Mark Schleifstein / NOLA.com 20 June 2020
A vast cloud of Saharan dust now crossing the Atlantic Ocean will arrive in south Louisiana by Wednesday, bringing with it the potential to dampen hurricane threats, create spectacular sunsets and exacerbate respiratory problems for people with health conditions, including children.
The dust, which consists of tiny mineral rock sand particles from desert areas in Africa, is a frequent summer visitor to North America. Though it’s nearly invisible, it makes its presence felt in a variety of ways.
“We can definitely expect an uptick in asthma and allergy conditions, especially in children,” said Dr. James Diaz, director of the environmental and occupational health sciences program at the LSU Health Sciences Center in New Orleans.
Asthma is a risk factor for COVID-19, and the increased risk from the dust could include increases in cases of the Kawasaki-like inflammatory syndrome that COVID-19 has caused in a small number of children, Diaz said.
Scientists group the dust into two size groups: PM 2.5, for particles measuring 2.5 microns or smaller, and PM 10, for particles 10 microns or smaller.
The U.S. Environmental Protection Agency has set a 24-hour standard of 65 microns per cubic meter for PM 2.5 and 150 microns per cubic meter for PM 10, and it’s likely that the Saharan dust, combined with particulate matter already in the air from chemical pollution, will require the state Department of Environmental Quality to issue air quality alerts for some state locations next week.
“Desert sand is microscopic quartz, compared to beach sand, which is microscopic seashell,” Diaz said. “The particles are charged and can carry pathogens into the lungs, like viruses, bacteria and fungi. Therefore, respiratory infections will increase as well.”
He said the particles also are likely to increase problems with patients with chronic obstructive pulmonary disease related to bronchitis, and could also cause heart attacks among those with coronary artery disease.
The health issues associated with Saharan dust have been documented in recent years in studies conducted among residents in the Caribbean and in Houston, according to Joseph Prospero, emeritus professor of atmospheric sciences at the University of Miami, who has been studying the dust outbreaks for 60 years.
Saharan sand outbreaks are common in the early summer months, Prospero said, typically forming over broad areas of the southern Saharan desert in Algeria, Libya and northern Niger. Intense heating of the Earth’s surface, combined with windy conditions, causes the particles to be lifted into the upper levels of the atmosphere, to heights of 3 1/2 to 4 1/2 miles. There, they can be blown west as part of a layer of hot, dry air atop easterly tropical waves crossing the Atlantic Ocean.
The present dust outbreak is streaming west from an 850-mile-wide stretch of the African coast, and its outer edge was already raining particles as small as 70 microns on Barbados and French Guiana, he said.
The most visible effect of the Saharan dust is likely to be more colorful sunrises and sunsets in southeastern Louisiana, said Kevin Gilmore, a meteorologist with the Slidell office of the National Weather Service.
“As the sun starts filtering through the haze caused by the particles, it can cause really brilliant sunrises and sunsets, with dark orange to pink colors,” he said.
Another positive effect stems from the warm, dry air carrying the Saharan dust and the high-wind layer in which it is contained. The winds can chop apart clouds trying to form thunderstorms, and the dry air can reduce the ability of the atmosphere to support thunderstorm creation, necessary to form tropical storms and hurricanes.
“The Saharan Atmospheric Layer is a bit like Big Ben,” said Jason Dunion, a research scientist with NOAA’s Hurricane Research Center in Miami, who tracks the dust to study how it reduces the ability of hurricanes to form in the Atlantic. “It’s actually quite amazing how, like clockwork, SAL activity — larger outbreaks that reach farther west — ramps up in June and keeps at a fast and furious pace from late June until early August.”
Data collected from satellites and instruments dropped from research airplanes into the dust layer can be uploaded into hurricane forecast models to help predict the layer’s effects. He said data collection efforts off the coast of Africa by NASA and around Barbados by NOAA have been rescheduled from this summer to next year because of pandemic issues.
“We’ll be bringing new instruments to look at the winds, moisture and dust in the SAL, and will also have our trusty dropsondes (instrument packages released from planes) to sample the full depth of SAL outbreaks,” Dunion said.
“Our plan is to transmit all of our dropsonde data in real time during the campaign so that modeling centers can ingest the data and so that it will be available to forecasters at the National Hurricane Center,” he said.
Other researchers have been studying the effects of the dust as it falls into the ocean. Recent studies have linked iron contained in the sand to blooms of trichodesmium, a one-celled cyanobacteria also known as sea sawdust, in waters off southwest Florida. The bacteria blooms die and sink to the bottom of the Gulf, where they decompose and release dissolved organic nitrogen.
The nitrogen, in turn, stimulates blooms of Karenia brevis, a type of toxic algae that causes red tides.
The freshwater of the Mississippi and Atchafalaya rivers help keep local salinity levels low enough for red-tide events from spreading west of the Mississippi border, said Nancy Rabalais, a marine biologist with LSU. A different cyanobacteria that is found in deeper waters off the Louisiana Gulf Coast might be affected by the dust, but has not been seen as a problem, she said.
The dust also stimulates growth of many groups of marine plankton, which can have both positive and negative effects, said Elizabeth Ottesen, a microbiologist at the University of Georgia.
“This can be both good and bad from our perspective,” she said. “It can increase productivity and feed the marine food web, but it can also stimulate the growth of harmful algae and bacteria.”
In a recent study, she and her team of researchers observed a series of blooms where most of the organisms were harmless to humans. But she said the blooms did include species of vibrio, which can cause disease.