O3Meter - Ozone measurement

AperCloud

About this app

O3meter turns a photograph of an ozonoscope paper strip into an
objective, numerical ozone value.

The ozonoscope is the oldest method for measuring surface ozone: paper
strips impregnated with potassium iodide and starch (Schönbein's method)
which, after exposure to outdoor air and immersion in distilled water,
develop a colour whose intensity depends on the amount of ozone
captured. Traditionally the reading depended entirely on an observer
comparing the strip by eye against a printed reference scale — a
subjective step that limits comparability between people, days and places.

O3meter removes that subjectivity. It analyses the photograph pixel by
pixel, computes the mean value of the red, green and blue channels,
converts the result to the HSV colour space and maps the hue — from
yellow (no ozone) through red to blue (maximum measurable ozone) — onto
a graduated internal scale from 0 to 180, which can be then converted to
the historical Schönbein (0–10) and Berigny (0–21) scales.

MAIN FEATURES
- Take a photo with the device camera or load an existing image from the
gallery.
- Region-of-interest selection, to restrict the analysis to part of the
strip when the coloration is not uniform.
- Numerical output on the internal 0–180 scale and its equivalent on the
Schönbein and Berigny scales.
- Mean RGB and HSV values displayed, so the computation is fully
transparent.
- Colour bar with a marker at the measured position.
- Designed to work with a low-cost set-up: a cardboard box, the phone
camera and an auxiliary light source.

WHO IS IT FOR
- Secondary school and university teachers and students. O3meter
supports a peer-reviewed laboratory practice on tropospheric ozone
already delivered to hundreds of students, and an openly available
teaching unit.
- Researchers recovering and homogenising historical surface ozone
records, for which the ozonoscope is the only direct source of data for
the second half of the nineteenth century and the early twentieth century.
- Citizen science, outreach and low-resource settings, where
UV-absorption analysers are not available.

FREE AND OPEN SOURCE
O3meter is released under the GNU General Public License v3. The source
code of both the Android and the desktop versions is available at
https://github.com/EPhysLab-UVigo/O3METER. The desktop version
additionally reads RAW camera files for high-accuracy measurements under
controlled illumination.

PLEASE NOTE
O3meter does not measure ozone on its own: it requires ozonoscope paper
strips prepared and exposed following Schönbein's method. The result is
a semi-quantitative value on the Schönbein and Berigny scales, not an
absolute concentration in ppbV or µg/m³.

Developed at EPhysLab (CIM-UVigo, Universidade de Vigo). The associated
project, "Ozone measurement practice in the laboratory using Schönbein's
method", was awarded a grant in the first edition of the European
Geosciences Union Higher Education Teaching Grant programme.