Determination of atmospheric ozone content using a calibrated integrating sphere

Research Group: Matthew Craig, Caleb Jones, Cameron Rutherford, and Dorothy Wang

Launch: Whitworth Spring 2017

''The goal of this experiment was to improve the ongoing design of a low-cost integrating sphere. The design improved on previous groups' designs, particularly in the coding and mechanical stability areas. The sphere relied on two red LED's to produce light, which would be absorbed by the ozone gas. The sphere collected some data, but, due to an inability to calibrate the data, it is merely proof of concept as the accuracy of the data is still questionable.''

Ozone and UV Radiation
Ozone has a relatively low abundance in the atmosphere. Primary atmospheric gases include oxygen (\(\mathrm{O}_2\)) and nitrogen (\(\mathrm{N}_2\)) gas. Ozone occurs in two layers of the atmosphere: the troposphere and the stratosphere. The troposphere is the first layer of atmosphere above the Earth's surface, containing approximately half of the Earth's atmosphere. In the troposphere, which extends from ground level to a level approximately 10 kilometers above sea level, ozone is an air pollutant that is harmful to breathe and damages sensitive crops and vegetation. At ground level, ozone is a harmful pollutant and a major component of urban smog, with a typical abundance of 20 ppb to 100 ppb. The natural concentration of ozone in the troposphere near the Earth's surface is 10 parts per billion. According to the Environmental Protection Agency, human exposure to ozone levels greater than 70 parts per billion for longer than 8 hours is unhealthy and can lead to a variety of harmful effects, including throat and lung irritation and aggravation of asthma or emphysema. The increasing prevalence of ozone as a pollutant in urban environments presents a human health hazard. The stratosphere is the second layer of the Earth's atmosphere, and extends from 10 km to 50 km above the ground at middle latitudes. About 90% of ozone in the atmosphere is found in the stratosphere, in which peak concentrations of ozone up to 15 parts per million occur at altitudes of approximately 32 km above the Earth's surface. Temperatures rise with increasing altitude through the stratosphere. Due to this temperature stratification, there is little convection and mixing in the stratosphere; thus, the layers of air in the stratosphere are quite stable. Figure 1 shows how ozone concentrations change with altitude in the atmosphere. As seen in the figure, the stratosphere contains the majority of atmospheric ozone.





Ozone (\(\mathrm{O}_3\)) is a toxic gas molecule formed as a result of UVC radiation from the sun. Ozone is formed when high-energy solar ultraviolet radiation strikes atmospheric oxygen molecules (\(\mathrm{O}_2\)) and splits the two oxygen atoms apart via photolysis. Each of these highly reactive oxygen atoms can then combine with an oxygen molecule to produce an ozone molecule. Ozone is found primarily in the stratosphere, and heats the stratosphere as it absorbs energy from ultraviolet radiation from the Sun. The ozone layer itself forms a natural layer in the atmosphere and varies in thickness and density as meteorological conditions change. The stratospheric ozone layer protects life on Earth by absorbing ultraviolet light, which damages DNA in plants and animals. Ozone absorbs harmful ultraviolet radiation in the wavelengths between 280 and 320 nm of UV-B radiation; radiation in this region of the electromagnetic spectrum can cause biological damage in plants and animals. Ozone also absorbs all UV-C radiation and approximately half of UV-A radiation, and is critical to life on Earth. Excessive UV-B and UV-A radiation can cause sunburn and lead to skin cancer and retinal damage. A depletion of stratospheric ozone causes increased amounts of harmful UV radiation to reach the Earth's surface, which can result in more cases of skin cancer, cataracts, and impaired immune systems. Ground-level ozone is generated from chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOC) in the presence of sunlight. Emissions from industrial facilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are major sources of NOx and VOC. Thus, the monitoring of ozone concentrations is essential.

