼25 km inland and the closest city, Merida, ∼50 km away.

Airborne particles were continuously monitored at the School of Chemistry at UADY, located in the central-western part of Merida (inset, Fig. 1). Table 1 lists the instrumentation used to characterize particle properties. Six IOPs complemented the continuous measurements: four in Merida (April 2017, July 2017, April 2018, and July 2018) and two in Sisal (January 2017 and July 2018). Documentation of the seasonality of BB and AD events in Yucatan was used to select optimal periods for the IOPs, as discussed further below.

Table 1.

List of the instrumentation deployed in the measurement site in Merida (located at 12 m above ground level) and the institution that provided them.

Table 1.

Elemental composition from filter samples determined by X-ray fluorescence (Espinosa et al. 2012) provide in situ confirmation of aerosol categories. Rodríguez-Gómez et al. (2020) discuss the sampling methodology for microorganisms, while Córdoba et al. (2021) and Ladino et al. (2021) document ice-nucleating properties of aerosol and seawater samples.

Climatological context and previous studies.

The Yucatan Peninsula is characterized by a dry season (December–May) and a rainy season (June–November) punctuated by intense cold fronts during December–February.

Dry-season BB activities peak between March and May (Rios and Raga 2018, 2019; Trujano-Jiménez et al. 2021), so IOPs were carried out in April 2017 and 2018 to maximize the chances of sampling BB particles. Real-time satellite information, forecast aerosol plumes (e.g., www.nrlmry.navy.mil/aerosol/) and HYSPLIT back trajectories confirmed the origin of the BB emissions.

Easterly waves and tropical cyclones affect the region during the rainy season, with rainfall modulated intraseasonally by the Madden–Julian oscillation (MJO) and interannually by El Niño–Southern Oscillation (ENSO). A relative minimum in precipitation is observed in July–August, known as the “midsummer drought,” which is associated with a strengthening of the Caribbean low-level jet accompanied by decreased probability of tropical cyclone landfall over Yucatan (Magaña et al. 1999; Karnauskas et al. 2013; Diaz-Esteban and Raga 2017; Perdigón-Morales et al. 2019).

The Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), is used to estimate a 17-yr climatology of the dust component of PM10. Figure 2 shows that during July the largest mean concentrations are over the eastern Atlantic and that values near Yucatan are higher than over Cuba and southern Florida. Thus, IOPs were conducted in July 2017 and July 2018 to maximize the chance of sampling AD plumes.

Fig. 2.
Fig. 2.

Mean monthly PM10 concentration associated with dust derived from MERRA-2 over 2003–19, for June–September. The two sampling sites are represented by a single black dot due to their proximity.

Citation: Bulletin of the American Meteorological Society 102, 8; 10.1175/BAMS-D-20-0172.1

Few studies have quantified the presence of BB plumes in Yucatan. Korontzi et al. (2006) analyzed 3 years of MODIS data and Yokelson et al. (2011) reported evidence of smoke from fires over Yucatan obtained from aircraft measurements during the MILAGRO campaign. Rios and Raga (2018, 2019) provide a 14-yr climatology of burned areas in tropical forests and agricultural lands, estimating their emissions and interannual variability and showing that BB activities peak during March–May. However, the properties of particulate BB emissions have not been systematically studied before.

Even less research has identified the potential presence of AD in the Yucatan Peninsula. Satellite lidar observations (Adams et al. 2012) suggested that during June–August AD particles reach Yucatan within the boundary layer, between the surface and 2 km. Das et al. (2013), from a few surface samples at a single inland location, suggested that phosphorus in the soil was potentially associated with dry and/or wet deposition of AD. Given the paucity of observations in this region, ADABBOY provides valuable, in situ data on the arrival of AD plumes over northern Yucatan.

Aerosol particle characterization.

Physical properties

Annual average PM2.5 and PM10 concentrations in Merida for 2016–18 are less than 9.7 and 24 µg m−3, respectively, close to the 10 and 20 µg m−3 recommended by the World Health Organization (Ramírez-Romero et al. 2021; Muñoz-Salazar et al. 2020). Average background number concentrations for particles >30 nm are <3000 cm−3 in Sisal and Merida (Muñoz-Salazar et al. 2020) and are significantly reduced upon the arrival of cold fronts (Ladino et al. 2019). Episodes of greatly enhanced concentrations (up to 55,000 cm−3) of ultrafine particles (20–200 nm) were observed in Merida, likely associated with new particle formation triggered by photochemistry (Muñoz-Salazar et al. 2020).

