On a global scale, African dust is known to be one of the major sources of mineral dust particles, as these particles can be efficiently transported to different parts of the planet. Several studies have suggested that the Yucatán Peninsula could be influenced by such particles, especially in July, associated with the strengthening of the Caribbean low-level jet. Although these particles have the potential to significantly impact the local air quality, as shown elsewhere (especially with respect to particulate matter, PM), the arrival and impact of African dust in Mexican territory has not been quantitatively reported to date.
Two short-term field campaigns were conducted to confirm the arrival of
African dust on the Yucatán Peninsula in July 2017 and July 2018 at the
Mérida atmospheric observatory (20.98
The second largest natural contribution of atmospheric particles, worldwide,
after sea spray, is mineral dust (Pey et al., 2013). Although volcanoes and
soil dust from agricultural activities are significant sources of mineral
dust (Walker, 1981; Tegen et al., 2004), the largest sources are the deserts that are
distributed around the world (Goudie and Middleton, 2006). Africa is
considered one of the most important of these sources as it emits ca. 800 Tg yr
African dust particles are efficiently transported far from their emission source (Perry et al., 1997; Chiapello et al., 1997). According to Middleton and Goudie (2001), there are different trajectories that African dust experiences around the world. Among the most important, African dust particles can be transported to the western Mediterranean and Europe (Karanasiou et al., 2012; Perez et al., 2008; Prodi and Fea, 1979; Salvador et al., 2014), to the eastern Mediterranean and the Middle East (Ganor and Mamane, 1982; Ganor et al., 2010; Athanasopoulou et al., 2016), and towards the southern African continent (d'Almeida, 1986; Resch et al., 2008). Additionally, African dust is transported across the Atlantic Ocean to the United States, Mexico, the Caribbean region, and South America (Prospero et al. 1981; Bravo et al., 1982; Perry et al., 1997; Chiapello et al., 1997; Prospero and Lamb, 2003; Venero-Fernández, 2016; Barkley et al., 2019; Kramer et al., 2020). The long-range transport of African dust over the Atlantic represents 25 % of the total emissions from the Saharan Desert (Shao et al., 2011). This transport is favored in the Northern Hemisphere during the summer (i.e., from June to September) within a dry and hot elevated layer called the Saharan Air Layer (SAL) (Carlson and Prospero, 1972; Prospero and Carlson, 1972; Karyampudi and Carlson, 1988; Tsamalis et al., 2013; Weinzierl et al., 2016).
During the summer, the SAL ascends to altitudes between 5 and 7 km through interactions with cool marine air masses (Adams et al., 2012; Chouza et al., 2016; Korte et al., 2018). Dunion and Velden (2004), Dunion and Marron (2008), and Dunion (2011) studied the characteristics of the air masses that reach the North Atlantic and the Caribbean region during the boreal summer months. They found that there are three distinct air masses: a moist tropical air mass (MT), the SAL, and midlatitude dry air intrusions (MLDAIs). Each type of air mass is associated with unique thermodynamic and kinematic characteristics, and they have a wide range of possible origins. However, the SAL and MLDAI air masses have distinct flow patterns across the North Atlantic, which allows one to differentiate between these masses by tracking their origin. In contrast, their distinctly unique moisture characteristics allow for the differentiation of the MT from SAL air masses (Dunion, 2011).
There are different methods for the detection of the long-range transport of African dust and its presence in different regions around the world. For several decades, the tracking of dust events has been studied using remote sensing (Chiapello et al., 1999; Dunion and Velden, 2004; Foltz and McPhaden, 2008; Prospero et al., 2002; Liu et al., 2008; Voss and Evan, 2020). Ground- and space-based tools such as light detection and ranging (LIDAR) and satellite sensors (e.g., the Moderate Resolution Imaging Spectroradiometer, MODIS, and the Visible Infrared Imaging Radiometer Suite, VIIRS) provide the aerosol spatial distribution with altitude in terms of the aerosol optical depth (AOD), mass concentration, and particle size distribution (Zhang and Reid, 2006; Jackson et al., 2013).