The natural level of ozone in the stratosphere is balanced by the production of stratospheric ozone and its destruction in chemical reactions. Though ozone is produced naturally in the stratosphere, it is gradually being destroyed by man-made chemicals, including chlorofluorocarbons, hydrochlorofluorocarbons, halons, methyl bromide, carbon tetrachloride, and methyl chloroform. These materials were formerly and sometimes still are used in aerosol sprays, coolants, foaming agents, fire extinguishers, and pesticides. Once released into the air, these ozone-degrading materials degrade very slowly, and can remain intact for years as they move through the troposphere and eventually into the stratosphere. Materials that get into the stratosphere can stay in this layer of atmosphere for long periods of time, due to the lack of vertical convection in the stratosphere. Chlorofluorocarbons (CFCs) are organic compounds are used in the manufacture of aerosols, foams and packing materials, solvents, and as refrigerants. While these chemicals are generally nontoxic, safe to handle, and inert in the lower atmosphere, they undergo significant reactions in the stratosphere, effectively destroying ozone in the stratosphere. These chemicals are broken down by the intensity of the sun's UV rays and release chlorine and bromine molecules, which then interact with and destroy the ozone in the stratosphere. Due to the catalytic nature of the reaction, 100,000 molecules of ozone can be destroyed per chlorine atom released from CFCs. Other materials that remain in the stratosphere for long periods of time may include large aerosol particles from volcanic eruptions and meteorite impacts, which may affect the Earth's climate.



Electron transitions within the ozone molecule allow for ozone absorption of radiation in the visible region. The ozone absorption spectrum stretches from 410 nm to 700 nm, with a peak absorption at 603 nm. The amount of ozone absorption decreases significantly with decreasing wavelengths of visible light; thus, our group chose to use green light at 565 nm as a control. Ozone is also highly absorptive at 254 nm, which falls within the ultraviolet region of the electromagnetic spectrum. However, the reflectivity of the integrating sphere’s internal coating is higher in the visible region (>99% reflectance using Spectralon) of the electromagnetic spectrum than at the lower wavelengths of ultraviolet radiation (<95%). Despite ozone’s higher absorptivity in the ultraviolet region, the decreased reflectivity of the integrating sphere will reduce the potential optical path length that can be achieved. Therefore, our team will be testing ozone absorption in the visible region of the electromagnetic spectrum using red and green light, with LEDs of 610 nm and 565 nm.

Measuring Atmospheric Ozone Concentrations


Atmospheric ozone concentrations can be measured by a variety of methods. The two major categories of ozone measurement techniques include local measurements and remote measurements. Remote measurements are obtained via the detection of ozone from large distances by measuring ozone's absorption of UV radiation. Common sources of UV radiation used in these remote methods include the Sun, lasers, and starlight. Ground-based stations often measure ozone by detecting changes in the amount of solar ultraviolet radiation reaching the Earth's surface. Ozone abundance can be determined by satellites in two ways: by measuring the absorption of solar UV radiation by the atmosphere, or by measuring the absorption of sunlight scattered from the Earth's surface. Lasers are also commonly used by ground-based stations and research aircraft to detect ozone over long distances along the laser light path. Local instruments measure ozone locally by continuously drawing air into a small detection chamber. Within the detection chamber, ozone is measured either by its absorption of ultraviolet light or by the electrical current or light produced in a chemical reaction with ozone. Ozonesondes are lightweight instruments suited for launching on high-altitude balloons that use this approach to measure ozone.

The purpose of this experiment was to further the development of a low-cost integrating sphere ozonesonde and expand upon the work done in 2008, 2010, 2012, 2014, and 2016 through this project. This iteration of the sensor involved improvements to mechanical durability and code stability.

Integrating Sphere Theory
An integrating sphere is a simple device used to measure optical radiation. An integrating sphere has four ports: two are used to input and output gas, while the other two ports allow for the input and output of optical radiation. The sphere must have a highly reflective internal coating to allow for its function as a multipass optical absorption cell. The integrating sphere can operate in the ultraviolet, visible and near infrared regions of the electromagnetic spectrum. The highly reflective internal coating of the integrating sphere causes input light flux to undergo several reflections before exiting the sphere, resulting in an optical path length that is longer than the sphere diameter. An important parameter of an integrating sphere is the sphere multiplier ($$M$$) which represents the average number of reflections a single photon will undergo before it exits the sphere. $$\rho$$: the reflectance of the sphere’s internal surface

$$f$$: ratio of the area of the port openings to the total internal sphere surface area

$$a(v)$$: level of attenuation of the optical radiation over a single pass through the sphere due to absorption and scattering by the test gas The average path length that a single photon travels within the sphere is the product of the sphere multiplier and the average distance travelled by a photon for a single pass through the integrating sphere. The average optical path length can be approximated to two-thirds of the sphere diameter. Thus, the average path length by a given photon within an integrating sphere containing an absorbing gas is \(L_\mathrm{effective} = \frac{2}{3} M D\), where $$M$$ is the sphere multiplier and $$D$$ is the diameter of the sphere.