AD and BB aerosol plumes are sporadic events, lasting 1–3 days per event, and exhibit large interannual variability. During ADABBOY there were four AD and seven BB events in 2017 and six AD and four BB events in 2018. PM10 in Merida increases significantly when AD plumes arrive, at times more than 300% above background (Ramírez-Romero et al. 2021). The PM10 24-h average observed during AD individual events in 2018 reached ∼38 µg m−3, not exceeding the WHO guideline of 50 µg m−3. Hourly values can be much higher, such as the “giant” AD event from 23 to 27 June 2020 when PM10 values reached as high as 317 µg m−3 (Mayol-Bracero et al. 2020). PM10 influenced by BB increases by as much as 200% above background (Ramírez-Romero 2019). These events deteriorate air quality but the interannual variability in duration and intensity modulates their health and environmental impacts.

The increase in total number concentration under BB and AD conditions is the result of large changes in number and volume particle size distributions. Neither BB nor AD plumes represent near-source, fresh emissions and the distributions have experienced processing in the atmosphere while advected into Yucatan. The number size distribution (Fig. 3a) shows that particles sampled during AD episodes dominate in the size range 0.05–0.3 µm, while BB-influenced are comparable to background. Since the size intervals increase with size, the size distributions are normalized by dividing the number or volume concentration in each size interval by the logarithm of that interval. The large concentration of small particles observed during AD is consistent with measurements made by aircraft near Cape Verde in 2015 (Liu et al. 2018); smaller AD particles with lower sedimentation rates are more likely to reach Yucatan. AD particles have larger concentrations than the background in all size ranges, but BB particles exceed AD concentrations for sizes above 0.3 µm. This is also evident in the volume size distributions (Fig. 3b) in which AD shows a bimodal distribution of comparable concentration, while BB volume dominates the size range from 1 to 5 µm. Volume size distributions are related to mass, assuming a nominal density for each aerosol type (MA, BB, or AD). Selected statistics are summarized in Table 2.

Fig. 3.
Fig. 3.

Particle (a) number and (b) volume concentrations as a function of equivalent optical diameter (EOD) for particles larger than 50 nm stratified by air masses, normalized by the logarithm of the channel size intervals. The distributions are the result of combining measurements from the PMS LasAir with those from the TSI CPC. The volume distributions are computed assuming particle sphericity at the EOD. The sizing uncertainty is on the order of ±20%. The AD distributions (yellow) and the BB distributions (red) were obtained from selected individual plumes that affected Merida. The average background distribution (blue) was obtained from samples not affected by AD or BB in Merida. Vertical error bars indicate one standard deviation and are only shown in the postive direction for clarity.

Citation: Bulletin of the American Meteorological Society 102, 8; 10.1175/BAMS-D-20-0172.1

Table 2.

Statistics derived from the number and volume concentration distributions from samples affected by BB and AD compared to average background samples in Merida (samplers located at 12 m above ground level).

Table 2.

Chemical properties

The percentage elemental composition of particles under background conditions, i.e., under no influence of either BB or AD plumes, are shown for Merida (Fig. 4a) and Sisal (Fig. 4b). Sodium (Na) and chlorine (Cl) are ubiquitous due to proximity to the Gulf of Mexico and Caribbean Sea (Córdoba et al. 2021). Calcium (Ca) is also part of the background composition since Cretaceous sediments form the Yucatan soil. Sulfur (S) observed in Sisal (Fig. 4b) is likely the result of dimethyl sulfide (DMS) emissions by phytoplankton; in fact, January samples in Sisal for marine background indicate that non-sea-salt sulfate makes up more than 50% of the composition (Córdoba 2019). Urban emissions in Merida modify the background composition indicating more S, associated with diesel fuel combustion for power generation and transport (Fig. 4a). Na, Cl, S, and Ca account for more than 70% of the composition of background samples.

Fig. 4.
Fig. 4.

