Science & Technology for Exploration

Sun Photometer

A sun photometer is a portable scientific instrument that measures the Sun’s energy as it reaches Earth’s surface. These measurements help scientists better understand atmospheric conditions, improve climate models, and validate satellite observations.

A person’s hand holds the controller of a MICROTOPS II sun photometer. A GPS is on the table to the left.

A Microtops II sun photometer (right) and GPS (left) used to collect measurements. Image courtesy of Shari Rohret, 2026 Pacific Islands Mapping Expedition. Download largest version (jpg, 1 MB).

How Does a Sun Photometer Work?

A sun photometer measures the direct solar radiation reaching Earth’s surface at multiple wavelengths. By pointing the instrument directly at the Sun, scientists can determine how much sunlight is lost as it travels through the atmosphere before reaching the sensor. As sunlight passes through the atmosphere, tiny particles called aerosols can scatter or absorb some of that energy. Aerosols include dust, smoke, pollution, and, over the ocean, sea salt. Sun photometer measurements are used to calculate aerosol optical depth (AOD), a measure of how much sunlight is blocked by aerosols in the atmosphere.

To ensure accurate measurements, the sun photometer must have an unobstructed view of the Sun. Even thin, high-altitude cirrus clouds can scatter sunlight and interfere with observations, making cloud-free conditions essential for collecting high-quality data.

On the left is an image of the sun shining in a blue sky. On the right is an image of the sun shining through thin, white cloud coverage.

Left: Ideal conditions for sun photometer measurements, with no visible cloud cover. Right: Thin cirrus clouds cover the Sun, impeding data collection. Image courtesy of Shari Rohret, 2026 Pacific Islands Mapping Expedition. Download largest version (jpg, 179 KB).

Why Are Sun Photometers Important?

Sun photometer measurements provide scientists with information about the amount and characteristics of aerosols in Earth’s atmosphere. Aerosols influence how much sunlight reaches the Earth’s surface, affect cloud formation, and can impact air quality, weather, and climate. By measuring AOD over time and across different regions, scientists can track events such as dust storms moving across oceans, smoke from large wildfires, or volcanic ash spreading through the atmosphere. These observations are also used to improve climate and air quality models and to verify the accuracy of aerosol measurements taken by satellites.

Sun photometer measurements collected aboard NOAA Ship Okeanos Explorer contribute to NASA’s AERONET (AErosol RObotic NETwork) program through the Maritime Aerosol Network. While AERONET stations on land provide long-term atmospheric observations around the world, measurements collected at sea help fill critical gaps over the ocean, where permanent observing stations are scarce.

A world map. Parts of the ocean are covered by dots in the colors blue, green, yellow, orange, red, and violet. Blue dots are the most common, while violet dots are the least common.

Maritime Aerosol Network global coverage, showing cruise tracks and daily averages of aerosol optical depth. Blue dots indicate the lowest measurements of aerosol optical depth (AOD) while violet dots indicate the highest measurements. Image courtesy of AERONET Maritime Aerosol Network, NASA. Download largest version (jpg, 581 KB).

The ocean covers more than 70% of Earth’s surface, yet atmospheric observations there remain limited. Research vessels and other ships of opportunity participating in the Maritime Aerosol Network help fill these gaps, providing data from parts of the world that would otherwise be difficult to monitor. These observations complement satellite measurements and land-based monitoring stations, providing scientists with a more complete picture of the distribution and properties of atmospheric aerosols. 

By Shari Rohret, 2026 John A. Knauss marine policy and science communication fellow