The deposition of phosphorus (P) from African dust is believed to play an important role in bolstering primary productivity in the Amazon Basin and Tropical Atlantic Ocean (TAO), leading to ...sequestration of carbon dioxide. However, there are few measurements of African dust in South America that can robustly test this hypothesis and even fewer measurements of soluble P, which is readily available for stimulating primary production in the ocean. To test this hypothesis, we measured total and soluble P in long-range transported aerosols collected in Cayenne, French Guiana, a TAO coastal site located at the northeastern edge of the Amazon. Our measurements confirm that in boreal spring when African dust transport is greatest, dust supplies the majority of P, of which 5% is soluble. In boreal fall, when dust transport is at an annual minimum, we measured unexpectedly high concentrations of soluble P, which we show is associated with the transport of biomass burning (BB) from southern Africa. Integrating our results into a chemical transport model, we show that African BB supplies up to half of the P deposited annually to the Amazon from transported African aerosol. This observational study links P-rich BB aerosols from Africa to enhanced P deposition in the Amazon. Contrary to current thought, we also show that African BB is a more important source of soluble P than dust to the TAO and oceans in the Southern Hemisphere and may be more important for marine productivity, particularly in boreal summer and fall.
Full text
Available for:
BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Soils in the Amazon Basin are deficient in phosphorus, essential to soil fertility. Previous studies suggested that African mineral dust deposited to Amazonian soils served as an important source of ...phosphorus that enhances soil fertility. These studies lacked the quantitative measurements essential to validate estimates. Here we present daily measurements of mineral dust and PM10 carried in the trade winds at Cayenne, French Guiana, during 2002–2017. MERRA‐2 model dust concentrations showed excellent agreement with measurements over this period. Consequently, we used MERRA‐2 to estimate temporal and spatial deposition rates to Amazonia. Our annual deposition rate, 8–10 Tg dust, is substantially lower than previous estimates. Deposition rates are greatest over the northern and northeastern regions of South America. In contrast, rates are low in central Amazonia. Our results raise questions about the impact of African dust on soil fertility in Amazonia. However, African aerosol transport carries other aerosol species that could play a role in soil fertility, including biomass‐burning products known to contain substantial concentrations of phosphorus. Our study highlights the need for more measurements of aerosol and precipitation chemistry over wider expanses of South America in order to better characterize aerosol chemical and physical properties, to identify aerosol sources, and to constrain model estimates. We show that over 2002–2017 dust transport to South America was negatively correlated to rainfall over the Sahel in North Africa. Long‐term monitoring is needed to capture changes in transport from Africa that might occur as a result of climate change.
Plain Language Summary
The Amazon Basin plays a major role in climate by removing huge quantities of carbon dioxide (CO2) from the atmosphere and storing the carbon in vegetation. This removal partially offsets the impact of CO2 emitted by humans and the consequent rate of global warming. There is a concern about decreasing soil fertility in the Amazon Basin because of the loss of phosphorus, an essential plant nutrient, due to intense tropical weathering and biomass burning. Previous work suggested that large quantities of dust are transported from Africa to South America every year and deposited to the Amazon. Dust contains phosphorus and other nutrients that could mitigate soil losses and increase Amazonian soil fertility. However, estimates of the amount of dust carried to the Amazon are uncertain because of the lack of aerosol measurements. Our multiyear measurements of African dust in the trade winds in French Guiana, coupled with a chemical transport model, suggest that significant quantities of dust reach the heart of the Amazon Basin although the amounts are substantially less than had been previously estimated. This raises questions about the long‐term status of soil fertility in the Amazon and the future impact on the carbon cycle. We also find that the quantities of dust transported to South America are inversely linked to rainfall in North Africa. Consequently, climate change will affect dust transport to South America.
