Estimating Ozone by Using Ground Instruments

Estimating Ozone by Using Ground Instruments

Nash Moore

CSGC CU - Boulder

A UV meter and an aerosol detector will be used jointly to estimate the amount of ozone over a certain location. The UV meter gives a reading of the amount of UV at Earth’s surface. The aerosol meter gives a value for the amount of green light that reaches Earth’s surface. This value will then be used to calculate the aerosol optical depth, which is the amount of aerosols that the light travels. Combining the UV reading, the aerosol optical thickness, and the intensity of sunlight prior to entering the atmosphere, an ozone thickness can be estimated.

As light travels through the atmosphere, it encounters many particles known as aerosols. Aerosols are small particles floating in the air, typically spherical, with the diameter ranging from .01 to 100 m. As light hits the particles, it is scattered. Due to their smaller wavelengths, blue and ultraviolet are reflected more than red and infrared. This light is ricocheted off of many particles and will eventually reflect back into space or towards the ground. Aerosol optical thickness (AOT), also referred to as aerosol optical depth, is a value related to the amount of light that remains after being scattered off of aerosol particles in the atmosphere.

An aerosol detector will correctly determine the AOT. This detector uses the amount of green light received by a green (LED) to display a voltage on the LCD screen. With this voltage and information about our location and time of day, we can calculate the AOT.

First, we must find the relative path length that the light had to travel through the atmosphere. We determine the relative path length by designating the relative path length to be one when the sun is directly above us. As the sun moves across the sky, the relative path length changes due to the light traveling through changing amounts of the atmosphere. The path length depends the zenith angle, which is the angle formed by an imaginary perpendicular line from the surface of the earth and an imaginary line from that same point on the earth to the sun. The zenith angle depends on the distance from the equator, elevation, and time of day. Sbdart has a solar angle calculator on their website that calculates the zenith angle with the location and time data mentioned. ( Once we know the zenith angle, the relative path length is calculated by the following formula:

This is an example of the path length changes. When the sun is practically at the horizon, a zenith angle is approximately 89 degrees. The relative path length is 57. This represents the relative path length as 57 times the length of what it would be if the sun were directly overhead. This is only an approximation and assumes that we are only working with zenith angels less than 90 degrees. Since the sun will be in a slightly different location each time data is collected, it is recommended that the relative path length is calculated each time for more accurate results.

Before we can calculate the AOT from the voltage reading, we must determine the extra terrestrial constant. This is determined if the relative path length is zero, meaning that light had not yet traveled though any of earth’s atmosphere. When we calibrate our aerosol meter, the value given will be designated as the extra terrestrial constant. Since each hand-built instrument is different, this constant varies slightly. However, it is only necessary for us to determine this constant for one particular instrument.

To find the extra terrestrial constant, we plot the natural log of the voltage read on the meter versus the relative path length. To obtain a clear graph, we should take a measurement at approximately each integer value for the relative path length. This means that we need measurements from either dawn to noon, or noon to dusk. Here in Boulder, Colorado, the mountains to the west prohibit us from obtaining good measurements as the sun is setting. Therefore, it is necessary for our calibration to be done early in the morning. For more accurate results, we should take daily data sometime within the period of time when calibration data was taken.

At large zenith angles, the relative path length changes rapidly as the angle increases or decreases. At smaller angles, the relative path length changes at a slower rate. In order to take one measurement at each integer multiple of the relative path length, we will need to take many ore measurements when the sun is closer to the horizon and take fewer as it approaches high noon. The Citizen Explorer program recommends the following times to take calibration data, with airmass representing the relative path length:

“For the first hour take data as often as possible but definitely

more than once every 3 minutes. At dawn and dusk the

AIRMASS is changing at the rate of more than 6 AIRMASS per minute

for the second hour take data every 2-3 minutes since the AIRMASS

is changing at the rate of 1 AIRMASS every 7 minutes.

for the third and fourth hour you can take data every 15 minutes

for the remainder you can take data every 20-30 minutes”[Citizen-Explorer Web Site].

After taking the calibration data, we plot this information using the natural log of the voltage reading versus the relative path length, and then draw a best-fit line. The y-intercept of the line is the extra terrestrial constant for that aerosol meter.

After obtaining the relative path length and the extra terrestrial constant, the aerosol optical thickness is calculated by the following formula:

We let ET represent the extra terrestrial constant, V represent the voltage reading on the aerosol meter, apply Rayleigh coefficient_of_scattering, which is 0.1158, L represent the relative path length, and P represent the pressure at the time of the measurement, in atmospheres.

Once we have calculated the aerosol optical thickness, we can determine the amount of UV light that is not reaching the ground due to reflection by aerosol particles.

The other ground instruments that is used is a small UV meter from Sunsor Inc. This UV meter was chosen because it was relatively cheap and was accurate enough for our purposes. The UV measurement is taken by aiming the meter towards the sun until a maximum reading is found. Fortunately, the reading is used as given unlike the aerosol meter reading.

However, we still need to determine the amount of UV light prior to entering the atmosphere before we are able to estimate the amount of ozone. Fortunately, the SOHO satellite records these measurements.

Ozone absorbs UV light. The amount of light absorbed is equal to the amount of UV light prior to entering the atmosphere minus the amount of UV light measured on the ground by the handheld UV meter, minus the amount of UV light lost due to scattering off of aerosol particles. Once we know the amount of UV absorbed by the ozone, we can estimate the amount of ozone between us and the sun.