Measurement of Aerosols Optical Thickness of the Atmosphere using the GLOBE Handheld Sun Photometer

Summary

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.

Abstract

Here, we describe the measurement of aerosol optical thickness using the GLOBE handheld sun photometer. Aerosol optical thickness (AOT) was measured at Xavier University of Louisiana (XULA, 29.96° N, 90.11° W and 3 m above sea level). The measurements were done at two different wavelengths, 505 nm and 625 nm. AOT measurements were done 6 times a day (7 AM, 9 AM, 11 AM, solar noon, 3 PM and 5 PM). The data shown in this paper are the monthly average AOT values taken at solar noon. During each measurement time; at least five values of the sunlight voltage V and the dark voltage V_dark are taken for each channel. The mean for these five measurements is taken as the average for that measurement time. Other meteorological data such as temperature, surface pressure, rainfall and relative humidity are also measured at the same time. The whole protocol is completed within a time span of 10–15 min. The measured AOT values at 505 nm and 625 nm are then used to extrapolate the AOT values for wavelengths 667 nm, 551 nm, 532 nm and 490 nm. The measured and extrapolated AOT values were then compared with values from the nearest AERONET station at Wave CIS site 6 (AERONET, 28.87° N, 90.48° W and 33 m above sea level), which is about 96 km south of XULA. In this study we tracked the annual and daily variations of AOT for a 12 month period from September 2017 to August 2018. We also compared AOT data from two independently calibrated GLOBE handheld sun photometers at the XULA site. The data show that the two instruments are in excellent agreement.

Introduction

Atmospheric aerosols are minute solid and liquid particles (ranging from submicron to millimeter size) suspended in the air. Some aerosols are produced through human activity and others are produced by natural processes^1,2,3,4. Aerosols in the atmosphere reduce the amount of solar energy reaching the earth’s surface by scattering or absorbing light and thermal radiation from the sun. The amount of aerosol in the atmosphere varies significantly with location and time. There are seasonal and annual changes as well as episodic changes due to events such as large dust storms, wild fires or volcanic eruptions^5,6,7,8.

The impact of aerosols on the climate and on public health are among the dominant topics in current environmental research. Aerosols affect the weather by scattering or absorbing light and thermal radiation from the sun and by acting as condensation nuclei in the formation of clouds. Aerosols also play a role in the dispersal of pathogens in the air and they can cause or enhance respiratory and cardiovascular diseases. Aerosol optical thickness (AOT) is a measure of the amount of sunlight that is absorbed or scattered by these aerosols. There are several ground-based methods for monitoring AOT^9,10,11. The biggest of the ground-based AOT monitoring system is the Aerosol Robotic Network (AERONET) project. AERONET is a network of over 400 monitoring stations spread all over the world^12,13. Despite this large number of monitoring stations, there are still large gaps world-wide that are not monitored for AOT. As an example, the nearest AERONET station from our study site is about 90 km away. This paper describes the use of a portable handheld sun photometer that can be used to bridge the gaps between AERONET monitoring stations. The portable handheld sun photometer is an ideal instrument for use by students around the world in a global aerosol monitoring network^14,15. The Global Learning and Observations to Benefit the Environment (GLOBE) program provides a platform for such a network, through thousands of schools in all the 50 states of the United States and in nearly 120 other countries^16,17. The primary idea of the GLOBE program is to use students all over the world to provide scientifically valuable measurements of environmental parameters using inexpensive equipment. With proper guidance, students and other non-specialist can form networks of handheld sun photometers to fill the gaps between the AERONET monitoring stations. The biggest advantage of the handheld sun photometer is that it can be taken to even the remotest parts of the world. AOT measurements with other small and transportable instruments have been successfully used in the past to carry out research studies in remote and hard to access areas^17,18

The main goal of this study is to use the GLOBE handheld sun photometers to track the annual, daily and hourly variation of AOT at our XULA study site and compare with measurements from a nearby AERONET station. This paper presents data for a 12 months period from September 2017 to August 2018. This is the first ever AOT recorded for the XULA site. The GLOBE sun photometer measures AOT at two wavelengths, 505 nm and 625 nm. The AERONET site at Wave CIS Site 6 measures AOT at 15 different wavelengths. For our comparison we focused on these 4 wavelengths, 667 nm, 551 nm, 532 nm and 490 nm. We chose these because they are the 4 AERONET wavelengths nearest to the GLOBE sun photometer wavelengths. To make the comparison, we extrapolated AOT values at these wavelengths for XULA site.

