Determination of atmospheric ozone concentration through absorption of five wavelengths of light between 375 and 950 nm

Research Group: Brendan Clark, Travis Plunkett, and Reed Sehorn Hurst

Launch: Whitworth Spring 2020

 Abstract:  The goal of this project is to develop an inexpensive, compact medium of determining ozone concentrations at varying atmospheric altitudes. This project is important because existing modes of testing ozone require either large or expensive devices. This project will continue to develop the 'integrating sphere' concepts that has been under development by previous years' Whitworth Near Space research teams-- in which various wavelengths of light are allowed to be absorbed by atmospheric ozone and the magnitudes of ozone absorption are recorded with respect to elevation and time. The design of the 2018 Whitworth research team appeared to yield the most complete results, so this project seeks to implement the 2018 design, while incorporating additional LED lights. This project does not incorporate the 2018 'control chamber' apparatus, which is a component that may be helpful for future teams' ozone research, and which is worth exploring. This WIKI entry contains useful information in the realm of continuing ozone research, even though this years' research efforts (due to COVID 19 matters) did not continue into completion.

= Background =

Atmospheric Ozone
Ozone is comprised of three oxygen atoms. The gas coexists with a plethora of other atmospheric gases, such as the most common and notable: oxygen gas (O2) and nitrogen gas (N2). O3 naturally occurs in Earth's atmosphere at the altitudes which encompass the atmospheric levels known as the troposphere and the stratosphere. In the troposphere (where most earthly life exists), O3's presence is typically undesirable and harmful. At such altitudes, O3 is known to be carcinogenic and damaging to life forms. Largely due to human practices which emit volatile organic compounds (VOC's), such as the use of gasoline engines, O3 (the main ingredient of smog) bears a larger stratospheric presence in the current day than it ever has before in recorded history. In the stratosphere, O3's presence is desirable. The gas, at such altitudes, comprises what is known as "the ozone layer" and effectively absorbs 97%-99% of incoming ultraviolet light, which would otherwise ensue havoc upon almost all known lifeforms. In recent history, the human usage of various chemicals is said to have caused significant depletion of stratospheric ozone. This depletion, of course, is not desirable. Measures have been taken to mitigate the usage of stratospheric-ozone-reducing chemicals (which have had an effect, but have not been said to have cured the stratosphere-ozone issue).

Ozone Measurement
Because of the importances surrounding ozone and its atmospheric presence, the ability to measure its consentrations with respect to atmospheric region is critically important. Scientists must perform the said measurements should they confront the related atmosphere issues. Ozone is measured in a number of ways; most notably:
 * By means of utilizing satellites, lasers, and long-distance sensors to measure the effect that ozone has on the light that passes through it. These methods fall into the category of remote measurement--which is a manner of measuring which is done in long-range from the measurement instrument.
 * By means of using methods such as chemical reactions and short-distance light absorption. These methods fall into the category of local measurement, which is a manner of measuring which includes physically capturing a sample before measuring it.

This project seeks to measure the concentrations of ozone as a function of altitude using an inexpensive data module in conjunction with a high-altitude balloon. The specific means of ozone measurement will be by means of Absorption Spectroscopy--or measuring the amount of ozone present by measuring the amount of light (at various wavelengths) which is absorbed by a sample of atmosphere. The rates at which ozone absorbs various wavelengths of light are known, and can be applied to the measurements to determine the concentrations of ozone. This will be done utilizing the local measurement apparatus known as the 'integrating sphere.'

=Theory=

The Integrating Sphere
The concept of an integrating sphere can be understood as follows. A light ray, being confined to the ray-reflection model of light, enters into an enclosed sphere. The ray of light, being modeled as an individual photon, must inevitably reflect off of the inner walls of the sphere many times before it becomes probable that it will contact a specific, sensory, point inside of the sphere. In an integrating sphere, light is allowed to enter and to assume an infinite amount of ray-model reflective paths. A light sensor is placed at a single location within the sphere. Most photons of light will reflect many times, while others will reflect fewer times before contacting the point where the sensor lies. This reflective process is known as integrating the luminous flux via the sphere walls. This method is meant to allow any given photon, on average, to travel along a maximally long path (therefore having a maximally long time to interact with the light-absorbing ozone) before being sensed by the sensor (which, in the case of this project, is an analogue photo diode).

