In the fields of atmospheric science and greenhouse gas research, radon (Rn-222) is an important observational indicator. Since 2026, the Hong Kong Observatory has regularly submitted outdoor radon activity concentration data monitored at the King’s Park Meteorological Station to the Global Atmosphere Watch programme under the World Meteorological Organization for use by meteorological agencies and scholars worldwide.

Radon is a colourless, tasteless and odourless radioactive gas, mainly resulting from the decay of natural radionuclide uranium-238 in soils, rocks and even building materials. It is dispersed from the ground into the air. Radon gas releases alpha particles during its decay process. Calculating the activity concentration of radon depends on measuring the number of particles released during its decay. The outdoor radon monitoring instrument at the King’s Park Meteorological Station employs an active sampling design. The instrument's sampling component continuously samples outdoor air at a preset airflow rate, drawing it into the scintillation counting chamber. Before entering the chamber, the air passes through filter paper to remove radon progeny particles (radon decay products), ensuring the results accurately reflect the radon activity concentration in the air.

The inner wall of the scintillation counting chamber is coated with a layer of silver-activated zinc sulphide crystals (ZnS:Ag). After radon decays and releases high-energy alpha particles in the scintillation counting chamber, these alpha particles collide with the crystals, which then release blue visible photons. Since these light signals are rather weak, the system uses photomultiplier tubes to convert the captured photons into electronic signals and amplify them step-by-step to form current pulses that can be recorded and statistically analysed. Finally, the radon activity concentration in the air sample can be calculated based on the current pulse data and the instrument's detection efficiency. The advantage of this monitoring method is that the silver-activated zinc sulphide crystal is selective for alpha particles, making it less susceptible to interference from background radiation such as cosmic rays, thus helping to maintain data consistency.

To ensure the reliability of the observational data, the equipment is regularly calibrated using internationally recognized metrological standards, such as traceable standard sources from the National Institute of Standards and Technology (NIST). In addition to instrument calibration, technicians also purge the equipment approximately every two weeks using high-purity nitrogen gas. This procedure removes residual decay substances from the scintillation counting chamber, resets the instrument's background values, and removes moisture. Regular maintenance ensures the accuracy of the measurements.

Radon monitoring data has significant scientific value in meteorological research. After radon is released from soil and rocks into the atmosphere, it diffuses upwards with air currents and is diluted. Therefore, changes in radon concentration near the ground are closely related to the thickness and stability of the atmospheric boundary layer (the layer of atmosphere closest to the ground). When the atmosphere is stable, vertical airflow is obstructed, often forming an inversion layer (where temperature increases with altitude). In this case, the inversion layer acts like a "lid" over the city, trapping radon released from the ground within a shallower atmospheric boundary layer. Due to limited space for dilution, radon concentrations rise. By monitoring diurnal radon concentration differences, scientists can gain a more precise understanding of atmospheric mixing processes. For example, concentrations rise overnight when the atmosphere is stable and decrease during the day when convection is strong. This "natural indicator" is crucial for improving atmospheric physics models and climate evolution research.

Furthermore, in atmospheric science research, radon is considered an excellent "natural tracer" and an important tool for interpreting greenhouse gas data. This is primarily based on the unique physical characteristics of radon: its source exclusivity (released almost entirely from terrestrial landmasses) and its short half-life (approximately 3.8 days). These two major properties allow radon to function as a key indicator for identifying air mass trajectories and distinguishing between continental and marine contributions.

In practice, scientists primarily identify the emission sources of greenhouse gases through the following methods:

If a monitoring station concurrently detects high concentrations of both greenhouse gases and radon, it indicates that the air mass has lingered over or travelled across land over the past few days, accumulating both local greenhouse gas emissions and radon emanating from the ground along its path. Based on this, it can be inferred that the greenhouse gas increase most likely originates from terrestrial emission sources in cities or industrial zones of neighbouring regions.

Conversely, if an increase in greenhouse gas concentrations occurs while radon activity remains at an extremely low level, it reflects that the air mass likely originated from a remote oceanic background. Since oceans release virtually no radon, and during the prolonged transport over the sea, most of the radon originally carried by the air mass has already decayed into other elements due to its short 3.8-day half-life. Therefore, the extremely low radon level indicates that the increase in greenhouse gases is not driven by local land emissions but rather stems from long-range sea path transport, or there is release source over the nearby ocean.

Processed radon data is regularly submitted to the World Data Centre for Greenhouse Gases. This centre, situated in Japan and operating under the Global Atmosphere Watch programme, is responsible for collecting greenhouse gas and related tracer data worldwide. Through this data sharing, scientists can combine observations from different locations to analyse air mass origins and movement paths, constructing a more complete global atmospheric transport model.