Measurements of AOT are done every day when the weather conditions permit. Measurements that are done when there are cirrus clouds within the vicinity of the sun are excluded in the analysis. Table 1 shows the number of days in each month that we had completely clear skies. Altogether, about 47% of the data taken was excluded.

Month	Sept	Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug
Number of Days	18	20	16	15	15	15	16	15	18	15	15	16

Table 1: AOT measurements were done 6 times a day (7:00 AM, 9 AM, 11 AM, solar noon, 3 AM, and 5 AM). The data shown on the plots are the monthly average AOT values taken at solar noon. During each measurement time; at least five values of the sunlight voltage V and the dark voltage V_dark are taken for each channel. The mean for these five measurements is taken as the average for that measurement time. The error in these measurements is calculated as the standard deviations of these five measurements. AOT values are obtained using the equation shown below¹⁶:

V₀ is the calibration constant of the sun photometer, R is the earth-sun distance in astronomical units, V_dark is the dark voltage recorded when light is blocked from passing through the hole on the top bracket of the sun photometer, V is the sunlight voltage recorded from the sun photometer when light passes through the hole on the top bracket, a_R represents the attenuation of light due to Rayleigh scattering, P and P₀ are the measured and standard atmospheric pressure, respectively, and m is the relative air mass. The relative air mass is calculated from data provided by the National Oceanic and Atmospheric Administration (NOAA). Other meteorological data such as temperature, rainfall and relative humidity are also measured at the same time. Equation 1 as given above includes the contributions of optical thickness from ozone. The effect of ozone on AOT values is calculated based on tabulated values of the ozone absorption coefficient and assumptions about the ozone amount in the atmosphere¹⁹. Bucholtz^20,21 has produced tabulated values of a_R based on standard atmospheres. For the 505 nm channel a_R ≈ 0.13813 and for the 625 nm channel it is ~0.05793.

The data presented here represents an example of how teams of students can be organized to take long and sustained AOT measurements. In this study, two student teams used two independently calibrated GLOBE handheld sun photometers to track the annual, daily and hourly variation of the aerosol optical thickness of the atmosphere at our XULA study site. The two Globe sun photometers used in this investigation were purchased from the IESRE (Institute for Earth Science Research and Education; one had serial number RG8-989 and the other had serial number RG8-990). Before the data from the two instruments could be combined, a regression analysis was carried out to ascertain the agreement

Protocol

1. Photometer Operation

NOTE: These protocols are best done by two people working together. One person holds and aligns the sun photometer while the second person record the measurements.

1. Measure the longitude and latitude for the site using GPS. At the site, the first step is to activate the GPS by choosing sensor set-up from the sensor menu and select GPS. Once GPS has acquired enough satellites, latitude and longitude values will be displayed. Once values are displayed press collect data and then press save.
2. Make sure the sun photometer is working well. A properly calibrated sun photometer should produce a stable voltage of ~0.03 V indoors and up to 5 V when light is directed on the detector. The voltmeter on the Globe sun photometer is in-built on the sun photometer
3. Record the air temperature. If using an alcohol in glass thermometer, give the thermometer 3–5 min to adjust to the outside temperature before recording the stable reading. If using the sun photometer’s in-built thermometer, turn the rotary switch to T and record the voltage reading on the voltmeter. The voltage reading multiplied by the 100 will give the air temperature in degrees Celsius at that time.
4. Set the rotary switch to the green channel of the sun photometer.
5. Have one person align the sun photometer so that light passing through the hole on the top bracket produces a sunlight spot centered over the colored dot on the bottom bracket. For best results, use a table and a chair. The person aligning the sun photometer should sit on the chair and rest his/her arms on the table in order to obtain a steady reading.
6. Have the second person record the reading on the voltmeter. Make sure the sun spot is stable on the dot before taking a reading. If voltage reading is fluctuating, just record the maximum value shown.
7. Record the time at which the reading was taken. Time must be recorded to the nearest 30 s. A digital watch serves this purpose better than an analogue one.
8. Obtain the dark voltage. Have the person sitting down keep the sun photometer aligned to the sun with one hand and then cover the hole on the top bracket with a finger from the other hand. The second person will record the voltage reading.
9. Set the rotary switch to the red channel and repeat steps 1.4–1.7.
10. Repeat steps 1.4–1.8 four more times to obtain five voltages readings for the green channel and five voltage readings for the red channel
11. Measure the air temperature again as in step 1.2.

