You're standing on a rooftop in Oxford, 1926. A man named Gordon Dobson is squinting at a homemade spectrometer, trying to coax a number out of the atmosphere. He's not measuring pollution. Here's the thing — he's not tracking climate change. He's just trying to figure out why the sky sometimes looks different — and whether that difference means anything at all.
That's where modern ozone monitoring begins. Not with satellites. Not with lasers. With a physicist, a prism, and a lot of patience.
What Is Ozone Monitoring in 1920
Ozone monitoring in 1920 wasn't a single technique. Day to day, it was a patchwork of chemical tricks, visual estimates, and early spectroscopy — each with its own blind spots. The goal was simple: figure out how much ozone sits in the column of air above you. Now, the execution? Anything but.
Most methods relied on ozone's reactivity. That property made it detectable, but also made it a nightmare to measure consistently. Fast. Even so, it oxidizes things. You couldn't just bottle air and analyze it later. Ozone decays in minutes. You had to measure it in situ, or at least preserve the signal the moment it formed Worth keeping that in mind. Turns out it matters..
The unit of measurement didn't even exist yet. The "Dobson Unit" — now standard — was decades away from being formalized. Day to day, in 1920, researchers reported results in "millimeters of ozone at standard temperature and pressure" or vague "relative intensity" scales. Comparing data across stations was basically guesswork.
The Schönbein Paper Method
This was the workhorse. Portable. Cheap. Used by meteorological services from London to Leningrad Small thing, real impact..
You take filter paper. Soak it in potassium iodide and starch. Plus, dry it. Plus, expose it to air. Ozone oxidizes iodide to iodine, which complexes with starch to form a blue-black color. The darker the stain, the more ozone.
Simple, right?
Not really. Humidity changes the reaction rate. Day to day, wind speed changes exposure. Other oxidants — nitrogen dioxide, peroxides — give false positives. The color develops over hours, not instantly. And judging "how blue" by eye? That's not data. That's an argument waiting to happen.
Still, it worked well enough for daily synoptic maps. The British Meteorological Office ran a network of these stations. So did Germany's Reichswetterdienst. They produced the first continental-scale ozone climatologies — flawed, but real.
Early Spectroscopic Approaches
A few labs tried something more precise: spectroscopy.
Ozone absorbs strongly in the ultraviolet, particularly the Hartley band (200–300 nm). If you split sunlight with a prism or grating, measure intensity at two wavelengths — one absorbed by ozone, one not — you can calculate the total column amount Practical, not theoretical..
The theory was solid. The practice? Brutal Not complicated — just consistent..
Glass absorbs UV. You need quartz optics. Quartz was expensive, hard to work, and rare. Detectors were photographic plates — slow, nonlinear, temperature-sensitive. You'd expose a plate, develop it, measure density with a microphotometer, then pray your calibration held.
Gordon Dobson at Oxford was the main player here. He built a double-prism quartz spectrograph, mounted it on a rotating table to track the sun, and spent years wrestling with stray light, thermal drift, and plate calibration. His 1926 paper in the Proceedings of the Royal Society laid the groundwork for everything that followed.
But in 1920? He was still debugging And that's really what it comes down to..
The Chemical Titration Method
A distant third option: bubble air through a potassium iodide solution, then titrate the liberated iodine with sodium thiosulfate Took long enough..
Accurate in the lab. Useless in the field.
You need a known flow rate, a known exposure time, a way to prevent ozone loss in the tubing, and a clean lab to run the titration. This leads to none of that existed at a remote weather station. A few research groups used it for calibration campaigns — but never for routine monitoring.
Why It Mattered Then
You might ask: who cared about ozone in 1920?
Meteorologists did. They'd noticed correlations between total column ozone and surface pressure patterns. High ozone often meant high pressure aloft — a clue to upper-air dynamics before radiosondes existed. In real terms, the Berliner Wetterkarte started publishing ozone contours alongside isobars. It was a proxy for stratospheric weather Worth keeping that in mind. Surprisingly effective..
Astronomers cared too. Ozone absorption shapes the solar spectrum reaching the ground. On top of that, if you're doing stellar spectroscopy, you need to correct for it. The "ozone hole" in the UV was a known nuisance long before it was an environmental crisis.
And there was a quiet debate: is ozone made in the stratosphere, or transported from the tropics? The answer required global measurements. Because of that, which required a reliable method. Which didn't exist yet And that's really what it comes down to..
So the stakes were scientific, not regulatory. No Montreal Protocol. But no sunscreen warnings. Just physicists and meteorologists trying to close a gap in the atmospheric puzzle.
How It Worked — The Daily Grind
Let's walk through a typical observation at a European weather station, circa 1922 Worth keeping that in mind..
