Around the Americas

Shooting the sun: Measuring sunlight at the Earth's surface

sunphotometer in use at seaIf it happens to be daytime when you are reading this you can get a sense of how bright the sun is by simply glancing out the window. The amount of sunlight you see will depend upon a number of factors including the time of day, season, amount of cloud cover, how close you live to the equator, and something else: tiny particles called atmospheric aerosol. Our eyes may be pretty good at determining solar brightness but they aren't able to measure the presence of aerosol, which is what scientists and crew onboard the Ocean Watch are doing with a small and highly sophisticated instrument called a sunphotometer.

Around the Americas' sunphotometer project, sponsored by the Joint Institute for the Study of the Atmosphere and Ocean in partnership with NASA, is being used to collect information about atmospheric aerosol. Aerosol, or tiny particles in the atmosphere, reflect solar radiation back into space and interfere with how much sunlight makes it to the earth's surface. Clouds, which are made up of small ice crystals and droplets of water, are perhaps the most obvious example of atmospheric aerosol. These tiny particles are also noticeable in the haze or smog that exists near big cities.

Atmospheric aerosol has both natural and human-made causes. Examples of natural causes include sea spray, volcanic eruptions, dust storms, and wild fires. Aerosol particles produced by humans include soot from unburned fuel from cars, pollution from factories and coal or oil-fired power plants, or smoke plumes caused by burning forests and clearing land for agriculture. No matter what causes the particles, whether they are natural or human-made, the sunphotometer has the ability to measure how much solar radiation is reaching the earth, which tells scientists about the total number of particles that are in the atmosphere.

How does the sunphotometer work and what does it measure?

The sun has to be visible for the sunphotometer to work so it is only useful when there is not a lot of cloud-cover. The instrument is used by pointing it directly at the sun. A small image of the sun appears on a bulls-eye screen that can be safely observed by the human eye. When the sun is steadily centered in the bulls-eye the photometer automatically records and stores the amount of sunlight.

The sunphotometer measures something called aerosol optical depth which is the clarity of the atmosphere between the sun and the point on earth where the sunphotometer is being held. Sun above the sailboat Ocean WatchThe instrument is calibrated to know the intensity of sunlight at the top of the atmosphere which the sunphotometer compares to the intensity measured at the earth's surface. The difference between the values at the top of the atmosphere and the surface of the earth is proportional to the aerosol optical depth. In calculating the aerosol optical depth, the sunphotometer accounts for the latitude, longitude, and date and time of the measurement. It also corrects for the known optical depth of air molecules in the path of the measurement so that the final result measures only the optical depth due to particles that are in the pathway of the sun.

Why is this important and what does it have to do with the ocean?

While the sunphotometer readings may seem to be the smallest and least glamorous of the scientific projects onboard the Ocean Watch, the data are of enormous significance to atmospheric scientists because aerosol optical depth has never been measured on a continuous voyage around the American continents. The sunphotometer information fills in gaps in NASA's data network and will be made available to scientists around the world. Sunphotometer readings help track pollution plumes from large cities, dust from existing deserts and new areas that are becoming deserts, the burning of trees and grasslands in wildfires, and deforestation and bio-mass burning for human development and agriculture.

The voyage of the Ocean Watch is primarily a project to raise awareness about the health of our oceans but the health of the world's oceans depends very much upon the health of the atmosphere. The ocean and atmosphere are referred to by scientists as being "coupled" because they behave in some ways like a single entity. The atmosphere and ocean exchange heat, water, and chemical compounds like carbon dioxide, which is contributing to ocean acidification. We cannot, for example, begin to understand the complexity of something like climate change unless we understand the interactions between the ocean and atmosphere.

More about the sunphotometer

Crew log: Sunphotometer in use»

NASA's Maritime Aerosol Network (MAN)»