2. Collection of Metadata

1. Use the Globe cloud chart to observe and record the clouds near the sun. This is done by looking into the sky and checking off observed features from the GLOBE cloud chart (https://www.globe.gov/documents/348614/24331082/GLOBE+Cloud+Chart.). Visible cirrus clouds are easy to observe because of their characteristic thin wispy strands. Invisible cirrus clouds are inferred if the sunlight voltage reading on an apparently clear day is less than 0.5 V.
   1. Use a hygrometer to measure and record the relative humidity: Hold the hygrometer with an extended arm away from the body, leave it in the air for about 3 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
   2. Use a barometer to measure and record atmospheric pressure.
2. Calculate AOT by plugging the measured values and the constants into Equation 1 given above.

3. Temperature Regulation

NOTE: The electronics of the sun photometer are sensitive to temperature. For optimal performance, the following steps are recommended.

1. If outside temperature is more than 5 degrees below room temperature, keep the sun photometer wrapped in thermal foam when not in use.
2. When taking measurements during the hot summer months, keep the sun photometer in the shade when not in use.

Results

The GLOBE sun photometer measures AOT at λ= 505 nm and λ= 625 nm. The AERONET site at Wave CIS Site 6 measures AOT at 15 different wavelengths. For our comparison we focused on these 4 wavelengths of the AERONET site: 667 nm, 551 nm, 532 nm and 490 nm. To make a comparison between the two stations, we extrapolated AOT at 667 nm, 551 nm, 532 nm and 490 nm for the XULA site. This is done using the XULA site’s Angstrom coefficients. For any given site and instrument, the optical thickness τ,...

Discussion

The first step in this protocol is to define the study site. This is done by using a GPS to find the longitude and latitude of the study site. The longitude and latitude values are critical in the calculation of AOT using equation 1. During measurement, it is crucial that the sun photometer is pointed directly and firmly at the sun. The tiny hole at the top bracket of the handheld sun photometer reduces the amount of scattered light reaching the LED detectors in the sun photometer. Equation 1 is an approximation that ass...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
A Calibrated GLOBE handheld sun photometer	IESRE, USA (GLOBE sun photometer) and TERNUM, UK (Calitoo sun photometer		The GLOBE sun photometer measures AOT at 505 nm and 625nm.
Barometer	Forestry suppliers, USA, Cat# 43316	43316	The aneroid barometer must have a clear scale with a pressure range between 940 and 1,060 millibars.
GLOBE cloud chart	Forestry Suppliers, USA Cat#33485	33485	A free cloud identification chart is obtained from www.globe.gov.
Hygrometer	Forestry suppliers, USA, Cat# 76254	76245	Any digital hygrometer which measures relative humidity in the range of 20-95% with an accuracy of 5% is acceptable.
Labquest2 GPS	Vernier, USA, Cat LABQ2	LABQ2	Vernier LabQuest 2 is a standalone interface used to collect sensor data with its built-in graphing and analysis application. GPS is one of its built-in sensors
Taylor Orchid Thermometer	Forestry Suppliers, USA Cat# 89129	89129
Watch	Forestry suppliers, USA, Cat# 39137	39137	The watch must be digital and capable of measuring time up to seconds.