Preliminary Testing
The previous group to attempt to test the ozone content using an integrating sphere in 2016 discovered during calibration that the foam used to construct the pod absorbed copious amounts of ozone. In an attempt to limit this absorption, the exposed foam was sprayed with a protective enamel. Two different types of enamel, Rust-Oleum Gloss Protective Enamel (White) and Plasti Dip Multi-purpose Rubber Coating (Red) were tested to see which hindered the absorption of ozone the best. The Rust-Oleum Enamel reacted with the foam and dissolved a large portion of the foam we had sprayed. The Plasti Dip Rubber Coating, on the other hand, made a significant decrease in ozone absorption by the foam without affecting the foam itself. It was determined that the pod would be sprayed with Plasti Dip Rubber Coating before the launch to limit that variable during our flight.

Project Definition
The purpose of this experiment was to expand upon the work of previous groups in developing a low-cost optical sensor to measure stratospheric ozone concentrations through the use of integrating spheres. This iteration of the sensor involved improving security and durability of the pod to ensure it returned with results. Multiple LED's will be used to measure the total absorption of light by the ozone layer over a large pathway in the integrating sphere.

Developing an optical sensor with a larger diameter integrating sphere will generate longer effective optical path lengths and allow the detection of lower concentrations of ozone (to the 10’s of ppm). In order to maximize sphere size for maximum optical path length, we will need to consider space allocation within our box. ABS plastic and 3D printing can be used to secure the wires as well as technology necessary to perform the experiment from the ground as well as maximize the space within the box.

Ozone is also highly absorptive at 254 nm. However, the reflectivity of the integrating sphere’s internal coating is much higher in the visible region of the electromagnetic spectrum than at the lower wavelengths of ultraviolet radiation. Thus, our team will be using LEDs in the visible spectrum; any potential advantages gained by using ultraviolet light in which ozone is more highly absorptive are negated by the reduction in potential optical path length due to lower reflectivity of the integrating sphere in the ultraviolet region.

Mechanical Design
Due to the incredible amounts of acceleration experienced during flight, especially at the breaking point of the balloon, a sturdy mechanical design was imperative to the overall success of our project. In effect, there were three necessary components to the overall design: the integrating sphere, the fans, and the pod itself.

Integrating Sphere
The most important aspect of the project was to create a functional integrating sphere. Due to our limited budget, it was necessary to build our own from scratch rather than purchase a commercial integrating sphere online. The integrating sphere consisted of two cake pans painted white on the inner surfaces and epoxied together, with four holes drilled strategically to insert the fans, lights, and phototransistor. Two Fat Daddio 4” Cake Pans (purchased off of Amazon.com) were used to build the sphere itself. When combined using Loctite epoxy and 3 layers of duct tape (to keep the light from coming through the epoxy), the semi-spherical shape of the pans created a nearly perfect sphere when placed on top of each other. Four total holes were drilled into the cake pans, two per pan. On each pan, two holes of diameters 3/16" and 1/2" were drilled into the sphere, with the larger 1/2" diameter holes corresponding to the holes for the fans to facilitate air circulation within the sphere, and the smaller 3/16" diameter holes corresponding to the holes for the phototransistor in the first pan and the PVC cap containing LEDs in the second pan. First, the holes for the fans were drilled, strategically placed so that when mounted inside the box the fan holders would be align with the outside of the box as well as the holes. This design not only made the integrating sphere easier to mount inside the box, but also helped to avoid interference of the fans with the straps used to lash the box to the balloon. A phototransistor was secured to the integrating sphere using clear Loctite epoxy and duct tape for additional stability. Four LED lights, two red and two green, were inserted into a small PVC end cap which was then mounted to the outside of the integrating sphere using tape. The cap was centered around one of the smaller holes so the light would enter the integrating sphere in small amounts. The inside of the integrating sphere was painted white using Rust-Oleum Flat Protective Enamel. Only one thin, even coat was applied to allow the light to be better deflected within the sphere.



