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Measurement of Aerosols Optical Thickness of the Atmosphere using the GLOBE Handheld Sun Photometer
Chapters
Summary May 29th, 2019
The goal of the methods presented here is to measure aerosol optical thickness of the atmosphere. The sun photometer is pointed at the sun and the largest voltage reading obtained on an in-built digital voltmeter is recorded. Atmospheric measurements such as barometric pressure and relative humidity are also performed.
Transcript
Using this protocol we can measure the aerosol optical thickness of the atmosphere using the handheld GLOBE sun photometer with an accuracy that is acceptable to the atmospheric science community. The advantage of this technique is that it can be used by everyone from high school students to college students. The GLOBE sun photometer is robust and can be used in remote and hard-to-access areas.
The aerosol optical thickness protocols are clearly outlined here and easy to follow. When taking AOT measurements it is always best to work in pairs. A visual demonstration helps to understand the technique of aligning the sun photometer with the sun.
At the sight, activate the GPS by choosing sensor setup from the sensor menu and select GPS. Once the GPS has acquired enough satellites, latitude and longitude values are displayed. Then, press collect data and press save.
If outside temperature is more than five degrees below room temperature, keep the sun photometer wrapped in thermal foam when not in use. When taking measurements during hot summer months, keep the sun photometer in the shade when not in use. Check if the sun photometer produces a stable voltage of about 0.03 volts indoors and between one volt and five volts when light is directed on the detector.
To record the air temperature, turn the rotary switch to T and record the voltage reading on the volt meter. Multiplying the voltage reading by 100 gives the air temperature in degrees Celsius. Then, set the rotary switch to the green channel of the sun photometer with one person sitting on the chair and resting his or her arms on the table.
Align the sun photometer so that the light passing through the hole on the top bracket produces a sunlight spot centered over the colored dot on the bottom bracket. Have the second person record the reading on the volt meter, making sure the sun spot is stable on the dot. If voltage reading is fluctuating, record the maximum value.
Record the time within 30 seconds. To obtain the dark voltage, have the person sitting down keep the sun photometer aligned to the sun with one hand then cover the hole on the top bracket with a finger from the other hand. Have the second person record the voltage reading.
Next, set the rotary switch to the red channel and repeat, recording light and dark voltages. Obtain five voltage readings for the green channel and five voltage readings for the red channel. Measure the air temperature again.
Observe the clouds near the sun and use the GLOBE cloud chart to check off observed features. Visible cirrus clouds are easy to observe because of their characteristic thin wispy strands. On an apparently clear day, if the cirrus clouds are not visible but the sunlight voltage reading is less than 0.5 volts, it is recommended to infer invisible cirrus clouds.
To measure the relative humidity, hold the hygrometer with an extended arm away from the body. Leave it in the air for about three minutes and then take the dry bulb reading first, followed by the wet bulb reading. Find the difference in the two readings and use the relative humidity chart to establish the relative humidity.
Then, use a barometer to measure and record atmospheric pressure. Calculate the aerosol optical thickness by plugging the measured values and the constants into the AOT equation. The monthly average AOT values measured at Xavier University of Louisiana over the 12 month period indicate AOT peaks in February and May.
The seasonal variation of the AOT at the Xavier University of Louisiana site categorizes December, January, and February as winter. March, April, and May as spring. June, July, and August as summer.
September, October, and November as fall. Based on extrapolation, AOT for four wavelengths, 667 nanometers, 551 nanometers, 532 nanometers, and 490 nanometers, at the Xavier University of Louisiana site was compared with the data at the AERONET station. The arrows show the AOT peaks in February and in May for both sites.
The reliability of the GLOBE sun photometers was checked by comparing two independently calibrated instruments against each other. The agreement between the two sun photometers is stronger for the 505 nanometer red channel than the 625 nanometer green channel. This difference is because the red LED has stronger sensitivity to temperature than the green LED.
The daily variation of AOT was between 0.265 in the morning and 0.06 in the evening for the red channel which corresponds to about 77%variation. The data shows a peak at 9:00 a.m. of 0.265 and another peak at 3:00 p.m.
of 0.182 for the red channel. The green channel showed similar peaks. The most important thing to remember is that these measurements do not work when the sun is obscured by cirrus clouds.
The presence of cirrus clouds can result in false values for AOT. The next logical step after this is to use the Calitoo sun photometer with its blue, green, and red channels to validate the GLOBE sun photometer and use both instruments into the AERONET sites. At present there are over 400 AERONET monitoring stations around the world but even these are not enough to cover the whole planet.
Our hope is that the GLOBE handheld sun photometers, using the protocols describe here, can be used to bridge the gaps left out by AERONET.
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