Featured image by NASA. All other images by Forrest M. Mims III.
The ozone layer envelops the entire Earth and protects us all — animals and plants — from most of the sun’s dangerous ultraviolet rays. While there is some ozone at the surface, the main ozone layer begins at the bottom of the stratosphere, just above the troposphere where we live and where weather occurs. This boundary is called the tropopause.
You can see where the tropopause merges into the stratosphere when a major thunderstorm is nearby. The anvil-shaped top of these storms flattens out at the tropopause.
Astronauts can see the ozone layer when the sun is just below the horizon. The featured image above is a 2014 photograph made by astronauts aboard the International Space Station (ISS) at sunset over Papua, New Guinea, that shows a very large thunderhead over Australia 1,500km (900 miles) away. The orange layer is the upper troposphere. The stratosphere begins at the tropopause just above the thunderhead, and the thick blue layer is the main section of the ozone layer. (For more details, search online for “ISS041-E-105277".) The lighter blue is the mesosphere.
The ISS photo suggests that ozone forms a thick, protective layer. But if all the ozone over your head were brought down to sea level, atmospheric pressure would compress it to a layer only 2.5 to 3.5 millimeters thick (0.1 to 0.14 inch). That’s around the thickness of a stack of two pennies!
In the central United States, the peak thickness of the ozone layer occurs at an altitude of around 25km (15.5 miles), and it thins out at around 40km (24.85 miles). Because of aerosol interference, a twilight photometer can measure the height of the ozone layer much more accurately than the full profile.
This shows how the peak altitude of the ozone layer can change significantly during a single day when stratospheric disturbances occur. At least that’s what Grok suggested when I reported this unusual pair of plots, which were made from my field in Texas the morning and evening of the same day.
You can see various ozone profiles measured by satellites by searching online or using Grok, ChatGPT, and other AI resources.
NOTE: Most ozone layer plots show the peak ozone on the right side of the chart. This is reversed when plotting twilight photometer data, which indicate a decline in the twilight signal when ozone
is present.
Atmospheric scientists track the ozone layer’s altitude because of its role in atmospheric circulation, rainfall, and studies of ozone’s recovery from losses caused by CFCs. I track its altitude for the same reasons, and to look for unexpected changes caused by unknown reasons.
Measuring the peak height of the ozone layer is the objective of this article. We will follow the footsteps of ozone pioneer F.W.P. Götz, who in 1930 made the first measurements of the height of the ozone layer over Arosa, Switzerland, with a bulky Dobson spectrophotometer pointed at the sun. I am very familiar with this instrument, for in 2016 I calibrated the world standard Dobson for NOAA during two months at Hawaii’s Mauna Loa Observatory.
Extracting the height of the ozone layer from Dobson measurements is not simple, so there is now much more interest in satellite ozone data. I have long wanted to develop a much simpler ground-based method based on twilight photometry.
While the strongest absorption of ozone occurs in the ultraviolet (UV) wavelengths, the French chemist and physicist James Chappuis discovered in 1880 that a broad band of visible light peaking at 575nm (yellow) and 603nm (orange) is also absorbed by ozone. In 1954, O.R. Wulf of the Smithsonian Astrophysical Observatory found variations in the ozone column during twilight caused by this Chappuis absorption.
I finally succeeded in measuring the height of the ozone layer with a twilight photometer on July 20, 2016, at the Mauna Loa Observatory using a 620nm LED for a detector (detection peak around 600nm) and an op-amp with a huge feedback resistor of 20 gigohms (GΩ). The peak ozone layer height from the photometer was 25.3km (15.7 miles) while a balloon-launched ozonometer launched from Hilo measured the peak height at 25.9km (16.1 miles), a difference of only 0.6km (0.4 mile) or about 2.5%.
Twilight Photometry
It is rather astonishing that a handheld instrument can accurately measure the height of the tropopause (10–15km or 6.2–9.3 miles), aerosol layers (5–25km or 3.1–15.5 miles), the ozone layer (25–35km or 15.5–21.8 miles), and even meteor smoke in the thermosphere (80–110km or 49.7–68.4 miles).
You can acquire these results during twilight by pointing straight up at the sky a filtered photodiode or LED connected to a very high-gain amplifier. This instrument is a twilight photometer, and construction details for one of my earliest were published in Make: Volume 44 (makezine.com/projects/twilight-photometer) and also in my book Make: Forrest Mims’ Science Experiments.
Here we will cover how twilight photometry can be achieved with a suitable LED mounted at the focus of a small telescope. The telescope is pointed straight up for an hour or more before sunrise, or after sunset. The LED will observe the light from the edge of Earth’s shadow as it rises or falls in altitude during twilight. The LED’s signal is amplified, stored in a data logger, and later processed in a spreadsheet program such as Excel.
My twilight photometry research described in Make: led to an assignment from NASA’s Dr. Dong Wu to develop twilight photometers to measure the altitude of aerosols from the historic eruption of the Hunga Tonga undersea volcano in 2022. For that project, I retained Hagerup Technical Services to construct five twilight photometers using LEDs as photodiodes.
While conducting Hunga Tonga aerosol measurements, I noticed that the NASA photometers found the altitude of the tropopause by detecting a subtle change in aerosols that occurs there. Earlier researchers mentioned this possibility, but never pursued it. Our many twilight measurements were within a few percent of the tropopause measured by weather balloons launched by the National Weather Service. This finding was so significant that Dong and I wrote a formal paper in 2024, “Tropopause Height Inferred from Twilight Photometry” in Bulletin of the American Meteorological Society (doi.org/10.1175/BAMS-D-24-0054.1).
The tropopause is the boundary between the troposphere, where weather occurs, and the very stable stratosphere, where the ozone layer occurs. It is typically 9–12km (5.6–7.5 miles) high over the U.S. When possible, aircraft pilots prefer to fly above the tropopause where the air is much more stable.
Ultra-Sensitive Ozone Layer Twilight Photometer
My first twilight photometers used conventional, low-noise op-amps with ultra-high-resistance feedback resistors.
The NASA-inspired photometers used a 1" telescope and an Analog Devices ADA4530, a relatively expensive surface-mount (SMD) op-amp that can amplify the photocurrent from an LED a trillion times. This ultra-high amplification provides less noise than my previous photometers.
The NASA twilight project led to a grant from Rolex to develop a much simpler photometer with even less noise than the NASA-sponsored units. While the NASA instruments worked well, I designed a much simpler circuit that used only an ADA4530, a 5GΩ or 10GΩ feedback resistor, and a small feedback capacitor (~10pF–20pF).
For the first time since I began twilight research in 2013, the new ultra-simple design sponsored by Rolex displayed a sharp negative spike at the tropopause rather than the slight dimple displayed by the NASA photometers. This spike was caused by a sharp drop in aerosols that clearly indicates the tropopause.
Building an ADA4530 Twilight Photometer
There are various ways to assemble a twilight photometer using the ADA4530 chip. Some are much less costly than the method described here. For example, you can use a $3 Fresnel lens instead of the expensive Thorlabs telescope I used.
Either way, the ADA4530 chip ($30–$40) and an LED are installed on a tiny 7/8"-diameter printed circuit board that fits inside a telescope tube. My friend Don Wilcher designed the board and arranged for its fabrication.