Fans
Two fans were used to help circulate the ozone through the sphere during flight. To mount these to the box, two fan holders were created using AutoDesk Inventor and then printed using PLA plastics. The fan holders were secured to the pod using clear Loctite epoxy. The basic design of these holders was to funnel the ozone back towards a small cylindrical opening that was then inserted into the integrating sphere directly. There was a small square area that the fan could be placed in and held firmly throughout the entire flight. The square fans measure 25 mm per side (measured using calipers accurate to the 0.05 mm) so the square hole used to hold the fans measured 25.6 mm to account for any variation during printing. The large square face measured 60 mm per side and the depth of the entire component was also 60 mm. The cylindrical knob on the back that would be inserted into the sphere tapered slightly from 10.9 mm in diameter down to 10.6 mm. The hole through that cylinder only measured 7.5 mm in diameter. For the insertion of the lights and the phototransistor, two smaller holes of diameter 3/16" were drilled into the integrating sphere at angles of approximately 90° that would allow the light to bounce around many times before reaching the phototransistor.

Pod


Preliminary testing suggested that the pod foam, a styrofoam-type material, readily reacts with ozone. Thus, it was necessary to modify the provided foam box to account for the rapid absorption of ozone by the pod foam. Further testing revealed that a red Plasti Dip sealant greatly reduced the amount of ozone absorption by the pod foam. All exposed foam surfaces were sealed with one even coat of the Plasticoat sealant.

Further modifications to the pod were necessary to account for the positioning of the fans on the outside of the box. Using a carpenter’s knife and files, two square holes were cut into the foam for the placement of the fan holders. The holes were adjusted to account for the angled back of the fan holders, and tapered to a small area that would eventually reach into the box and the integrating sphere. A small area was cut out of the lid of the pod in order to fit the integrating sphere with the cap inside the pod. Two 9V batteries were attached to the sides of the pod using two zip ties per battery. Four zip ties were used to hold the electric board to the base of the pod.

Electrical Design
The three main circuit components that we used to measure the ozone in terms of electrical design are the red and green LEDs, the Photransistor along with the Operational Amplifier, and the SD card mount. With the circuit diagrams included, there are a few things to note regarding the choices made and why they were made.





LEDs
Research on the absorption spectra of ozone and other common atmospheric gases revealed that 603 nm was the wavelength at which peak absorption by ozone occurred. Red LEDs at 610 nm were chosen for the main indicator of ozone presence, as these LEDs were the closest to the targeted 603 nm and easiest to obtain commercially. Green LEDs were chosen at a wavelength of 565 nm, as visible light of wavelength 565 nm was found to have little to no absorption by ozone as well as other common atmospheric gases. Thus, the green LEDs were chosen as an adequate control light source. No light from the LEDs was also chosen as a control for this experiment.

It is also not seen on the circuit diagram, but two LEDs are actually connected to each section where an LED should be connected in an attempt to greatly increase the amount of light being produced. This totals 4 LEDs: two red and two green.

The two 100 ohm resistors are ceramic resistors connected in parallel. The ceramic resistors were chosen to minimize the heating and changing of resistance within the resistors over the course of the three hour flight. The resistors were connected in parallel in attempts to minimize the resistance and maximize the current (and by extension the brightness) going through the LEDs. This was deemed to be necessary because the LEDs are rather dim compared to what would be desirable for such a test, and connecting the resistors in parallel offered a better solution to maximizing the brightness of the LEDs.

Phototransistor and Operational Amplifier
The phototransistor choice was made as it was the most sensitive to change in the wavelengths of light that we were planning on using to investigate the ozone concentration. Since the phototransistor only produces varying current as a result of the varying levels of light, an operational amplifier that was configured into a transimpedance amplifier.

A transimpedance amplifier is simply a device that transforms current into voltage. A choice of 1.5 kilo ohm 4.7 kilo ohm produces a usable output range of about 0.8 V to 2.6 V (3.3 V – 0.7 V) when set up as shown in the diagram for the operational amplifier. The third resistor provides the negative feedback loop and is the component most directly responsible for the transformation of current to voltage. Using a 1 Mega ohm resistor proved to produce very desirable voltage ranges during testing with the red LEDs producing a reading of approximately 0.786 (2.59 V) with no ozone present.

Another option to boost the signal was to use the other part of the dual operational amplifier. However, further testing revealed that changing other variables was more effective in boosting the small signal obtained in preliminary tests. Thus, to boost the signal, other variables were changed, such as the position of the LEDs in the PVC cap and the diameter of the hole in the integrating sphere for the PVC cap. Maintaining the same amplifier setup will reduce the amount of noise in the collected data. Boosting the signal by changing the position of the LEDs and increasing the diameter of the hole in the integrating sphere was sufficient in boosting signal, though changing the amplifier setup may be considered for future projects.