Another useful tool is the reanalysis from global climate models that assimilates, in a statistically optimal way, satellite and ground observations. The reanalysis produces continuous, four-dimensional fields of different atmospheric variables of interest, contrasting with the observations that may be spatially and temporally sparse (Cohn, 1997; Kalnay, 2003; Rienecker et al., 2011; Schutgens et al., 2010). The use of reanalysis, considering its inherent uncertainties, has become an essential tool in the atmospheric research community (Gelaro et al., 2017). For example, the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model has been successfully used to track the transport of African dust particles (e.g., Ashrafi et al., 2014; Prospero et al., 2005). HYSPLIT uses meteorological data from different modeling sources, including the NCEP–NCAR (National Centers for Environmental Prediction–National Center for Atmospheric Research) reanalysis model (Stein et al., 2015).
The transport of African dust can also be evaluated with the NASA Global Modeling and Assimilation Office (GMAO) MERRA-2 reanalysis (Prospero et al., 2020). MERRA-2 (Version 2 of the Modern-Era Retrospective analysis for Research and Applications) is the first multidecadal reanalysis that assimilates both meteorological and aerosol data from various ground- and space-based remote sensing sources (Gelaro et al., 2017; Randles et al., 2017). Despite some deficiencies, previous studies have demonstrated that the MERRA-2 aerosol assimilation system does indeed show considerable skill in simulating numerous observable aerosol properties (e.g., Buchard et al., 2015, 2016, 2017; Randles et al., 2017). MERRA-2 has been previously used to study the effects of aerosol particles in the Earth system in several studies focused on dust-related phenomena. For example, Buchard et al. (2017) showed the benefit of the MERRA-2 assimilation for the retrieval of the seasonality, vertical distribution, and magnitude of the dust surface concentrations during an episode of dust transport from Africa to the Caribbean. Later on, Veselovskii et al. (2018) showed the consistency of the MERRA-2 aerosol products with Mie–Raman lidar observations performed in West Africa during a smoke and dust mixing event. Similarly, Grogan and Thorncrof (2019) studied the characteristics of African easterly waves and their relationship with synoptic-scale plumes of Saharan mineral dust. More recently, Bibi et al. (2020) studied atmospheric dust load and deposition fluxes along the North African coast of the Mediterranean Sea, and Aldhaif et al. (2020) studied dust events impacting the East Coast of the United States.
The in situ monitoring of aerosol properties, such as aerosol size and mass
distribution, is very useful to determine their influence on local air
quality and human health (Querol et al., 2019). Hence, different studies
have been carried out in the Caribbean islands and Florida to quantify the
impact of African dust on the local air quality (Prospero, 1999; Prospero
and Mayol-Bracero, 2013; Prospero et al., 2014). In Barbados, the monitoring
of the atmospheric aerosol mass began in 1965, whereas in Miami, Florida, it
began in 1974 and continues to the present (Prospero and Mayol-Bracero,
2013). In Barbados, it is estimated that 50 % of the PM
The chemical and mineralogical composition of particles also plays an
important role in the identification of African dust in the receptor regions
(Nenes et al., 2014). The most abundant minerals present in these particles
are silicates (quartz), clay minerals (kaolinite, illite, chlorite,
palygorskite), feldspars (albite, anorthite), and carbonates (calcite)
(Goudie and Middleton, 2006; Querol et al., 2019; Broadley et al., 2012).
The major oxides in Saharan dust are SiO
Although the arrival of African dust in Mexico has been suggested for decades (e.g., Bravo et al., 1982; Prospero, 1999; Lenes et al., 2012), to our knowledge, there has not been a comprehensive study, published in the open, peer-reviewed literature, that documents this atmospheric phenomenon. For the first time, in this study we document the arrival of African dust on the Yucatán Peninsula for two consecutive years (i.e., 2017 and 2018) using in situ and remote sensing measurements, reanalysis, back trajectory analysis, and complementary meteorological observations.