Percentage elemental mass concentration for background samples in (a) Merida and (b) Sisal. (c) Enrichment factors calculated for samples obtained under the influence of BB (red bars) and AD (yellow bars) plumes with respect to the background conditions sampled in Merida. The dashed horizontal line corresponds to enrichment factor equal to 1.

Citation: Bulletin of the American Meteorological Society 102, 8; 10.1175/BAMS-D-20-0172.1

The elemental composition of BB particles depends on the type of fuel burned (Akagi et al. 2011). The red bars in Fig. 4c show the enrichment factor for each element associated with BB emissions, relative to the background samples in Merida. Such samples indicate the expected enrichment of potassium (K), a key plant nutrient released during the combustion of vegetation (Ackerman and Cicek 2017). There is good correlation between the time series of PM2.5 and K for the BB periods. (Pearson correlation coefficient, CCP = 0.6). Analysis also indicates increased S associated with BB. A large fraction of BB particle composition would be carbonaceous so selected samples from 2017 in Merida were analyzed for total carbon (TC =organic + elemental). A significant increase in TC (by ∼70%–80%) is seen in samples of BB versus AD (Córdoba et al. 2021). Elemental components associated with fossil fuel combustion in Merida do not exhibit seasonality, so the difference in TC between BB and AD samples is directly linked to differences in organic carbon, as would be expected from the combustion of vegetation.

The elemental composition of AD has been studied utilizing different methodologies and distances from the sources, e.g., near sources in Africa and downwind over Cape Verde (e.g., Kandler et al. 2011; Bozlaker at al., 2018; Liu et al. 2018), Barbados (Trapp et al. 2010; Kandler et al. 2018), Miami (Prospero et al. 2010), and the Gulf Coast of Texas (Bozlaker et al. 2019). The arrival of AD over Yucatan in July is marked by the large increase in aluminum (Al), silicon (Si), and iron (Fe) as seen in Fig. 4c (yellow bars), compared to the background composition, and modest enrichment of magnesium (Mg) and titanium (Ti), all key elements in mineral dust (Linke et al. 2006; Marsden et al. 2019; Querol et al. 2019). During the AD period, there is excellent correlation between the time series of PM2.5 and Mg (CCP = 0.82), Al (CCP = 0.94), Si (CCP = 0.94), Ti (CCP = 0.89), and Fe (CCP = 0.88). Our results are consistent with the AD elemental profile reported by Bozlaker et al. (2018) from samples in Barbados. Moreover, Bozlaker et al. (2019) measured PM2.5 in Houston and Galveston, Texas, during an AD event between 17 and 25 August 2014 that led to mass doubling, which is also observed in our study. Enrichments of Mg, Al, Si, Ca, Ti, Mn, and Fe in our samples are consistent with their results.

Biological properties

Maritime and continental microorganisms are ubiquitous in Yucatan since temperature and relative humidity favor their proliferation and survival (Ponce-Caballero et al. 2013). Culturable bacteria, fungal propagules and actinos (marine and continental actinobacteria) were detected during ADABBOY (Rodriguez-Gomez et al. 2020). The concentration of fungal propagules is ∼3 times higher than of bacteria, both in Sisal and in Merida (Fig. 5). A threefold to fivefold increase in number concentration of bacteria and fungal propagules is observed between MA and AD, associated with African air masses. Nevertheless, the impact of the local meteorology in the concentration and diversity of bioparticles in the Yucatan Peninsula requires careful evaluation (Rodríguez-Gómez et al. 2020).

Fig. 5.
Fig. 5.

Boxplots of the concentration [CFU m−3] of (a) bacteria and (b) fungal propagules. BB, AD, and MA refer to biomass burning, African dust, and marine aerosol IOPs during 2017 and 2018. Samples are exposed for 5 min and results are counts after 48 h (modified from Rodriguez-Gomez et al. 2020).

Citation: Bulletin of the American Meteorological Society 102, 8; 10.1175/BAMS-D-20-0172.1

While similar bacteria genera were observed during all IOPs, different species were present in MA and AD, e.g., Vibrio spp. were identified in samples of MA during cold fronts, typical of marine environments, while Microbacterium, Sphingomonas, Bacillus, and Streptomyces were isolated and identified during AD episodes, in agreement with Griffin (2007). Cladosporium and Penicillium were the dominant fungi under background conditions whereas during the influence of the AD plumes additional genera, such as Alternaria, Fusarium, and Tricomona/Monillia, were identified by Rodriguez-Gomez et al. (2020).