SD card mount
This was rather simple with SPI protocol being used to communicate between the mbed and the SD card. Comments will be made about making sure the SD card properly mounted and recorded data throughout the flight, as well as making sure it could be removed at any 5 minute interval without completely ruining the data in the Code section of this wiki. The one thing to note electrically however is that the internal LEDs on the mbed were the main way we ensured that the device was either done reading data, or that something had gone wrong and no data was being recorded (the SD card had not been mounted properly).

Fans
The above mentioned fans were 5V computer cooling fans. They were put on a separate circuit from the mbed, to avoid electrical noise getting into our signal from the phototransistor. To power them, they were wired in parallel to a voltage divider, which would convert the 9V flow from the battery to the 5V required for the fan.

Other
Initially, a PCB board was going to be used to avoid the need for a perforated board and a lot of soldering. However, an error was made in the design of the board and the LEDs were not connected to the mbed electronically, and were constantly powered. There was also no way to power the system, as the batteries could not be connected to the mbed itself to provide power. Ideally a PCB board should be printed in the future to minimize the risk of parts becoming detached during flight.

Code
Here is the link to our code which is commented as to the more intricate details in terms of how it works.



The code outputs the data in TSV format, '''which is very important. If the data is not properly formatted for easy input into Mathematica, it will make things significantly more difficult in the long term to analyze data.'''

The main idea of the code is that there is a ticker that goes off every 5 seconds that triggers the collection of data. There is a timer that keeps track of the time, so that we can output the time to the text file. The extended timer library created by Dr. Larkin is used as the library that already existed did not work for long enough and so data would not record for the whole flight. Each data point consists of a reading from the photo transistor with the red light on, with the green light on, and with none of the lights on. Each reading averages the input from the photo transistor 10 000 times in an attempt to avoid any electrical noise.

In the code the data file is constantly opened and closed after every reading, with the file always being appended instead of created. This means that if the power were to cycle for any reason during the flight or a connection were to fail for some reason, we would only lose up to 5 seconds of data, as opposed to all the data. Without this we would be running a high risk of collecting no data at all over the course of the flight.

Something else to note is the internal LED s are used to indicate the success or failure of the mounting of the SD card. This is extremely important as the SD card not mounting/the file not opening correctly would be fatal to the collection of data during the flight. Another LED is used to indicate the completion of the collection of data.

Validation and Calibration
In observing the data, the light vs altitude has a correlation to the previously determined ozone vs altitude, somewhat validating our data, but the light levels decrease at altitudes lower than ozone should be present. We have attributed that to the lower temperatures as the pod rises, causing changes in the sensitivity of the phototransistor, and a lowering of the resistance of the circuit, which leads to lower readings.

Unfortunately, we could not calibrate the sphere, due to several factors. First, the testing tank does not reach sufficient amounts of ozone because of leaks and absorption of ozone by the wooden lid for the tank. Second, the resistors which amplify the signal from the phototransistor heat up in the testing chamber, which causes greater electrical resistance in the circuit, leading to inaccurately elevated readings.

A sealed phototransistor and light would need to be added to the pod as a control to find changes in light level as the pod goes up, to act as a control. Also, the testing chamber needs to be re-built to be both smaller and have less ozone-absorbing materials.

Data and Analysis
The command module altitude vs. flight time data and raw data for flight time vs. photodiode signal are attached below for reference.



Figure 13 shows the raw data, where photodiode signals were recorded for the red LED, green LED, and no light throughout the duration of the flight. The high-altitude balloon burst at approximately 107 minutes into the flight, which corresponds to the aberrant photodiode activity observed in the raw data graph around t=107 min.