The Yucatán Peninsula is located in the southeast of Mexico. It borders
the GoM to the north; the Atlantic Ocean to the east; and
the Caribbean Sea, Guatemala, and Belize to the south. The Yucatán has
characteristics that are unique to this region (Plasencia, 1998). For
example, its uniform terrain, the absence of rivers, and the type of soil,
which is formed by Cretaceous sediments that do not present mineralization and are
rich in calcium, commonly called “Laja de Yucatán” (Plasencia, 1998)
sets the Yucatán aside from other regions of Mexico. The average
temperature of the Yucatán Peninsula ranges from 25 to
35
Location of the sampling site at the School of Chemistry of the Autonomous University of Yucatán (FC-UADY) and the three World Meteorological Organization (WMO) radiosonde stations located on the Yucatán Peninsula: Mérida International Airport, Mexico (WMO index: 76 644); Cancún, Mexico (WMO index: 76 595); and Philip S. W. Goldson International Airport, Belize (WMO index: 78 583).
Summary of the measured variables and the instrumentation used.
Aerosol particles were continuously monitored with sensors installed at the School of Chemistry of the Universidad Autónoma de Yucatán (FC-UADY), located in the central-western part of the city (Fig. 1), as part of the African Dust And Biomass Burning Over Yucatan (ADABBOY) project. Table 1 lists the instrumentation used to characterize particle physical, optical, and chemical properties. The Partisol and MiniVol were installed on the rooftop of the FC-UADY, whereas the other instruments were maintained in an environmentally controlled area where they sampled from inlets connected to a ventilated chimney that extended approximately 1.5 m above the roof. This measurement site is part of the University Network of Atmospheric Observatories (RUOA) supported by the National University of Mexico (UNAM). Two intensive sampling periods were conducted between 11–31 July 2017 and 30 June–17 July 2018.
The particulate mass concentration was monitored continuously with
PM
The total number concentration of particles with sizes approximately larger
than 50 nm was measured by a condensation particle counter (CPC 3010, TSI)
at a sampling rate of 1 Hz with a flow rate of 1.0 L min
PM
Elemental analysis was performed on each filter using X-ray fluorescence
(XRF) with the X-ray spectrometer at Laboratorio de Aerosoles, Instituto de
Física, UNAM (Espinosa et al., 2012). The X-ray tube was made by Oxford
Instruments (Scotts Valley, CA, USA), and a Rh anode and an Amptek X-123 SDD
spectrometer (Bedford, MA, USA) were used. The samples were irradiated for
900 s working with a current of 500
The local and regional meteorological conditions were monitored using
different approaches. The RUOA meteorological sensors were placed at the
rooftop of the FC-UADY (Table 1) and continuously measured the wind speed and
direction, air temperature, relative humidity, solar radiation, and
precipitation. To derive the regional and vertical distribution of
meteorological conditions, radiosondes and reanalysis were used. The
information provided by the radiosondes launched was from three World
Meteorological Organization (WMO) stations on the Yucatán Peninsula, as
shown in Fig. 1, located at Mérida (Mérida International
Airport, Mexico (WMO index: 76 644), Cancún (WMO index: 76 595), and
Belize (Philip S.W. Goldson International Airport, Belize; WMO index: 78 583).