Key Points
We use 15 years of aerosol measurements in trade winds at Cayenne, French Guiana, to quantify the transport of African dust to South America in boreal spring
Dust transport may be supplying significant amounts of nutrients, including phosphorus, that are important for maintaining soil fertility in the Amazon Basin
The MERRA‐2 model yields deposition rates to the Amazon that are substantially smaller than rates from previous studies
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The accumulation of ultraviolet (UV)‐absorbing compounds (flavonoids and related phenylpropanoids) and the resultant decrease in epidermal UV transmittance (TUV) are primary protective mechanisms ...employed by plants against potentially damaging solar UV radiation and are critical components of the overall acclimation response of plants to changing solar UV environments. Whether plants can adjust this UV sunscreen protection in response to rapid changes in UV, as occurs on a diurnal basis, is largely unexplored. Here, we use a combination of approaches to demonstrate that plants can modulate their UV‐screening properties within minutes to hours, and these changes are driven, in part, by UV radiation. For the cultivated species Abelmoschus esculentus, large (30–50%) and reversible changes in TUV occurred on a diurnal basis, and these adjustments were associated with changes in the concentrations of whole‐leaf UV‐absorbing compounds and several quercetin glycosides. Similar results were found for two other species (Vicia faba and Solanum lycopersicum), but no such changes were detected in Zea mays. These findings reveal a much more dynamic UV‐protection mechanism than previously recognized, raise important questions concerning the costs and benefits of UV‐protection strategies in plants and have practical implications for employing UV to enhance crop vigor and quality in controlled environments.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
North African dust reaches the southeastern United States every summer. Size-resolved dust mass measurements taken in Miami, Florida, indicate that more than one-half of the surface dust mass ...concentrations reside in particles with geometric diameters less than 2.1 µm, while vertical profiles of micropulse lidar depolarization ratios show dust reaching above 4 km during pronounced events. These observations are compared to the representation of dust in the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) aerosol reanalysis and closely related Goddard Earth Observing System model version 5 (GEOS-5) Forward Processing (FP) aerosol product, both of which assimilate satellite-derived aerosol optical depths using a similar protocol and inputs. These capture the day-to-day variability in aerosol optical depth well, in a comparison to an independent sun-photometer-derived aerosol optical depth dataset. Most of the modeled dust mass resides in diameters between 2 and 6 µm, in contrast to the measurements. Model-specified mass extinction efficiencies equate light extinction with approximately 3 times as much aerosol mass, in this size range, compared to the measured dust sizes. GEOS-5 FP surface-layer sea salt mass concentrations greatly exceed observed values, despite realistic winds and relative humidities. In combination, these observations help explain why, despite realistic total aerosol optical depths, (1) free-tropospheric model volume extinction coefficients are lower than those retrieved from the micro-pulse lidar, suggesting too-low model dust loadings in the free troposphere, and (2) model dust mass concentrations near the surface can be higher than those measured. The modeled vertical distribution of dust, when captured, is reasonable. Large, aspherical particles exceeding the modeled dust sizes are also occasionally present, but dust particles with diameters exceeding 10 µm contribute little to the measured total dust mass concentrations after such long-range transport. Remaining uncertainties warrant a further integrated assessment to confirm this study's interpretations.
African dust is transported to South America (SA) every winter and spring. Hypotheses suggest that either Western or Central North Africa (e.g., Bodélé Depression) is the main source of transported ...dust, yet these notions remain largely untested with geochemical data. Using 2 years of isotopic measurements (strontium and neodymium) of African dust collected in SA integrated into a statistical model, we identified strong interannual variability in dust source region. Central North Africa supplied 44% of long‐range transported dust in winter 2016 while the Western region accounted for 53% of dust in winter 2014. We propose the variability is due to differences in the strength of the Libyan High and precipitation over the Gulf of Guinea and Atlantic Ocean between the 2 years. Our findings can improve constraints of dust nutrient deposition and predictions of how changes in climate impact the source and magnitude of dust transported to the Amazon.
Plain Language Summary
Dust is blown from North Africa to the Amazon and the western Atlantic Ocean in winter and spring. Identifying the area within North Africa where dust originates is the subject of intense debate, with leading hypotheses disagreeing on whether the Bodélé Depression (Central North Africa) or Western North Africa provide dust that fertilizes the Amazon. Here, we present a more nuanced hypothesis: dust from both the Central and Western North African dust source regions are important with the dominate source depending on the prevailing meteorological factors and precipitation patterns. Our hypothesis is supported by isotopic fingerprinting that was used to determine the source area of dust transported to South America (SA). We then integrated our data into a statistical model, which quantifies the proportion of dust from each North African region that contributes to the dust we collected in SA. Because dust source location dictates the amount and solubility of associated nutrients, changes in dust source impacts nutrient deposition to the Amazon and Tropical Atlantic Ocean. Additionally, identifying the factors that control dust source location can provide clues into how past and future changes in climatological conditions affect dust transport.
