Atmospheric ozone, a gas, plays a significant role in absorbing the Sun’s high-energy ultraviolet (UV) radiation before it reaches the Earth’s surface. Before the 1920s, understanding this layer was largely qualitative, based on indirect observations of the solar spectrum. The 1920s marked a major turning point, as scientists began seeking a reliable, quantitative method to measure the total amount of ozone overhead. This pursuit shifted ozone science from a historical curiosity to a field of systematic, global monitoring.
Early Attempts at Ozone Detection
For decades before the 1920s, ozone detection relied on the simple chemical Schönbein method, named after its discoverer. This technique utilized strips of filter paper coated with a mixture of potassium iodide, starch, and distilled water. When exposed to the air, ozone would react with the potassium iodide, causing the starch to turn a shade of purple or blue.
The resulting color change was compared against a numerical scale, giving a relative indication of surface-level ozone concentration. However, this method had severe limitations because the chemical reaction was highly sensitive to atmospheric humidity. High moisture levels often exaggerated the color change, meaning the result was not a true quantitative measure of ozone and was completely unreliable for measuring the stratospheric ozone layer. These measurements were limited to the lowest part of the atmosphere and were ultimately replaced by more sophisticated physical methods.
The Development of Spectroscopic Measurement
The application of spectroscopy brought a major advancement in ozone monitoring, as it was a physical method independent of chemical reactions or humidity. The breakthrough involved measuring the amount of solar UV light absorbed by the ozone layer. British physicist Gordon M. B. Dobson pioneered this approach starting in 1924 by developing a specialized instrument. This device, the precursor to the modern Dobson spectrophotometer, was designed to measure the total column ozone, which is the total amount of ozone in a vertical column of air extending from the ground to the top of the atmosphere.
The principle is based on ozone strongly absorbing UV light at specific wavelengths. The instrument compares the intensity of UV light at a wavelength strongly absorbed by ozone with the intensity at a nearby, weakly absorbed wavelength. By calculating the ratio of these two intensities as light passes through the atmosphere, scientists precisely determine the total amount of ozone present. Dobson’s first models used photographic plates to record the spectra. Later in the 1920s, he adapted the design to use photoelectric cells, allowing for more immediate and accurate readings.
Establishing the First Monitoring Stations
The spectroscopic method, established around 1924, laid the foundation for the world’s first systematic network of ozone observation stations. Dobson secured funding to build five additional spectrographs, which were deployed internationally between 1925 and 1930. These initial instruments were sent to various locations, including the measurement site at Oxford, UK, and an observatory in Arosa, Switzerland.
The establishment of these stations marked the beginning of long-term, standardized data collection. The Arosa station began its total column ozone time-series in 1926, which remains the longest continuous record in the world. The initial goal was to study the natural variability of the ozone layer, including daily, seasonal, and geographical fluctuations. This network provided the first reliable quantitative data, shifting atmospheric science from speculation to empirical observation.