Low concentrations of bacteria and fungal propagules were found during the BB period (Fig. 5), especially for fungal propagules, in agreement with Ponce-Caballero et al. (2013). BB activities in the tropics are dominant during the dry season with relatively lower relative humidity, a factor that limits the production of microorganisms.

ADABBOY highlighted the interannual variability of the AD plumes arriving in the Yucatan Peninsula that impacts the sampled microbiota (Fig. 5: Merida, AD17 vs Merida, AD18), supporting the hypothesis that African plumes may transport foreign microorganisms onto Mexican territory.

Ice-nucleating properties associated with immersion freezing.

Since most of the precipitation over Yucatan forms in deep clouds at temperatures well below 0°C (Diaz-Esteban and Raga 2019) it is relevant to determine the ice-nucleating abilities of MA compared with BB and AD particles. Figure 6 summarizes the INP concentrations as a function of temperature and aerosol type. The BB particles do not activate at temperatures warmer than −15°C, consistent with other studies (Kanji et al. 2017). In contrast, MA and AD are ice active between −15° and 0°C. AD as INP have been extensively studied by airborne sampling in the eastern Atlantic near Africa (e.g., DeMott et al. 2003; Twohy et al. 2009; Price et al. 2018, among many others) and in the laboratory (Augustin-Bauditz et al. 2014; DeMott et al. 2015). At −15°C the AD plumes sampled in Yucatan contained higher INP concentrations than MA. Nevertheless, the most efficient particles were those collected during the passage of cold fronts under predominantly maritime conditions, with onset freezing temperatures as warm as −3°C (Ladino et al. 2019) and higher active site densities (Córdoba et al. 2021). The activation fraction of most of the AD samples collected in Yucatan are slightly less efficient than airborne filter samples near Cape Verde reported by Price et al. (2018), with AD samples in Yucatan at −20°C about two orders of magnitude less in concentration. Below −20°C the highest INP concentrations were found in AD, followed by MA and BB particles. These results are in excellent agreement with results from other locations as summarized by Kanji et al. (2017). The AD was identified as the main source of INPs in the Yucatan Peninsula during the midsummer season, with a high potential to affect local and regional precipitation. Nevertheless, the MA particles are ubiquitous all year-round, contributing significantly to INPs and precipitation development.

Fig. 6.
Fig. 6.

INP concentration as a function of temperature for three distinct air masses: marine aerosol (MA, blue), biomass burning (BB, red), and African dust (AD, yellow) for particles with sizes ranging between 0.32 and 10 µm. The vertical black lines indicate the standard deviation (modified from Córdoba et al. 2021).

Citation: Bulletin of the American Meteorological Society 102, 8; 10.1175/BAMS-D-20-0172.1

Seawater around Yucatan is a large source of airborne particles, as a result of bubble bursting and wave breaking at the surface, including salts, microorganisms and exudates. Water samples were obtained from the sea surface microlayer (SML) and from the bulk-surface water (BSW) near the Gulf of Mexico coast. The ice-nucleating abilities of water samples from the SML and the BSW were determined using the UNAM-Droplet Freezing Assay. Surface layer waters contain ice active material (Fig. 7), with INP concentrations ranging between 6.0 × 101 and 1.1 × 105 L−1 water at temperatures below −16.5°C (Ladino et al. 2021).

Fig. 7.
Fig. 7.

Frozen fraction curves for the sea surface microlayer (SML, red line) and bulk surface water (BSW, blue line) samples collected at the Gulf of Mexico. The solid black line denotes the homogeneous freezing curve and the shaded areas denote the standard deviation for each sample type (modified from Ladino et al. 2021).

Citation: Bulletin of the American Meteorological Society 102, 8; 10.1175/BAMS-D-20-0172.1

Microorganisms are good INPs (Maki and Willoughby 1978; Yankofsky et al. 1981) and airborne microorganisms were sampled and identified during ADABBOY. Analysis of the 13 bacteria species isolated from air samples—firmicutes, actinobacteria, beta-proteobacteria and gamma-proteobacteria phylum—indicate that they are inefficient INPs (onset freezing temperatures < −18°C) compared to other bacteria, such Pseudomonas syringae (Yankofsky et al. 1981; Wex et al. 2015).