Morning Setup
The observer — usually a junior meteorologist — retrieves the Schönbein papers from a desiccator. Day to day, humidity above 80%? Now, they're pre-cut, pre-soaked, numbered. He checks the hygrometer. Now, he notes it. The correction tables assume dry paper.
He mounts two papers on a wooden frame: one exposed, one control (wrapped in foil). But the frame sits on a stand, 1. 5 meters above grass, away from buildings. Orientation matters — downwind of the station's coal stove is a bad idea But it adds up..
Exposure
Eight hours. Even so, note it. Note it. The observer records wind, temperature, cloud cover every hour. That's why passing truck? Standardized to 8–16 local time. The paper integrates everything. That's why any fog? There's no "undo" button.
Development
At 16:00, he collects the papers. He picks the closest match. Back inside, he compares the exposed strip to a reference chart — a set of numbered blue shades, calibrated months ago in a lab. "Number 7." That's the raw index Nothing fancy..
Then he applies corrections. Humidity factor. Temperature factor. That said, wind speed factor. Each from a different table, each derived from different lab experiments. The final number: "3.2 mm O₃ at STP Worth keeping that in mind..
He writes it in the logbook. That's why telegraphs it to the central office. Repeats tomorrow That's the part that actually makes a difference..
The Spectrograph Session (Oxford Only)
At Dobson's lab, the routine is
At Dobson’s laboratory the work began long before sunrise. Even so, the instrument — a brass‑mounted spectrograph fitted with a quartz prism and a series of interchangeable filters — was calibrated each week against a mercury‑arc standard that emitted a line at 253. 7 nm, the wavelength most strongly absorbed by molecular oxygen and ozone. A thin, silver‑coated photodiode, housed in a light‑tight tube, recorded the intensity of the solar beam after it passed through the filter; the difference between the unfiltered and filtered readings yielded the optical density attributable to ozone.
The observer first aligned the instrument so that the sun’s rays entered the entrance slit at a fixed angle, then opened the shutter for a predetermined exposure — typically three to five minutes, long enough to average out fleeting cloud transients yet short enough to avoid saturation. That said, the resulting trace, a narrow spike on a moving chart recorder, was read at the nearest millimetre mark. By comparing the height of the spike to a pre‑compiled calibration curve, Dobson derived the total column ozone in Dobson Units (DU), a unit that aggregated the integrated absorption across the entire atmospheric column.
Because the spectrograph measured the wavelength‑dependent attenuation directly, it could detect subtle shifts in the spectral shape that the Schönbein paper method could not. A change in the UV band between 300 nm and 320 nm, for instance, hinted at variations in the ozone‑to‑oxygen ratio, while a flat, featureless trace suggested a stable, well‑mixed layer. These spectral nuances were logged alongside the paper‑based index, creating a dual record that allowed cross‑validation of the two independent techniques Still holds up..
The routine was not without friction. Now, seasonal variations in temperature altered the prism’s refractive index, demanding periodic re‑zeroing. On top of that, humidity in the lab could fog the quartz windows, forcing the technician to clean the optics with lint‑free swabs and distilled water before each set of measurements. Beyond that, the spectrograph required a clear sky; any overcast condition rendered the solar beam too diffuse for reliable quantification, so Dobson’s team often scheduled observations for the clearest mornings of the week.
When the two records — paper strips and spectrograph traces — were merged in the central archive, a more strong picture of the atmosphere emerged. The paper method supplied a daily, site‑specific column amount that could be telegraphed to neighboring stations, while the spectrograph offered a portable, spectral check that could be repeated at different locations or even from a moving balloon platform. Together they formed a modest but effective network, enabling the first truly global perspective on stratospheric ozone long before the advent of satellite remote sensing.
The meticulousness of these procedures did more than fill logbooks; it cultivated a culture of precision that would later underpin the sophisticated instrumentation used to monitor the ozone hole. The early 20th‑century observers understood that every decimal place mattered, because the data they gathered would become the benchmark against which future anomalies — sharp declines in the 1970s, sudden recoveries after the turn of the millennium — could be measured.
In hindsight, the daily grind of the Schönbein paper assay and the Dobson spectrograph session was the quiet engine of atmospheric science. By turning a simple colour change into a quantifiable index and by converting solar absorption into a calibrated unit, these pioneers transformed an invisible gas into a measurable component of Earth’s system. Their disciplined approach turned scattered observations into a coherent, worldwide dataset, laying the groundwork for the scientific scrutiny that eventually led to the Montreal Protocol and the modern quest to understand climate‑ozone interactions. The legacy of that painstaking routine endures in every satellite sensor that scans the stratosphere today, reminding us that even the smallest, most methodical steps can reshape our comprehension of the planet Not complicated — just consistent. Which is the point..
