Automatic (commercial) and hand held (made in-house) sun photometers are used to study air quality by generating many in situ measurements against which satellite algorithms may be validated.

The Fire Science Lab uses sun photometry to study atmospheric aerosols. We have a range of instruments. Our fully automatic instrument participates in an ongoing global effort to establish aerosol climatology, taking readings only at the Fire Science Lab. We deploy our hand held instruments during the burning season to regions identified as requiring further study.

PRINCIPLE INVESTIGATOR

Wei Min Hao, Supervisory Research Chemist

FUNCTION AND OBJECTIVES

A sun photometer measures the brightness of several colors of light coming from the sun. When the sun photometer is used on the surface of the earth, these colors (known as "spectral bands") tell us about the atmosphere. It can do this because we know that 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 the small, seasonal changes in earth-sun distance. Therefore, differences between the brightness we record on the surface and the brightness we would expect to see if no atmosphere were present must be due to the atmosphere.

Once we know what the atmosphere did to the sunlight, we can start to infer what the atmosphere contains. There are two primary methods to do this. The first is to measure absorption of light in a single band, specific to a particular substance. Ozone, for instance, is known to very strongly absorb a set of very specific spectral bands. Water vapor can also be detected with this method. Measurement of these spectral bands can tell us how much ozone or water vapor is between the instrument and the sun.

The second method involves the measurement of absorption in many spectral bands at once. This method is typically used to detect the presence (and relative abundance) of aerosols in the atmosphere. Aerosols are very tiny solid particles which are suspended in the air. These aerosols come in a whole distribution of sizes, even if they are made from the same substance. For instance, there may be tiny crystals of sea salt suspended in the air, and each crystal will have its own unique size. Likewise, if an aerosol made of desert dust or smoke particles were to be present, each particle would have its own size.

Even though aerosols have their own unique sizes, aerosols of a common type do tend to all be clumped around one or two average sizes. These clumps, known as 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 enables us to calculate the atmosphere's transmission spectrum. Once we subtract out all of the effects we know are not due to aerosols, we have an aerosol transmission spectrum.
The aerosol transmission spectrum can be used to calculate the size distribution of all the aerosols in the entire atmospheric column between the instrument and the sun. Depending on the objective, this approach could be considered beneficial or detrimental. Because the sun is a very strong source of light, transmission measurements can be made through fairly dense haze. On the other hand, no information about the altitude of specific aerosols is available (i.e., they could be very near the ground or in the stratosphere).

Obviously, sun photometry is better suited for the study of widespread hazy conditions than for the study of smoke plumes. A smoke plume tends to be very dense, and it is also very localized. To try and get a reading of the smoke plume, you would have to drive around until you were in the plume's shadow. Even then, a plume from a substantial fire may be thick enough to completely obscure the sun.
Our atmospheric investigations are typically regional or national in scope, are focused on smoke from biomass burning (trees and grass), and are concentrated in areas most severely impacted by biomass smoke events (tropical areas in South America and Africa). For a single study, our hand held instruments were used in collaboration with NASA to examine human-caused aerosols in the Baltimore area. For all of these studies, we try to maximize the number of locations from which we collect data, which in effect means that we try to maximize the number of instruments.
Two different styles of sun photometer are employed by the Fire Science Lab: fully automatic and hand held. Commercially manufactured units are available for both styles. Prices are roughly $25000 for an automatic unit and $5000 for a hand held unit, depending on the options desired. To maximize the number of instruments available for these studies, we designed our own hand held unit which could be built for about $500 in parts. After several years service, we upgraded these units for approximately $1000 each. More than 100 of these units were produced. One of these units is pictured above.

Automatic sun photometers

The Fire Science Lab owns an automatic sun photometer which participates in a global effort to establish an 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, also on a prescribed schedule. NASA then processes the data and posts it on the web.

Current data from our Missoula station.

In addition to merely participating in the AERONET effort, our automatic sun photometer can serve as a calibration reference for our many hand held units. By including our AERONET sun photometer in our calibration plan, we can ensure that the results from our hand held units yield data which may be intercompared with data from the AERONET system.

Hand held sun photometers

The Fire Science Lab owns one commercial hand held sun photometer and more than one hundred hand held units of our own design (pictured below). Historically, we have collaborated with NASA on intensive data collection campaigns for a specific region and for a specific season. NASA supplies five to ten automatic sun photometers and we supply 30 to 40 hand held sun photometers. To ensure that the 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 try and capture how aerosols are distributed around the area.

Our hand held sun photometer is a rather simple design. It requires a substantially greater level of operator involvement than does the automatic sun photometer, and the operators are typically nonspecialists hired directly from the local populations. The instrument has been designed with only one button. Operators push the button to turn the instrument on. Then they point the sun photometer at the sun (in the picture above, the right side of the instrument is toward the sun; in the picture at the top of the page, the brass protrusion contains the detectors, which face the sun). The metal alignment jig (at the top of the above picture) helps the operator aim the unit without forcing them to look into the sun. Once roughly aligned, the operator "wiggles" the unit around until the reading reaches a maximum. Pressing the button again stores the maximum reading into memory, along with the time. Operators are asked to take three readings each time they take data, and are also required to keep a log book of weather, sky, and haze conditions. Data are taken at several times during the day.

Once every one or two months, someone involved with the study travels around the region and visits each hand held. Data are downloaded out of the unit, batteries are replaced, and the unit is reset to make room for readings over the next two months.

Sun photometer calibration

Each sun photometer has one of two types of calibration applied to it. A transfer calibration is where two instruments take data at the same time. One instrument is "trusted", and is considered to have a known good calibration. The other instrument is the one being calibrated. This type of calibration is also known as an inter-comparison calibration because the unknown instrument is compared with the trusted instrument. The advantage of this type of calibration is that it can be accomplished anywhere, so long as the sky is clear. The disadvantage is that it requires the presence of a trusted instrument.

A second type of calibration, known as a Langley Plot calibration, does not require the presence of a trusted instrument. Any stable instrument may be calibrated directly, without reference to any other device. This method essentially requires that the instrument make many observations of the sun within approximately two hours of sunrise or sunset. One or two measurements during midday are also desirable. Atmospheric conditions must be stable for all of the observations, so they must be on the same day, and smoke or smog cannot be blowing in and out of the area. Since much of the variability is in the lower atmosphere, performing the calibration on a mountaintop is preferable to performing it in a valley. NOAA's Mauna Loa Observatory in Hawaii is the preferred location for these calibrations. As such, this is where NASA performs Langley plot calibrations for the AERONET automatic sunphotometers, and this is where we calibrate one or two "trusted" hand held sun photometers prior to a deployment.

Currently, we have no sun photometry deployments scheduled, so we are not maintaining a calibration on the hand held sun photometers. Our automatic sun photometer is returned to NASA on a predetermined schedule in order to receive a transfer calibration from one of their trusted instruments.

Specifications