UNDERSTANDING THE TRANSPORT AND IMPACT OF AFRICAN ON THE BASIN

by Joseph M. Prospero and Olga L. Mayol-Bracero

The huge quantities of African dust carried into the Caribbean Basin warrant the formation of an observational program that tracks the impacts of this transport and future changes linked to climate variability.

Fig. 1. A National Aeronautics and Space Administration (NASA) Sea-viewing Wide-Field Sensor (SeaWiFS) satellite composite image of an African dust outbreak on 3 Apr 2010. Dust extends from the coast of to the western Caribbean. The dust outbreak first emerged from Africa in late March; heavy dust transport continued across the region for several weeks. [Image courtesy of Norman Kuring, NASA Goddard Space Flight Center, and GeoEye.]

FRICAN DUST TRANSPORT, METEOROLOGY, AND CLIMATE. The arid regions of North Africa are estimated to emit about 800 Tg yr–1 of dust each year, 70% of A the global total and six times more than the next largest source, (Huneeus et al. 2011). A large fraction of these emissions are carried to the west over the . Satellite images of individual dust outbreaks (Fig. 1) often show dust extending in a continuous plume from the coast of Africa into the Caribbean Basin. Satellite measurements of aerosol properties (e.g., aerosol optical depth; Hsu et al. 2012) clearly show the temporal and spatial distribution of dust 

AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER 2013 | 1329 Unauthenticated | Downloaded 09/24/21 04:11 PM UTC over this huge region. While satellites serve as a climate indices, such as El Niño–Southern Oscilla- valuable tool for studying dust transport, our detailed tion (ENSO) (Prospero and Lamb 2003), the Atlantic understanding of the long-term record of this trans- multidecadal oscillation (AMO) (Evan et al. 2011; port, its link to climate, and the specific impact on the Wang et al. 2012), and the North Atlantic Oscilla- Caribbean Basin rests largely on the aerosol records tion (NAO) (Ginoux et al. 2004; Evan et al. 2006). accumulated at (Prospero and Lamb 2003), Since the late 1980s, rainfall has improved in the Puerto Rico (Gioda et al. 2013; Reid et al. 2003a), , and in some years has been plentiful. Indeed, and Miami (Prospero 1999) . The longest records are there is evidence of “greening” across the region (de from Barbados, where studies began in 1965, and Jong et al. 2011). Nonetheless, Barbados dust con- from Miami, starting in 1974. These measurements centrations have remained higher than pre-drought continue to this day. A portion of these records, from levels. Moreover, in contrast to the first 25 years of 2004 to 2009, is shown in Fig. 2. Concentrations are the record, there is no clear relationship to Sahel greatest in Barbados and show a strong seasonal cycle rainfall or to the cited (and other) climate indices. with a summer maximum and winter minimum. The absence of such linkages makes it difficult to Miami dust concentrations track those in Barbados predict how dust emissions and transport might in a general way. The Miami record differs in that change over the coming decades as climate changes. concentrations are generally less than those in The problem is exacerbated by the inability of models Barbados and the dust transport season is shorter. (Seneviratne et al. 2012) to agree on future rainfall There are large year-to-year differences between trends over large areas of North Africa (including the two records, which are attributed to changes in the Sahel) that are known to be major dust sources large-scale winds and removal by precipitation. The today and in the recent past. similarities and differences between these two sites However, there are factors other than rainfall provide a sense of what we might expect to see across and dust source activity in Africa that could impact the entire basin. transport to the Caribbean (Engelstaedter et al. 2009). The Barbados measurements show that large If we wish to understand present-day variability and changes have occurred over the 48-yr record. Initially to predict future trends, we need a better under- these changes seemed to be linked to African cli- standing of the entire dust cycle, starting with the mate (Prospero and Lamb 2003). Concentrations processes that affect dust mobilization in Africa, the were low in the late 1960s, at the end of a long wet meteorological environment that controls transport phase in West Africa. Transport dramatically in- including the transit time, and finally the removal en creased in the early 1970s in apparent response to route, especially by precipitation. drought and again in the early 1980s when drought There is continuing debate about the nature of was most intense. Over this period trade-wind dust the most active dust sources and the relative im- concentrations were highly anticorrelated to Sahel portance of small-scale sources (dust “hot spots”) precipitation, used as a proxy for general changes in (Ginoux et al. 2012; Okin et al. 2011) such as dry source-region climate (Prospero and Lamb 2003). lakes, playas, and wadis as contrasted to larger-scale There were also suggestions of relationships to major terrain characteristics and climate (Bullard et al. 2011; Muhs 2013; Prospero et al. 2002). Of particular importance, and still highly uncertain, is the role AFFILIATIONS: Prospero—Cooperative Institute for Marine of anthropogenic land disturbance, which could and Atmospheric Studies, Rosenstiel School of Marine and account for a large fraction of current emissions Atmospheric Science, University of Miami, Miami, ; Mayol- (Ginoux et al. 2012; Mahowald et al. 2010). There Bracero—Institute for Tropical Ecosystem Studies, University of have been a number of major field campaigns in Puerto Rico, Río Piedras Campus, San Juan, Puerto Rico North Africa during the past decade that have led to CORRESPONDING AUTHOR: Joseph M. Prospero, Rosenstiel School of Marine and Atmospheric Science, University of Miami, a better grasp of dust mobilization processes, most 4600 Rickenbacker Causeway, Miami, Florida 33149-1031. notably the African Monsoon Multidisciplinary E-mail: [email protected] Analyses program (AMMA; Mari et al. 2011) and the Saharan Dust Experiment (SAMUM; The abstract for this article can be found in this issue, following the table of contents. Heintzenberg 2009). While these and other studies DOI:10.1175/BAMS-D-12-00142.1 have greatly broadened our knowledge of dust pro- cesses in North Africa, we nonetheless continue to In final form 24 January 2013 ©2013 American Meteorological Society lack a good understanding of the meteorological process that are the most important drivers of dust