Our data analysis primarily focuses on the photodiode activity for the red LED, which was used to measure ozone abundance. The data showed no significant deviations of the photodiode readings for green light from that of no light throughout the duration of the flight, validating our choice of the green LED as a control wavelength of light. Our data indicates that the balloon reached peak ozone abundance at approximately 70 minutes into flight. This finding suggests that the balloon passed through the layer of peak ozone abundance during the ascent, which is consistent with the decreasing concentrations of ozone shown thereafter in the graph. Though the photodiode signal readings were interrupted by the bursting of the balloon at a flight time of approximately 107 minutes, a similar set of activity can be seen at a faster rate on the balloon's descent: the photodiode signal readings again reached minimum values at around 125 minutes into the flight, which corresponds to the balloon passing through the layer of maximum ozone abundance, before increasing as the balloon descended into the lower atmosphere. As the balloon passed through to the lower atmosphere, the photodiode signal for red light leveled out at a notably higher reading compared to the readings before launch.

Our data suggests that the red LED used has a wavelength (610 nm) that is well-absorbed by ozone. Thus, an absorbance of the red light corresponds to a significant presence of ozone. A decrease in the red light photodiode readings corresponds to an increase in ozone concentration.

- Figure 14 shows the data for Voltage (Red light photodiode readings - no light photodiode readings) vs. Altitude. Our data shows that as altitude increased, the photodiode readings for red light decreased. As expected, higher concentrations of atmospheric ozone were detected by the sensor with increasing altitude. It is worth noting, however, that our sensor began detecting ozone absorption almost immediately after launch. Our research (see Figure 1) shows that ozone is not naturally abundant in the atmosphere until the troposphere meets the stratosphere, which occurs at approximately 15 km in altitude. This discrepancy could indicate a variety of different things, but it is difficult to draw concrete conclusions without calibrating the sensor properly.



Ideally, the data for the ascent and descent of the pod should look identical, as atmospheric ozone concentrations should be relatively constant at particular altitudes. However, the ascent data and descent data for the red light photodiode readings are significantly different, suggesting that the tropospheric ozone concentrations were substantially higher near the end of flight compared to at the beginning of the flight. Refer to Figure 15 for a graph of the photodiode readings for red light as a function of time.

The differences between the ascending data and the descending data may be attributed to temperature differences caused by the rapid descent of the balloon. The temperature of the pod was likely significantly colder during the descent compared to the ascent, as the pods were falling at approximately 2 miles per minute. During our calibration testing, it was noted that the readings of the sensor increased as the resistors and other parts of the circuit warmed up; thus, it is logical to think that the accuracy of the sensors and photodiode readings are compromised by substantial temperature changes.

Conclusion
We were successful in creating a low-cost integrating sphere which retrieved expected data trends. We were able to improve the stability of the integrating sphere, an issue that previous groups had struggled with, by using a large amount of epoxy, duct tape, and zip ties. Improvements to the code as well as the electrical design were also made. The data showed an increase in ozone concentration based on the absorption of light as the elevation increased throughout the flight. Our results suggest that there is an increase in ozone concentration as altitude increased, with a peak concentration of approximately 20 kilometers above sea level. However, because we were unable to calibrate the integrating sphere before or after the launch, we do not know the accuracy of our system.

Recommendations for Future Work
During testing we noticed that the longer the LEDs were on for, as well as other aspects of the circuit, the higher the readings we were receiving from the sphere. For future testing we would recommend attempting to correlate the temperature inside the box/of the circuit to changes in readings from the integrating sphere

As seen in the Mechanical Design section of the wiki, there is a somewhat unusual setup for the LEDs in the PVC cap. This is because during testing we found that the green LED signal was very low in comparison to the RED signal. By doing the setup seen in the image, and placing the LED s almost directly over the hole, we minimized the problem as best as we could. In future we would recommend either a complete rework of the setup, or purchasing of more powerful LED s so that the green LEDs would be useful.

During testing we found that the current setup for testing how our sensor reacts to ozone was significantly flawed, and should definitely be reworked. We are certain there are ozone leaks in multiple places, along with multiple different parts of the tank potentially absorbing significant amounts of ozone. In future we suggest a complete rebuild of this testing tank with proper seals and materials that will with 100% certainty not absorb ozone. This will allow for proper testing of the sensor and will actually provide a way to validate the data.

For our flight we had to do a makeshift solution for the phototransistor and sealing it from the outside. In future we recommend finding a way to properly seal in the phototransistor from not only light, but also from letting any gas out of the sphere through the hole it is coming in through

Building off of what was said in regards to the green LED and the LED setup needing work, potentially different/more LEDs could be used to track the concentrations of other atmospheric gasses such as Nitrogen. Research would have to be done to find the appropriate wavelength of light, as well as making sure the same problem doesn't arise of having low power LED bulbs.