The processed radiosonde data were obtained from the University of Wyoming
(
Hourly total precipitable water vapor and three-dimensional 3-hourly aerosol
mixing ratio data were obtained from the MERRA-2 reanalysis (GMAO, 2015a, b). The aerosol properties in MERRA-2 were simulated with the Goddard
Chemistry Aerosol Radiation and Transport model (GOCART), which takes the sources, sinks, and chemistry of 15 externally mixed aerosol
mass mixing ratio tracers into
account: dust (five noninteracting size bins), sea salt
(five noninteracting size bins), hydrophobic and hydrophilic black and
organic carbon (BC and OC, respectively; four tracers), and sulfate
(SO
The air mass back trajectories were calculated using the HYSPLIT model from
the National Oceanic and Atmospheric Administration (NOAA). In conjunction
with the in situ measurements, the back trajectories were calculated considering
the maximum concentration of PM reported by the PM
Mass concentrations of PM
Several studies have shown that air quality (PM
MERRA-2 precipitable water (black solid line) and precipitable water values estimated from
radiosonde measurements (blue dashed line), for the three WMO radiosonde
stations located on the Yucatán Peninsula for
Figure 2 also shows the elemental composition obtained from the XRF analysis
(16 elements) for five ADPs observed during the 2017 and 2018 field
campaigns. In addition, 1 d from each field campaign was selected to
determine the elemental background composition. The selected days are 10–11 July
in 2017 and 6 July in 2018. These days were chosen because the
PM
High levels of sodium (Na, pink), chlorine (Cl, turquoise blue), sulfur (S,
dark orange), and calcium (Ca, light green) were found in the background
samples, corresponding to
Interestingly, the elemental composition of the airborne particles collected
during the ADPs showed higher concentrations of silica (Si, dark yellow),
aluminum (Al, light purple), and iron (Fe, dark purple) than those in the
background particles (Fig. 2). While the Si concentrations are approximately 3
times larger than the baseline, Al and Fe increased by 8 and 12 times,
respectively. To corroborate the relationship between the increase in PM and
the African dust, each of the 16 elements analyzed by XRF were correlated
with the PM
Additionally, it is important to note that Al, Si, K, and Fe are common
oxides found in African dust composed of minerals and clays such as quartz
(SiO
The 3 h time series of the vertical profile of the estimated dust
content from MERRA-2 for the 1 July–14 August period for
To confirm that no aerosol sources other than the African dust were the
origin of the high PM peaks observed in Mérida in July 2017 and 2018, the
PM
Daily mean of PM
Finally, although none of the different meteorological variables monitored
at the surface level were found to correlate with the PM
To evaluate the source of the ADPs observed in Mérida from a large-scale
perspective, we focus on the classification of tropical air masses in the
North Atlantic and the Caribbean region during the boreal summer months
proposed by Dunion (2011). We used HYSPLIT to estimate the trajectories of
different air masses that reached Mérida during the July–August periods of 2017
and 2018. HYSPLIT trajectories for the 2017 and 2018 ADPs point to an
African origin and, therefore, suggest that these air masses are either MT
or SAL (See Fig. S6). To differentiate the MT from SAL, we focused on
their distinctly unique moisture characteristics. Dunion (2011) proposes
that a threshold of 45 mm of total precipitable water vapor (PWV), which
corresponds to the total amount of water vapor contained in the atmospheric
column from the surface to the top of the troposphere (AMS, 2000), can be
used to differentiate dry from moist air masses. This value is consistent
with other studies that use PWV to identify dry-air days (e.g., Hankes and
Marinaro, 2016), and as deep tropical convection begins to increase above
a critical PWV value of 50 mm (Holloway and Neelin, 2009). Note that PWV is
given by the vertical integral of the mixing ratio
Dispersion diagrams of the surface dust mixing ratio from MERRA-2
(
Figure 3 shows the time series of PWV for the July–August 2017 and 2018 periods at each WMO radiosonde site. The black solid line shows PWV from MERRA-2, available from the Vertically Integrated Diagnostics (GMAO, 2015a), together with PWV estimated using Eq. (1) from the available radiosonde profiles shown as the dashed blue lines. One caveat is that there is a striking lack of radiosonde data in the periods of interest. Nevertheless, we can see good agreement between the available observed PWV and that of PWV from MERRA-2. Therefore, the latter can be used as a good approximation for PWV in the region to differentiate moist from dry air masses. In Fig. 3, the periods where PWV is less than 45 mm are highlighted in red. These periods show dry air masses that coincide with air mass trajectories with an African origin (i.e., 22–24 July, 27–28 July, and 4 and 6–7 August in 2017, and 10–12 July, 13–15 July, 16–17 July, 23–26 July, and 9–12 August in 2018), allowing us to conclude that these dry air masses have mainly SAL characteristics.