1330 | SEPTEMBER 2013 Unauthenticated | Downloaded 09/24/21 04:11 PM UTC emissions (e.g., frontal systems, haboobs, mesoscale cyclonic sys- tems, and small-scale turbulent processes; Knippertz and Todd 2012). The Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Obser- vations (CALIPSO) dust product shows a strong link to the seasonal migration of the intertropical con- vergence zone over West Africa and monsoon dynamics (Adams et al. 2012). All these processes could play a fundamental role in dust emissions; the relative importance of these processes could change with season and with changing climate (Engelstaedter and Washington 2007; Williams 2008). Moreover, dust can itself impact Fig. 2. Monthly mean dust concentrations at Barbados and Miami, atmospheric and ocean processes to 2004–09. a degree that could affect weather and climate over the Atlantic and the Caribbean most notably the annual dust cycle at Barbados and Basin. Of particular interest is the evidence that high Miami and the southward shift of the plume in boreal dust concentrations and the meteorological environ- winter that results in transport to ment associated with Saharan air outbreaks could (Prospero et al. 1981) and the Amazon basin (Swap modulate the growth of tropical cyclones (Dunion et al. 1992). Model development is hampered by the and Velden 2004). Evan et al. (2011) suggest that dearth of dust measurements over the oceans. Given variations in dust transport over time are negatively the wide impact of dust over the Atlantic and the correlated to changes over Caribbean Basin, measurements in this region would the tropical Atlantic. These changes, in turn, could contribute greatly to model development both to bet- be linked to the suggested negative correlation ter constrain emission estimates in North Africa and between Atlantic dustiness and hurricane activity. to better characterize transport and deposition over Cloud microphysics could also be affected by African the receptor region. dust, which can serve as both condensation (Twohy Improved model performance will require a more et al. 2009) and freezing nuclei (Cziczo et al. 2013; accurate description of all processes controlling dust Heymsfield et al. 2009). Because of the complexity of mobilization and distribution including the evolution the ocean–atmosphere processes in this region and and persistence of the strong stratification of dust the impact on cloud processes (Rauber et al. 2007) it layers. Early research (Carlson and Prospero 1972; is difficult to quantify the role of dust in this context Prospero and Carlson 1972) postulated the existence at this time. Nonetheless, there is a clear need to focus of the (SAL)—a hot, dry, elevated on these relationships. layer within which the highest dust concentrations are usually found (Reid et al. 2003a). The SAL is a DUST MODELS. Models are essential to the persistent feature in sonde profiles over the region development of an understanding of the entire dust in summer (Dunion 2010). Long-term lidar studies cycle. However, dust models are in an early stage of on Barbados carried out by the University of Miami development. A recent intercomparison of 15 global (http://mplnet.gsfc.nasa.gov/index.html; select models in the Aerosol Comparisons between Obser- “Ragged Point”) and the Max Planck Institute for vations and Models (AEROCOM) project (Huneeus Meteorology in Hamburg (http://barbados.zmaw et al. 2011) shows large disparities. Estimates of global .de/) confirm the often-layered structure of dust emissions span a wide range, from 514 to 4313 Tg yr–1, events although there is considerable variability in and those from North Africa range from 204 to dust vertical distributions. An example of a dust 2888 Tg yr–1. On regional scales, models had difficul- event (Fig. 3a) shows a complex but well-defined ties in reproducing concentrations and deposition, series of layers above the surface mixed layer. Sea-salt

AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER 2013 | 1331 Unauthenticated | Downloaded 09/24/21 04:11 PM UTC fields is clearly evident in the CALIPSO lidar aerosol product (Adams et al. 2012). Figure 4 shows the distribution of dust in boreal summer over Africa, the Atlantic, and Caribbean. Dust appears primarily as an elevated plume that is consistent with the canonical con- cept of a SAL distribution. The layer is clearly seen to persist, although progres- sively weakened, across the Atlantic and into the western Caribbean. The persistence of this layer over such a great distance presents challenges to our understanding of how dust and other aerosols are transported over great distances. Dust models are being used to produce routine forecasts of dust emissions Fig. 3. NASA Micropulse Lidar (MPL) aerosol vertical profile, Ragged Point, and transport. The WMO Barbados, 24 Jul 2009. (a) MPL backscatter (532 nm); (b) aerosol extinc- tion profile at 1235 UTC based on concurrent AERONET measurement of Sand and aerosol optical depth; (c) sonde profile, Grantley Adams Airport, 1200 UTC. Warning Advisory and The sonde shows a classical SAL profile (very dry air associated with a near- Assessment System (SDS- isentropic temperature layer) with the base at about 1500 m and a top at WAS) website (http://sds- 3600 m. Although surface-level dust concentrations were relatively low was.aemet.es/forecast- (7.4 µg m–3), optical depths were moderately high (0.286, 500 nm) and the products/dust-forecasts) Angstrom exponent low (0.15), which suggests relatively high dust concentra- has links to 14 daily fore- tions aloft. The MPL profile shows a well-defined, but complex, aerosol verti- cast products. Of these, cal structure including the presence of (presumed) dust at the top of the SAL. (Barbados sonde data, University of Wyoming Department of Meteorology) 10 include the Caribbean Basin and the Americas in their coverage. These prod- aerosol dominates the lidar backscatter below about ucts, coupled with ground-based aerosol measure- 500-m altitude although dust is also present based on ments in the region, could serve as the basis of an air measurements at Ragged Point. The lidar data cou- quality alerting system as discussed below. pled with concurrent aerosol optical depth measure- ments (http://aeronet.gsfc.nasa.gov/; select “Ragged DUST PARTICLE CHARACTERIZATION. Point”) enables the extraction of the vertical profiles The impact of dust particles on climate, ocean pro- of extinction; in the example (Fig. 3b) the low-level ductivity, terrestrial ecosystems, and human health sea-salt layer and the elevated dust layers are clearly will ultimately depend on the concentrations of dust distinguished. Concurrent sonde data from the local and the chemical and physical properties of indi- airport show the presence of a classical SAL profile of vidual particles. These properties will change during temperature and water vapor. transit, most notably because of the loss of large On a larger scale, the complex and dynamic particles and the shift of the particle size spectrum to relationship between dust source activity in North smaller sizes, but also because of chemical processing Africa, vertical distributions, and meteorological (“aging”) during transit (Prather et al. 2008; Usher

