Dust and aerosol plumes often blow from Africa into the Atlantic in the spring. They cover thousands of square miles and transport tons of particles. They reach to the forests of Brazil and the Caribbean islands. They have been linked to pollution alerts, soil quality in Brazil, and impact on coral reefs. Images from NASA.
The dust and aerosols plumes also arrive from Asia. Prevailing westerlies in the middle latitudes brings tons of particles from China and other countries. Here is an image of a common situation over China. The land mass is obscured from view due to dust and aerosols, in addition to the clouds. Cloud formation is enhanced by particles in the atmosphere. They provide nucleation centers for water droplet formation, hence clouds.
Measurements have been made of the tiny airborne particles that arrive by air in North America each year according to NASA and university researchers. At least 64 million tons of dust, pollutants, and other particles mix into the air over North America each year. Over 69 million tons of aerosols are produced within North America alone by natural processes, transportation, and industrial sources. This new estimate of airborne aerosols nearly equals our own domestic production according to this study published in Science.
The study estimated that dust from Asia makes up 88% (56 million tons) of the particles reaching North America. These images show modeled dust moving from Asia to North America in April 2010. Credit to Goddard Earth Observing System Model, Version 5 (GEOS-5). Note the density of the model in central China near the Taklimakan Desert. Aerosols are carried by large dust storms usually in the spring. In addition, North America receives many aerosols from Africa and the Middle East.
Aerosol data came from instruments on the Terra satellite and the Cloud-Aerosol LIDAR and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. CALIPSO provides high-resolution vertical profiles of aerosols and clouds. Below is an example of a vertical profile from CALIPSO as it passed over a fire region in Arizona in 2011. The satellite uses LIDAR, light detection and ranging. It is similar to radar but employs light instead of microwave energy. The LIDAR measures the thickness and composition of clouds and aerosol layers and their altitudes.
Significance of the Findings
Scientists are beginning to quantify how many airborne particles reach North America. Determining their impact on warming or cooling the planet is one of the most difficult challenges in climate science.
“This is a crucial step toward better understanding how these tiny but abundant materials move around the planet and impact climate change and air quality,” says Hongbin Yu, lead author and an atmospheric scientist at the University of Maryland and NASA’s Goddard Space Flight Center.
Yu and colleagues observed different particle types and their heights in the atmosphere. That information along with upper atmosphere wind speed data allowed them to estimate the amount of pollution and dust arriving in North America. Dust and pollution particles rise into the atmosphere and can travel for days before settling to Earth. In doing so, they cross many political boundaries.
According to the research, about half of the particles above North American come from someplace else. They come in from high in the atmosphere. And, most of it is dust, not pollution, from sources in other countries. This has important implications for international treaties and agreements about pollution and its passage from nation to nation. Turning off all industry in the nations of the world would still leave us with a large amount of aerosol particulates because it is a larger naturally occurring process than was realized before.
This 3 minute video summarizes the findings and contains some commentary by the scientists involved. It tells of some of the impacts that aerosols have on weather and climate.
What are Aerosols?
About 90 percent by mass, most aerosols have natural origins such as volcanoes, forest fires, and certain plants. Some plants can produce gases that react with substances in the air to cause aerosols such as the “smoke” in the Great Smoky Mountains. Ocean micro-algae produce a sulfurous gas called dimethylsulfide that turns into sulfate aerosols in the atmosphere. Sea salt and dust are two of the most abundant aerosols. Sandstorms blow mineral dust from deserts into the atmosphere. Ocean waves send sea salt aloft. Both of these sources make particles which are typically larger than man-made aerosol particles.
About 10 percent by mass of aerosols are man-made. They are common in the air downwind of urban and industrial areas. Burning fossil fuel makes large amounts of sulfur dioxide that reacts and makes sulfate aerosols. As people clear land and burn off the biomass and organic waste, it makes smoke with a lot of organic carbon and black carbon soot.
What Are Some Implications?
• We don’t fully understand how aerosols affect climate. This study estimates that aerosols from overseas may result in one third of the total solar radiation reduction in North America. Aerosols and clouds seeded by them reflect about a quarter of the Sun’s energy back to space. It also says dust and pollution can change atmospheric circulation patterns. These changes impact weather and climate. How these changes occur and their impacts requires further study. More discussion of these impacts can be found in this link.
• Dust and pollution aerosols can ‘seed’ clouds. This affects rainfall and snowfall patterns. The tiny particles act as a nucleus, or seed, for water droplet formation, or snow flakes. More particles are thought to increase precipitation by this seeding.
• Dust and soot particles land on snow and cause it to become darker and melt sooner. This has implications for the quantity, quality, and timing of our water supplies.
• Changes in weather and climate affect how pollutants get mixed and removed by the atmosphere. A robust modeling system is needed to assess the climate and air quality implications of these new findings.
• Most of the aerosols that cross the Pacific Ocean are high in the atmosphere, and not in the air that we breathe. Only about 5% of that dust enters the continent within the lowest 1.2 miles (2 kilometers) of the atmosphere. So air quality implications may not be significant. The researchers believe controlling pollution near the surface is a more effective way of controlling the health effects of aerosols in our immediate surroundings.
According to Dr. Hongbin Yu …
There has been concern about an emerging Asian economy and the increased pollution that will influence North American air quality and climate. But we found that dust makes large contributions here. So we cannot just focus on pollution. We need to consider dust.
The take-home message is that this is a small world and we share one atmosphere. Every country is influencing other countries’ back yards.