Key Points
Western North African dust sources dominated transport to the Amazon in winter 2014 while Central dominated in winter 2016
Interannual variability in dust source is likely due to differences in meteorology and precipitation
Sources of North African dust transported to the Atlantic Ocean and Amazon could change as climate changes altering nutrient deposition
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Disentangling inputs of aeolian dust, ice-rafted debris (IRD), and eroded continental detritus delivered by ocean currents to marine sediments provide important insights into Earth System processes ...and climate. This study uses Sr-Nd-Pb isotope ratios of the continent-derived (lithogenic) fraction in deep-sea core TN057-6 from the subantarctic Southern Ocean southwest of Africa over the past 150,000 y to identify source regions and quantify their relative contributions and fluxes utilizing a mixing model set in a Bayesian framework. The data are compared with proxies from parallel core Ocean Drilling Program Site 1090 and newly presented data from potential South America aeolian dust source areas (PSAs), allowing for an integrated investigation into atmospheric, oceanic, and cryospheric dynamics. PSA inputs varied on glacial/interglacial timescales, with southern South American sources dominating up to 88% of the lithogenic fraction (mainly Patagonia, which provided up to 68%) during cold periods, while southern African sources were more important during interglacials. During the warmer Marine Isotope Stage (MIS) 3 of the last glacial period, lithogenic fluxes were twice that of colder MIS2 and MIS4 at times, and showed unique isotope ratios best explained by Antarctic-derived IRD, likely from the Weddell Sea. The IRD intrusions contributed up to 41% at times and followed Antarctic millennial warming events that raised temperatures, causing instability of icesheet margins. High IRD was synchronous with increased bioavailable iron, nutrient utilization, high biological productivity, and decreased atmospheric CO 2 . Overall, TN057-6 sediments record systematic Southern Hemisphere climate shifts and cryospheric changes that impacted biogeochemical cycling on both glacial/interglacial and subglacial timescales.
Coastal southeast Florida experiences a wide range of aerosol conditions, including African dust, biomass burning (BB) aerosols, as well as sea salt and other locally-emitted aerosols. These aerosols ...are important sources of cloud condensation nuclei (CCN), which play an essential role in governing cloud radiative properties. As marine environments dominate the surface of Earth, CCN characteristics in coastal southeast Florida have broad implications for other regions with the added feature that this site is perturbed by both natural and anthropogenic emissions. This study investigates the influence of different air mass types on CCN concentrations at 0.2% (CCN0.2%) and 1.0% (CCN1.0%) supersaturation (SS) based on ground site measurements during selected months in 2013, 2017, and 2018. Average CCN0.2% and CCN1.0% concentrations were 373 ± 200 cm−3 and 584 ± 323 cm−3, respectively, for four selected days with minimal presence of African dust and BB (i.e., background days). CCN concentrations were not elevated on the four days with highest influence of African dust (289 ± 104 cm−3 0.2% SS and 591 ± 302 cm−3 1.0% SS), consistent with high dust mass concentrations comprised of coarse particles that are few in number. In contrast, CCN concentrations were substantially enhanced on the five days with the greatest impact from BB (1408 ± 976 cm−3 0.2% SS and 3337 ± 1252 cm−3 1.0% SS). Ratios of CCN0.2%:CCN1.0% were used to compare the hygroscopicity of the aerosols associated with African dust, BB, and background days. Average ratios were similar for days impacted by African dust and BB (0.54 ± 0.17 and 0.55 ± 0.17, respectively). A 29% higher average ratio was observed on background days (0.71 ± 0.14), owing in part to a strong presence of sea salt and reduced presence of more hydrophobic species such as those of a carbonaceous or mineral-dust nature. Finally, periods of heavy rainfall were shown to effectively decrease both CCN0.2% and CCN1.0% concentrations. However, the rate varied at which such concentrations increased after the rain. This work contributes knowledge on the nucleating ability of African dust and BB in a marine environment after varying periods of atmospheric transport (days to weeks). The results can be used to understand the hygroscopicity of these air mass types, predict how they may influence cloud properties, and provide a valuable model constraint when predicting CCN concentrations in comparable situations.