Summary and outstanding questions

ADABBOY identified sources of aerosol particles in the Yucatan Peninsula, documented their properties, and evaluated the impact on local air quality of regional BB emissions and long-range transport of African dust, sporadic sources with different seasonality, revealing significant differences:

  • Under the influence of BB and AD plumes, PM10 mass concentrations increase up to 200% and 300% above background, respectively.

  • The BB number concentrations are comparable to the background at sizes smaller than 0.3 µm while AD concentrations are a factor of approximately two higher than BB and background in the size range from 0.05 to 0.3 µm.

  • BB volume concentrations dominate in the size range 1–5 µm while AD volume size distributions are bimodal.

  • Background aerosol composition is dominated by Cl, Na, and S.

  • BB particles are enriched in K and S, as well as in total carbon and significantly correlated with PM2.5.

  • AD particles are highly enriched in Al, Si, Fe, Mg. and Ti and significantly correlated with PM2.5.

  • Alternaria, Fusarium, and Tricomona/Monillia were identified in AD while Cladosporium and Penicillium are dominant fungi genera in background.

  • BB particles are inefficient INP at temperatures warmer than −20°C whereas both AD and MA are active INP below −10°C.

Data collected during ADABBOY will help constrain models of emission and transport of BB and AD. The temperature dependency of INPs is important for validating current parameterizations of ice crystal formation in tropical regions as a precursor to mixed-phase precipitation in weather and climate models. The broad range of bacteria, fungi, and actinobacteria, known sources of INPs, suggests that they might play a role in cloud formation and precipitation in this region.

This study significantly expands the existing dataset of particle properties in this region and raises several issues to be addressed in future studies:

  • Evaluate mitigation measures for anthropogenic BB activities that degrade air quality and threaten sensitive ecosystems in natural protected areas.

  • Analyze the interannual variability of AD intrusions related to regional air quality.

  • Assess the role of AD related nutrients on biogeochemical processes with expanded sampling of bulk deposition in the Yucatan.

  • Evaluate the role of endemic microorganisms versus those transported to Yucatan by cold fronts, tropical cyclones, and AD plumes.

  • Evaluate the role of regional phytoplankton blooms as INPs.

ADABBOY has documented the near-pristine environment that is polluted yearly by emissions from BB and AD intrusions. BB is an anthropogenic activity that can and should be mitigated in the future. AD events may increase as climate changes and desertification in Africa expands (Liu and Xue 2020). Given that Yucatan is representative of much of the western Caribbean and Central America, in that BB is a common practice and AD also reaches those other areas, long-term monitoring at one or more sites in the region will provide in situ evidence of the future evolution of BB and AD. This first detailed study of particle properties in this region provides a valuable baseline for future studies.

Acknowledgments

ADABBOY was supported by a large number of students that contributed valuable time and effort towards their undergraduate and graduate-level theses, contributing to publications cited. We hereby explicitly acknowledge the important contributions of Manuel Andino, Diego Cabrera, Aline Cruz, Sofia Giordano, Javier Juarez, Aimee Melchum, Joshua Muñoz, Diana Pereira, Zyanya Ramírez, and Camila Rodríguez. The authors also thank to Allan Bertram, Elizabeth García, Gabriel García, Manuel García, Victor García, Wilfrido Gutiérrez (deceased), Jorge Herrera, Lisa Miller, Juan Carlos Pineda, Rosario Pool Chi, Miguel Robles, Alfredo Rodríguez, and Ma. Isabel Saavedra. Funding was provided by Conacyt (FC-2164 and SNI-I1200/16/2019), PAPIIT-UNAM (IN102818, IA108417, and IN111120), RUOA-UNAM, and UADY (SISPROY-FQUI-2018.0003). NASA and NOAA are acknowledged for making MODIS data, MERRA-2 data, and HYSPLIT available.

Data availability statement.

The ADABBOY dataset is available upon request to L.A. Ladino (luis.ladino@atmosfera.unam.mx).

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