1332 | SEPTEMBER 2013 Unauthenticated | Downloaded 09/24/21 04:11 PM UTC et al. 2003). The impact of these processes would be dust, is an ideal location for the study of properties most noticeable in dust-laden air masses that mix of trace metals and other nutrients in dust and how with pollutants (Shi et al. 2012; Meskhidze et al. they might change with time in the atmosphere; 2005)—for example, European pollutants mixing the Atlantic transit time is relatively long—over a with dust over the . Of particular interest is week—and during transit the dust-laden air in the the effect on the solubility of Fe in dust particles SAL is relatively isolated from mixing with inputs and, after deposition, the impact on ocean primary from other sources (Trapp et al. 2010), which could productivity, the carbon cycle, and climate (Jickells complicate the interpretation of results. et al. 2005). At present, this assessment is limited by our lack of understanding of the properties that IMPACT OF DUST ON HEALTH. The con- render dust-Fe “bio-available.” In the absence of a centration of African dust and other aerosols in the specific understanding of those factors, Fe solubility Caribbean Basin often exceeds the air quality stan- is often used as a proxy. However, there is no general dards linked to health effects (Prospero and Lamb agreement as to a specific protocol for determining 2003). The inhalation of mineral dust is known to solubility; in practice, aqueous solutions of various produce a wide range of physiological responses and compositions and pH levels are used in an effort to impact human health (Plumlee et al. 2006; Morman simulate natural environments ranging from that of and Plumlee 2013). On a worldwide basis, mineral seawater to relatively acid cloud droplets to wetted dust is estimated to be a major health threat (Liu aerosol particles (Buck et al. 2010). African dust is also et al. 2009). However, in the basin there has been a significant, often major, contributor to soil forma- little research on health effects. Of particular interest tion in the Caribbean and the Bahamas (Muhs et al. is the possible role of dust on asthma. Asthma rates 2007, 2012). Trace species, especially phosphorus, are high throughout the Caribbean, comparable to could contribute to soil fertility. In particular, those in urban/industrial environments. Despite African dust is believed to supply critical nutrients this evidence, there has been little research on the to the Amazon basin (Swap et al. 1992); dust could causative factors. Furthermore, there are few data be playing a similar role throughout the Caribbean. on aerosol properties that might help to identify Therefore, there is a great need for a more coordinated linkages to health. We lack even the simplest of met- approach that combines laboratory, field observa- rics for “respirable” particles as defined by the U.S. tion, and modeling experiments to test hypotheses Environmental Protection Agency (EPA) [i.e., par- regarding the mechanisms of iron dissolution and ticles less than 2.5-µm diameter (PM 2.5) and those to assess their relative importance in different envi- less than 10-µm diameter (PM 10) as determined ronments. The Caribbean, as the receptor of African by EPA criteria; www.epa.gov/air/criteria.html].

Fig. 4. The temporal and spatial distribution of June–August dust as classified by the CALIPSO Vertical Feature Mask (VFM) retrievals over the period 16 Jun 2006–16 Jun 2010. The scale shows the frequency of occurrence (FoO) as a percentage of total retrievals during that period. The vertically averaged FoO values are projected on the horizontal plane. The averaged vertical profiles across the latitudes are projected on the panel at the right of the domain and the averaged profiles along the longitudes are shown on the back plane. Note the cutoff at FoO 15%. Thus the fact that CALIPSO shows only an elevated plume over the Atlantic should not be interpreted as due to the absence of dust (Adams et al. 2012).

AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER 2013 | 1333 Unauthenticated | Downloaded 09/24/21 04:11 PM UTC Measurements made in field programs (Li-Jones It is important to link measurements of aerosol and Prospero 1998; Prospero et al. 2001; Reid et al. properties over the Caribbean Basin to those made in 2003b) show that about half of the African dust mass the source region, North Africa. These measurements conforms to PM 2.5 and over 90% to PM 10. Based should be related to those made in other continental on the daily measurements on Barbados we would regions where pollutant species are usually dominant, expect that dust concentrations will frequently exceed and also to other dusty regions so as to better eluci- the EPA and World Health Organization (2006) 24-h date dust-specific health effects. guidelines for PM 2.5 and PM 10 aerosols. To this end, aerosol “super sites” should be estab- There is a clear need for controlled studies of lished with instrumentation comparable to the super human health effects at the population level and at sites designed for air quality studies such as those in the individual level. These should focus on vulner- the United States (Hersey et al. 2011). One site should able populations (i.e., the young and the aged) for be located on, or near, the coast of Africa to better both acute and chronic exposures. Also of interest characterize the “source” aerosol and the meteoro- are the mechanisms by which dust and other aero- logical processes related to source activity. There are sols act to impact health and the identification of several sites where research facilities and infrastruc- bio-active components: allergens, biological materi- ture support are available: at Dakar, Senegal (Drame als, fungal spores, metals (iron, aluminum, arsenic, et al. 2011); at the Surface Ocean Lower Atmosphere lead, cadmium), endotoxins, and organics. Despite (SOLAS) station on Sao Vicente, Islands the widespread occurrence of dust around the globe, (Müller et al. 2010); and on Tenerife, Canary Islands, a literature survey by de Longueville et al. (2013) where there is a well-developed World Meteorological shows remarkably few studies of the health impacts Organization (WMO) site that follows an extensive in dust-rich natural environments, globally a total of aerosol protocol (Rodríguez et al. 2011). In addition 50 papers. Of these, 11 dealt with African dust, and to aerosol measurements, lidars and aerosol optical almost all focused on the impact on . Health depth measurements would provide detailed infor- studies in North Africa coupled with parallel studies mation on vertical distributions and the relationship in the Caribbean Basin would be useful in assessing to the meteorological setting associated with dust the scope of dust impacts. To this end, there should outbreaks (Drame et al. 2011). be an effort to devise protocols that could be used in A second site should be located in the Caribbean both regions so that the results can be compared and to characterize aerosols in the receptor region. The contrasted in a quantitative manner. preferred site would be Barbados because of its loca- Finally, we need to develop means of communi- tion in the axis of the dust plume in summer, the cating risk to impacted populations and to design season when the entire Caribbean basin is impacted interventions based on the human health effects. by dust (Adams et al. 2012). Barbados is also favored Approaches include the issuance of air quality alerts because of its long aerosol record and the presence of based on real-time monitoring networks, the use of infrastructure and active research facilities. In addition remote sensing data, modeling forecasts such as those to the University of Miami presence at Ragged Point, cited above, and the development of response plans. Barbados, at an adjacent site the Max Planck Institute for Meteorology (Hamburg) carries out cloud and SUMMARY AND CONCLUSIONS. In light of aerosol studies (http://barbados.zmaw.de/) that are the changes observed in the Barbados dust record, expected to continue for many years. It is also impor- there is a clear need for long-term measurements in tant for the aerosol community to establish working the Caribbean Basin that focus on characterizing relationships with those focusing on environmental trends in aerosol concentrations and in critical aero- health. Barbados is of particular interest; because of the sol properties. Sites should be established at various relatively homogeneous population, it is increasingly locations to define the temporal and spatial variabil- becoming the focus of research on the genetic aspects ity of African dust and also of aerosols from other of health, including asthma (Mathias et al. 2010). sources (e.g., pollutants, biomass burning). Sampling There is a need to facilitate education and training should conform to PM 2.5 and PM 10 protocols so in aerosol measurements in the region. The University that the potential for health impacts can be better of Puerto Rico–Río Piedras could serve this function assessed. Precipitation collectors should be collocated based on their long experience and ongoing research with the aerosol samplers so as to relate deposition activities. These take place at two sites: the aerosol rates and chemistry to aerosol concentrations and monitoring program at Cape San Juan site located properties. on the easternmost end of Puerto Rico (in operation

1334 | SEPTEMBER 2013 Unauthenticated | Downloaded 09/24/21 04:11 PM UTC since 2004; www.esrl.noaa.gov/gmd/aero/net/cpr aerosol Fe and other trace elements in the North /index.html) and a cloud-forest site, Pico del Este, Atlantic Ocean: Observations from the A16N

