Radon-222 monitoring at German ICOS atmosphere stations

<p>Atmospheric measurements of the short-lived radioactive noble gas radon-222 (<span class="inline-formula">²²²Rn</span>) have many applications. Its concentrations are driven by atmospheric mixing as well as seasonal variations, which follow the seasonality of <span class="inline-formula">²²²Rn</span> exhalation from continental soils, with lower values during the wet winter and spring months compared to more dry summer conditions. Hence, it can be used as a tracer to distinguish marine from continental air masses or for transport model validation. The Heidelberg Radon Monitor (HRM) is a static filter detector measuring atmospheric <span class="inline-formula">²¹⁴po</span>lonium (<span class="inline-formula">²¹⁴Po</span>), which is a progeny of <span class="inline-formula">²²²Rn</span>. These measurements can be used to infer atmospheric <span class="inline-formula">²²²Rn</span> activity concentrations if the radioactive disequilibrium between <span class="inline-formula">²¹⁴Po</span> and <span class="inline-formula">²²²Rn</span> at the measurement site is known. In this study, <span class="inline-formula">²¹⁴Po</span> activity concentrations measured with the HRM at eight stations in the Integrated Carbon Observation System (ICOS) Germany network are presented, along with guidelines for evaluating these data to estimate atmospheric <span class="inline-formula">²²²Rn</span> activity concentrations. In addition to the established line loss and disequilibrium corrections applied when sampling through long tubing or from air intake heights close to the ground, respectively, an upper limit for relative humidity (RH) is suggested, where secular equilibrium can still be assumed. At higher RH, aerosol scavenging effects can cause disequilibrium between <span class="inline-formula">²¹⁴Po</span> and <span class="inline-formula">²²²Rn</span>. Using comparison with the model this threshold is determined to be at about 98 <span class="inline-formula">%</span> RH and was applied uniformly at all measurement sites. A clear diurnal cycle of <span class="inline-formula">²²²Rn</span> is observed at all German tower stations during the summer and autumn months as is a seasonal cycle with a maximum during summer and autumn months. Overall, our results demonstrate that the <span class="inline-formula">²¹⁴Po</span>-based <span class="inline-formula">²²²Rn</span> measurements with the HRM are reliable if the equilibrium conditions between <span class="inline-formula">²¹⁴Po</span> and <span class="inline-formula">²²²Rn</span> can be ensured, i.e., for air intake heights above 80–90 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">m</mi><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">a</mi><mo>.</mo><mi mathvariant="normal">g</mi><mo>.</mo><mi mathvariant="normal">l</mi><mo>.</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="36pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="8e966d80b90499d50817f03759d7e713"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-17-6173-2025-ie00001.svg" width="36pt" height="12pt" src="essd-17-6173-2025-ie00001.png"/></svg:svg></span></span> during conditions with <span class="inline-formula">RH&lt;98</span> <span class="inline-formula">%</span>. The corrected and ready-to-use dataset of 10-<span class="inline-formula">year</span> radon activity concentrations from the eight ICOS stations is published alongside this paper (<a href="https://doi.org/10.18160/Q2M8-B1HJ">https://doi.org/10.18160/Q2M8-B1HJ</a>, <span class="cit" id="xref_altparen.1"><a href="#bib1.bibx16">Fischer et al.</a>, <a href="#bib1.bibx16">2024</a></span>).</p>