You are here

Sun Photometer

Automatic (commercial) and hand-held (made by FFS staff) sun photometers are used to study air quality by generating many measurements against which satellite algorithms (a set of rules for solving a problem) may be validated.

Function and Objective: 

A sun photometer measures the optical properties of the atmosphere. It measures the brightness of several wavelengths or bands of light coming from the sun. When a sun photometer is used on the earth’s surface, brightness in distinct spectral bands tells us about the atmosphere. Above the atmosphere, the brightness of light in each spectral band is more or less constant, with most of the variation being accounted for by small, seasonal changes in earth-sun distance. Differences between the brightness recorded on the surface and the brightness one would expect to see if no atmosphere were present must be due to the atmosphere. Once researchers know how the atmosphere affects sunlight, they can start to infer what the atmosphere contains.

While each aerosol has its own unique size, aerosols of a common type tend to occur in one or two average size ranges, known as size distributions. The size distributions are typically recognizable because they take the form of the familiar "bell curve", one bell curve for each average size. Sun photometry is based on the premise that the size distribution's peak and width will produce a distinctive transmission spectrum. Measuring the sun's brightness in many spectral bands from the surface enables researchers to calculate the atmosphere's transmission spectrum. Once all of the effects not due to aerosols have been subtracted out, the result is an aerosol transmission spectrum.

The aerosol transmission spectrum can be used to calculate the size distributions of all the aerosols in the entire atmospheric column between the instrument and the sun. Because the sun is a very strong source of light, transmission measurements can be made through a fairly dense haze. On the other hand, no information is available about the altitude of specific aerosols.

The Fire Lab’s atmospheric investigations are typically regional or national in scope, focused on smoke from biomass (e.g., trees and grass) burning, and are concentrated in areas most severely impacted by biomass burning events (e.g., tropical areas in South America and Africa).

Automatic sun photometers

The Fire Lab owns an automatic sun photometer as part of a global effort to establish aerosol climatology. This effort, the Aerosol Robotic Network (AERONET), is led by NASA. The instruments participating in AERONET are identical and their calibration is maintained by NASA. These systems require minimal operator intervention: once or twice a month the on-board clock needs to be set, and once every year or two it needs to be sent back to NASA for calibration. During normal operation, these units charge their batteries with a solar panel, take readings on a prescribed schedule, and uplink their data to satellites in geosynchronous orbit. NASA then processes the data and posts it on the AERONET website.

Current data from the Missoula station.

Hand-held sun photometers

The Fire Lab owns one commercial hand-held sun photometer and more than 100 hand-held units designed by the Fire Lab. Historically, the Fire Lab has collaborated with NASA on intensive data collection campaigns for a specific region and for a specific season. NASA supplies 5 to 10 automatic sun photometers and the Fire Lab supplies 30 to 40 hand-held sun photometers. To ensure that data from the two instrument types are comparable, one hand-held unit is typically co-located with each automatic sun photometer. The remainder of the units are deployed across the region of study to capture how aerosols are distributed around the area. The Fire Lab’s hand-held sun photometers were designed with only one button. Operators push the button to turn the instrument on, and then point the sun photometer at the sun. A metal alignment jig helps operators aim the unit without looking directly into the sun. Once roughly aligned, an operator wiggles the unit until the reading reaches a maximum. Pressing the button again stores the maximum reading into memory along with the time. Data are taken at several times during the day.

Photo: The Fire Lab’s automatic sun photometer is part of a network of such devices used to establish a global aerosol climatology.

Photo: automatic sun photometer
Modified: Apr 11, 2014