in El Yunque National Forest (1051 m ASL) (Gioda CLIVAR /CO 2 repeat hydrography section. Mar. et al. 2013), which focuses on aerosol–cloud interac- Chem., 120, 57–70. tions. The Caribbean Institute of Meteorology and Bullard, J. E., S. P. Harrison, M. C. Baddock, N. Drake, Hydrology (CIMH), located on Barbados, could play T. E. Gill, G. McTainsh, and Y. Sun, 2011: Preferential a role in research and education through its close dust sources: A geomorphological classification de- links with the meteorological community. CIMH, signed for use in global dust-cycle models. J. Geophys. an organ of the Caribbean Meteorological Organi- Res., 116, F04034, doi:10.1029/2011JF002061. zation (CMO), coordinates the joint scientific and Carlson, T. N., and J. M. Prospero, 1972: The large-scale technical activities of the meteorological and hydro- movement of Saharan air outbreaks over the northern meteorological services of 16 Caribbean countries; it equatorial Atlantic. J. Appl. Meteor., 11, 283–297. provides training to this community and also, more Cziczo, D. J., and Coauthors, 2013: Clarifying the broadly, through its relationship with the University dominant sources and mechanisms of cirrus cloud of the West Indies system, located throughout the formation. Science, 340, 1320–1324, doi:10.1126/ Caribbean. science.1234145. Given the scope and complexity of the African de Jong, R., S. de Bruin, A. de Wit, M. E. Schaepman, dust phenomenon, it is clear that the full assessment and D. L. Dent, 2011: Analysis of monotonic greening of its impact on the Caribbean Basin will require a and browning trends from global NDVI time-series. coordinated research effort. We see the need for an Remote Sens. Environ., 115, 692–702. organizational mechanism that would facilitate such de Longueville, F., P. Ozer, S. Doumbia, and S. Henry, a broad-scale integrated research program across the 2013: Desert dust impacts on human health: An basin that, ideally, should be closely linked to the exten- alarming worldwide reality and a need for studies in sive research programs now being carried out in North West Africa. Int. J. Biometeorol., 57, 1–19. Africa. Such a program will be essential to character- Drame, M., G. S. Jenkins, M. Camara, and M. Robjhon, ize and track the changes in dust aerosol emissions 2011: Observations and simulation of a Saharan air in Africa that we might expect with climate change. layer event with a midtropospheric dust layer at Dakar, Senegal, 6–7 July 2010. J. Geophys. Res., 116, ACKNOWLEDGMENTS. This paper is based in part D21204, doi:10.1029/2011JD016368. on the proceedings of the First International Workshop on Dunion, J. P., 2010: Rewriting the climatology of the the Long-Range Transport and Impacts of African Dust on tropical North Atlantic and atmo- the Americas, October 6-9, 2011, San Juan, Puerto Rico. We sphere. J. Climate, 24, 893–908. thank the International Global Atmospheric Chemistry —, and C. S. Velden, 2004: The impact of the Saharan Project (IGAC), the primary sponsor of the workshop, and air layer on Atlantic activity. Bull. the University of Puerto Rico for hosting the workshop Amer. Meteor. Soc., 85, 353–365. (UPR’s Resource Center for Science and Engineering, Engelstaedter, S., and R. Washington, 2007: Atmospheric Center for Hemispherical Cooperation in Research and controls on the annual cycle of North African dust. J. Education in Engineering and Applied Science, Research Geophys. Res., 112, D03103, doi:10.1029/2006JD007195. Initiative for Scientific Enhancement, and Long-Term — , — , and N. Mahowald, 2009: Impact of Ecological Research program). We also acknowledge the changes in atmospheric conditions in modulating workshop support of the NOAA Center for Atmospheric summer dust concentration at Barbados: A back- Science and Merck. O. L. Mayol-Bracero and J. M. Prospero trajectory analysis. J. Geophys. Res., 114, D17111, acknowledge support from U.S. National Science Founda- doi:10.1029/2008JD011180. tion Grants AGS 0936879 and AGS-0962256, respectively. Evan, A. T., A. K. Heidinger, and P. Knippertz, 2006: Analysis of winter dust activity off the coast of West Africa using a new 24-year over-water ad- REFERENCES vanced very high resolution radiometer satellite Adams, A. M., J. M. Prospero, and C. Zhang, 2012: dust climatology. J. Geophys. Res., 111, D12210, CALIPSO-derived three-dimensional structure of doi:10.1029/2005JD006336. aerosol over the Atlantic basin and adjacent conti- —, G. R. Foltz, D. Zhang, and D. J. Vimont, 2011: nents. J. Climate, 25, 6862–6879. Influence of African dust on ocean–atmosphere Buck, C. S., W. M. Landing, J. A. Resing, and C. I. variability in the tropical Atlantic. Nat. Geosci., 4, Measures, 2010: The solubility and deposition of 762–765.

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