The arrival of African dust in Mérida was also explored from the MERRA-2
dataset. Figure 4 shows the time series of the estimated vertical profiles
of the 3 h time series of the dust mixing ratio from MERRA-2 at Mérida for
2017 and 2018. It shows that the events corresponding to the arrival of dry
air masses from Africa displayed in Fig. 3 nicely correlate with high dust
mixing ratios, strongly supporting the hypothesis of SAL air reaching the
Yucatán Peninsula. Figure 4 also shows that the July–August period of 2018
was particularly active with frequent arrivals of dust in the region, which is in
agreement with the higher PM
Finally, the arrival of African dust plumes over the Yucatán Peninsula was confirmed by investigating the AOD detected by the MODIS Aqua and Terra satellites for July 2017 and 2018, as shown in Figs. S8 and S9. Although this information cannot be used to perform quantitative analysis, the AOD images allow us to confirm the arrival of African dust plumes on the Yucatán Peninsula. Additionally, as in Figs. 2, 3, and 4, the AOD images also show that the African dust plumes activity was higher in 2018 than in 2017. Previous studies have also used the AOD from MODIS to identify the arrival of African dust. For example, Koren et al. (2006) tracked the long-range transport of dust from the Bodélé Depression (north central Africa) to the Amazon Basin. Similarly, Kalashnikova and Kahn (2008) demonstrated that it is possible to observe the evolution of African dust plumes over the Atlantic Ocean with MODIS. Additionally, Kaufman et al. (2005) identified and quantified the transport and deposition of mineral dust over the Atlantic Ocean using MODIS data.
The daily mean PM
Despite these differences, Fig. 5 shows that the observations at the RUOA
station have variations similar to those of MERRA-2. Figure 6 shows the
dispersion diagram of the daily mean surface dust mixing ratio from MERRA-2
vs. PM
For the first time, the arrival of African dust into Mexican territory is
quantitatively verified. The arrival of African dust particles in Mérida
significantly degraded the local air quality as PM
As shown in the present study, combining ground-based off-line and online sensors provides robust evidence of the arrival of African dust; however, we also show that the combination of back trajectories with radiosondes as well as the estimated surface dust mixing ratio from MERRA-2 are powerful tools that can be exploited when in situ information is missing, especially in developing countries where the necessary instrumentation is scarce.
Continuous monitoring of the arrival of African dust is of high importance not only in the Caribbean islands but also at other sites in Latin America such as Mexico, Belize, Guatemala, and Honduras. Additionally, epidemiological and statistical studies to track down the number of hospital admissions caused by respiratory issues before and after the arrival of African dust is urgently needed on the Yucatán Peninsula. This will allow policy-makers and local authorities to understand how strong the African dust impact is on local health and the need for better forecasting of such events.
Data are available upon request from the corresponding author.
The supplement related to this article is available online at:
CRR, GBR, and LAL designed the field campaigns and the experiments. CRR, MFC, HAO, DR, TA, and LAL carried out the aerosol measurements. CRR and AJ analyzed the remote sensing data. JM and HAO performed the chemical analyses. GBR, DB, DR, JSK, JYH, and LAL installed the equipment and provided the infrastructure for the ADABBOY project. CRR, AJ, and LAL wrote the paper, with contributions from all coauthors.
The authors declare that they have no conflict of interest.
The authors thank the University Network of Atmospheric Observatories (RUOA) for providing meteorological and criteria pollution data. Alejandro Jaramillo acknowledges the fellowship from DGAPA at UNAM. The authors also wish to express their gratitude to Elizabeth Garcia, Juan Carlos Pineda, Aline Cruz, and Javier Juarez for their invaluable help and support.
This research has been supported by the Consejo Nacional de Ciencia y Tecnología (Conacyt; grant no. FC-2164) and the Universidad Autónoma de Yucatán (grant no. SISPROY-FQUI-2018-0003).
This paper was edited by Sachin S. Gunthe and reviewed by Cassandra Gaston and two anonymous referees.