It should be noted that a mechanical apparatus would need to be constructed that will intake into and expel atmosphere out of the sphere so that the air inside of the sphere will be representative to the air outside of the sphere.

The average path that a photon of light will take under the conditions of this model can be determined mathematically-- one essential value is a multiplier "M" that depends various measurements within the sphere and a particular constant. This is shown below, whee M can be derived using this equation: \begin{equation} M = \frac{\rho}{1-\rho(1-a(\lambda))(1-f)}, \end{equation} where \(\rho\) is the reflectivity of the sphere's inner walls, \(f\) is the ratio of the area of the area of the opening where light enters the sphere to the area of the sphere's inner walls, and \(a(\lambda)\) is a constant representing "the wavelength dependent attenuation of the optical radiation over a single pass through the sphere due to absorption and scattering by the [specific gas of interaction]." ###REFERENCE### The average total length of the path that any given photon will travel between entering the sphere, reflecting of inner surfaces, and contacting any specific point in the sphere ("L_effective") is given by multiplying the multiplier "M" by the average distance that a photon will travel between entering the sphere and contacting its first surface of reflection (which can be reasonably approximated to be equal to 2/3 of the diameter of the sphere--thus "2/3*D"), as seen below. \begin{equation} L_\mathrm{effective} = \frac{2}{3} M D, \end{equation}

A relationship between quantity of ozone present and light absorbed would be developed after experimentation, using known values of average atmospheric concentrations of ozone per unit of altitude.

All of this, if done successfully, would theoretically provide a valid module of experimentation that can be used to accomplish ozone measurement.

NO2 Research and Finding new LED's
To determine another LED to integrate, wavelength-absorption graphs for NO₂ and for ozone were compared. It was determined that ozone absorbs ≥≈675nm light and NO2 does not. Thus, including LED's which emit light in this range would correct the issue of inconclusive data due to NO2 absorption interference.

This years' group chose to include five LED's which covered the electromagnetic spectrum from the wavelengths of about 300nm to 1000nm. Thus, the group could be confident that many data points could be compared in instances of data complication.

Definition of this Experiment
This experiment seeks to develop an inexpensive module which will utilize an integrating sphere and absorption spectroscopy to determine concentrations of atmospheric ozone with respect to altitude. The hypothesis of this experiment it that little to no ozone will be detected in the troposphere, ozone detected will decrease as altitude within the stratosphere increases, and zone detected will increase as altitude within the stratosphere decreases.



= Design =

Mechanical
This section covers the physical design of the research module, pertaining to the assembly and orientation of its material components.

Integrating sphere
The integrating sphere was constructed by attaching two hemispherical cake pans, using epoxy, after all components had been secured to the cake pans. The cake pans were each painted with five coats of white spray paint as to create a maximally reflective surface. This painting was done after all holes had been drilled into the pans, and before any components were connected to the pans. At the center point of the top cake pan a hole was drilled, measuring 21/64" in diameter. This light-entry hole functioned as the point where light entered the sphere. The LED Capsule was later fastened to the sphere, encompassing this hole. A hole is drilled 45° from the light-entry hole. This was the position of the light-sensor hole. The position of this hole was chosen as described so that light would 'bounce around' for a maximal distance before 'hitting' the light-sensor.' This principle can be understood by the ray model of light. The photo-diode was placed into this hole, pointing toward the inside of the pan, and was secured to place with epoxy. The bottom cake pan had two holes drilled, each measuring 39/128", and each 45° above their respective horizontal, and each positioned directly across from each other (pictured right). Each hole functioned as a point where atmosphere was taken either in or out of the sphere.