•African dust plumes do not perturb CCN concentrations in southeast Florida.•CCN concentrations are greatly increased on days influenced by biomass burning.•Heavy rainfall reduces CCN concentrations.•The duration varies for CCN concentrations to restore after periods of heavy rain.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The equatorial North Atlantic Ocean (NAO) is a nutrient‐limited ecosystem that relies on the deposition of long‐range transported iron (Fe)‐containing aerosols to stimulate primary productivity. ...Using microscopy, we characterized supermicron and supercoarse mode African aerosols transported to the western NAO in boreal winter/spring. We detected three particle types including African dust, primary biological aerosol particles, and freshwater diatoms (FDs). FDs contained 4% Fe by weight due to surficial dust inclusions that may be susceptible to chemical processing and dissolution. FDs were typically larger than dust particles and comprised 38% of particles between 10 and 18 μm in diameter. The low density, high surface‐area‐to‐volume ratio, and large aspect ratios of FD particles suggest a mechanism by which they can be carried great distances aloft. These same properties likely increase the residence time of FDs in surface waters thereby increasing the time for Fe dissolution and their potential impact on marine biogeochemical cycles.
Plain Language Summary
Atmospheric aerosols from Africa are transported by the trade winds to the western equatorial North Atlantic Ocean every winter and spring and can contain nutrients, such as iron (Fe). In this study, we measured the size and composition of supermicron (diameter (d) > 1 μm) aerosols collected at a site on the northeast coast of South America. Using electron microscopy, we found three distinct Fe‐containing particle types: mineral dust, freshwater diatoms from African paleolakes, and pollen grains; all three particle types extended into the supercoarse mode (d > 10 μm). Particle asphericity increased with increasing particle size and could explain in part the long‐range transport of supercoarse particles. Electron mapping of freshwater diatoms also revealed surficial Fe‐rich inclusions. Once deposited in the ocean, the asphericity and light density of freshwater diatom particles likely increases their residence time and therefore, the time for Fe dissolution in the surface ocean compared to dust.
Key Points
Three types of iron‐containing particles were observed including freshwater diatoms that comprised 38% of particles between 10 and 18 μm
Aspect ratios increased with particle size suggesting asphericity may aid in transporting supercoarse particles
Low density and high aspect ratios of diatoms can increase residence times in surface waters and the likelihood of iron dissolution
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The accumulation of UV-absorbing compounds (flavonoids and other phenylpropanoid derivatives) and resultant decrease in the UV transmittance of the epidermis in leaves (TUV), is a primary protective ...mechanism against the potentially deleterious effects of UV radiation and is a critical component of the overall acclimation response of plants to changing UV environments. Traditional measurements of TUV were laborious, time-consuming and destructive or invasive, thus limiting their ability to efficiently make multiple measurements of the optical properties of plants in the field. The development of rapid, nondestructive optical methods of determining TUV has permitted the examination of UV optical properties of leaves with increased replication, on a finer time scale, and enabled repeated sampling of the same leaf over time. This technology has therefore allowed for studies examining acclimation responses to UV in plants in ways not previously possible. Here we provide a brief review of these earlier studies examining leaf UV optical properties and some of their important contributions, describe the principles by which the newer non-invasive measurements of epidermal UV transmittance are made, and highlight several case studies that reveal how this technique is providing new insights into this UV acclimation response in plants, which is far more plastic and dynamic than previously thought.
•We review various approaches to studying UV sunscreen protection in plants.•New optical techniques can non-destructively measure epidermal UV transmittance.•Key insights have been gained on the nature of plant acclimation to UV radiation.•Findings indicate that UV-shielding is more flexible than previously considered.•Many plants can adjust UV-screening protection on a diurnal basis.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
Marine aerosols are a globally significant contributor to aerosol-cloud-climate interactions; however, the impact that different sources of pollution and natural emissions from the ocean have on the ...water uptake properties of marine aerosols remains largely underexplored. Here we present measurements of the cloud condensation nuclei (CCN) activation of marine aerosols taken in a coastal, marine environment impacted by sea spray aerosol and different sources of pollution. The hygroscopicity parameter, κ, was found to range from <0.1 up to 1.4 with a campaign-average value of 0.22 ± 0.12. Smaller particles were less hygroscopic than larger ones, and κ varied diurnally and temporally as a function of air mass transport conditions. Measurements made using aerosol time-of-flight mass spectrometry (ATOFMS) revealed that heterogeneous reactions, sulfates, and temporal differences in the observed particle types had the largest impacts on the observed κ values. The aerosol mixing-state was also found to affect κ. Temporal differences between freshly-emitted soot and aged soot internally mixed with sulfates, likely emitted from ships, had the largest impact on diurnal variations in κ. Our results further demonstrate the significant impact that pollution and the aerosol mixing-state have on aerosol-cloud interactions in the marine boundary layer.
PDF