LED capsule
A PVC Pipe Cap was outfitted with five holes. Each hole was meant to house its respective LED light. Each hole's diameter should be equal to the pictorially specified diameter of its respective LED light, should be positioned 72° from each of its adjacent holes, and should be positioned at the vertical midpoint of the PVC Pipe cap. Each of the LED lights mentioned in the "Parts List" section [ADD LINK] were secured into their respective holes with epoxy, as pictured. The LED Capsule was secured to the integrating top cake pan.

Intake/Outtake Fans and Mounts
Two fan mounts were 3D printed using this file [GET LINK]. Two brush-less fans (component link in "Parts List") [MAKE THIS A LINK] were each secured into a mount as pictured (pictured right). Each resulting apparatus was secured to the bottom pan of the integrating sphere as pictured (pictured right).

Ozone Testing Chamber
The group in 2018-2019 school year built a ozone testing chamber for the calibration of their sensors, this has been used in every group sense it's initial create and serves the same purpose, it not only that it allows for each group to get a base line test that they can compare their experimental results to helping create a more fleshed out and accurately analyzed data set. It is a nearly air tight chamber that ozone is pumped into by and exterior generator, harboring a power port into it to allow for power while maintaining a airtight chamber. Several precautions in terms of materials had to be made to ensure the longevity of the chamber due to the highly absorbic nature of ozone. In theory that is, unfortunately no group has made substantial enough progress to do anymore than test their sensors due to circumstances each year. Fairly simple design, a rectangular cube with glass sides and a semitransparent top made from silicon. The glue used to build the chamber is ozone-phobic, so is silicon hence why it was used as the top cover, this is also so it could have cuts made into the top of it for a power port. It must be entirely, or at least as air tight as possible. Two holes for fans were cut into the top to be hooked up to a ozone generator, allowing one to have a fan pumping ozone into the camber from the generator and then the other pumping out of it. Due to the fans being large, a 3D printable cone was made to reduce the size to a tube size allowing a silicon tube to be used to connect the ozone generator to the cone, then into the fan on both sides. The plastic used is PETG to due to it's fairly high resistance to ozone absorption. Then the connection points between the tube and the generator / cone was secured with circular clamps. A recent addition is a shelf to house the ozone generator above the container to help flow of air and to secure it better than having it simply rest upon the top of the lid.

Phototransistor
A phototransistor with a peak sensitivity at 570 nm and half-voltage range from 440 nm to 800 nm was chosen in order to get usable readings from both LED's. The phototransistor readings were first amplified through a dual operational amplifier, then the resulting voltage was filtered to reduce electrical noise. Two separate but identical amplifier and filter sets were constructed, one for each phototransistor. See Figure 7 for a diagram of the phototransistor and LED circuits.

Whitworth Launch Computer
The circuit was assembled on a pre-prepared Whitworth University Launch Computer. The computer contained internal circuitry to connect an SD card mount, external power switch, and external indicator LEDs. Our circuit was contained on a breadboard attached to the Launch Computer. A rechargeable LiPo battery was mounted on the underside of the computer in order to reduce the number of batteries that would have to be replaced for both testing and launch.

LEDs
All LEDs were connected through a resistor to keep them from breaking, and a transistor to give them more current so that they will be brighter. We will have five different types of LEDs: 370nm wavelength (the peak absorption of ozone), 600nm wavelength (the peak absorption of ozone in the visible light spectrum), 490nm wavelength (the peak absorption of nitrogen dioxide),. T851nm and 940nm. The group that worked to detect ozone two years ago had the problem of not being able to distinguish between absorption from nitrogen dioxide and ozone since they somewhat overlap, so we have taken the effort to use specific wavelengths of LEDs and light sensors that can detect specific wavelengths to distinguish between the peak absorption of nitrogen dioxide versus ozone. We are using a infrared LEDs that are not absorbed by nitrogen dioxide. Only one LED will be on at a time, toggling between them and capturing data with the sensors each time a new LED turns on.

All of the LEDs share a common ground connection. There is only one of each color of LED, and each color has its own power connection and their own resistor in their own circuit instead of being connected in parallel to maximize the amount of current going through each one.

Fans
The circuits for the fans were reused from last year's project. The fans do not need to be controlled in the code because they will just be running the entire time, so they are connected to an external 9V battery that will keep them running.

Software
Due to COVID 19 all work is incomplete but detailed reports on what exists as of writing the wiki along with future plans will be included.

Start Up
The beginning of the software consists of a starting a running clock that will be used latter on see "Data Management / Data Storage" section for more information and several layers of error light codes. The first check in the code is right after this timer has been started and is necessary for data storage, reporting back a short "red" blink indicating an error within starting the clock and will terminate the program. This was to help make sure that any potential error in the clock could be tested and as a layer to help ensure that the project recorded data. If successfully check and running we would get a green light. Following that check was a check for a storage device, this would be where all our data is saved to, and once again we would get an error and report back red flashing light followed up with a closure of the program to help indicated a failure. After the flight computer makes it through the second check and reports back we get a blue light, the following checks are for the editing of .txt files on the storage device. In this case there were two, one for the data file and one for the error file. If the error file failed to load the software would report back a solid red light, and if it passed the blue light would begin to flash. For the final step in the update process we had planned for a solid red light to be lit if the data file was loaded properly but unfortunately the final two steps had yet to be completed.

Data Management / Data Storage
One point was core to the set up of the data management and storage system, that being the utmost care to properly saved to the storage no matter what the circumstances, this was due to previous flights having issues completing the final step saving data to the storage device. To begin, after the storage device was saved and the files were loaded, we planned to have the sensors upload their data into the ram and over a set period of time the main class would take the data from ram and format it then load it into the data file after a given set of time and then reset the timer mentioned above. This time was the core to timing the data save and data upload (to ram), it was what allowed for the craft to periodically gather data and then saved it eventually to the data file. Hence why the individual data error if the timer didn't start. After the timer hit a specific number it would wright the ram data to the file, within the text saved into the file it would include the time the data was taken and the time the data was written to the file. Doing this would allow us to locate any potential inconsistencies within our data helping us to tune it later after the flight had taken place. After the data was successfully uploaded from ram into the data device the ram would be cleared and then the file would be closed to help preserve the data, this would help protect the data from being corrupted if our craft ran out of power or a potential electrical error occurred during the flight. Also, clearing the ram would help keep reduce potential latency in the software (even if minimally) and allow for as much storage as possible of our recorded flight data at any time.

Start Up / Data Management Code

= Validation and Calibration =

LEDs
We used a simple program that toggled between each of our five types of LED's. This was done without the LED's in the integrating sphere to ensure that all of the LED's worked as expected. Since the higher wavelengths are not visible, we used the photo transistor to make sure it was emitting light, so we were able to ensure that the IR LED's were working as expected. We then put both light sensors and the LED's inside of a cardboard box together (they were not yet inside the integrating sphere). We ran the same code that toggled between the LED's and ensured that the photo transistor was able to detect which LED was on at which time, since the sensor measures specific wavelengths of light. The values given by the photo transistor increased when the LED's were on as expected. We then tried to find the way to get the LED's as bright as possible because they would need to be fairly bright for our design to work once they were in the integrating sphere. We looked at the data sheets for each type of LED and calculated the amount of resistance necessary for each one based on the maximum current for each LED and using Ohm's law. It was also determined based on this information that a separate 9 volt battery would work well to power the LED's. We would also be able to make the LED's brighter by putting each one in a separate circuit instead of having them in parallel with each other so that each LED has more current going through it. So, we ended up with a separate circuit for each individual LED, each with their own resistor.

Photo-transistor
We were not able to complete calibration for this, but we would have done so by setting its zero value to correspond to the amount of light present in our integrating sphere when no LED's are shining (which, ideally, would have been light-less).

Data and Analysis
Due to the disruption of the semester caused by the Covid-19 pandemic of spring 2020, this experiment was never completed or sent to the stratosphere.

Conclusion
Due to COVID 19 matters, this expirement was not able to be finished or launched.