Exploring atmospheric exchange with natural tracers

This study is focused on the application of radon (222Rn; half life time of 3.82 days) as an atmospheric tracer for evaluating the exchange of trace compounds between the land surface and the atmosphere and for understanding atmospheric mixing processes. For this purpose established methods for detecting the flux and the atmospheric concentration of 222Rn were compared. Based on the measurements of the 222Rn flux and concentration, we explored: (1) the exchange between land surface and the air near surface during nighttime to understand the characteristics of the very stable nocturnal boundary layer, and (2) the exchange between the air near surface and the air at high altitude to estimate the flux density of total bacterial cells from surface to high altitude.
The accuracy of 222Rn flux measurements is an important issue for the reliability of 222Rn tracer method. During summer of 2008 the 222Rn flux from soil was determined with different direct and indirect measurements at four eastern Spanish sites with different geological and soil characteristics. Direct 222Rn flux measurements with continuous and integrated monitors were in good agreement with the results obtained by indirect methods, which are based on correlations between 222Rn flux and the terrestrial γ dose rate, or the 226Ra activity in soil.
Another issue concerning the reliability of the 222Rn tracer method is the accuracy of atmospheric 222Rn measurements. For an inter-comparison the atmospheric 222Rn concentration was measured using either two-filter or one-filter instruments at the Schauinsland station in the Black Forest, Germany. These two types of detectors measured either 222Rn or its progeny activity concentration in the atmosphere. Generally, the values observed by the two detectors largely show a parallel behavior. However, occasionally meteorological and local conditions lead to differences in the concentrations determined by the two detectors. The 222Rn progeny, which was attached to aerosols, was negatively influenced by precipitation and forest canopy contact. Hence the disequilibrium between 222Rn and its progeny became larger during rainy conditions and longer surface contact. Progeny-derived 222Rn concentration compared to directly measured 222Rn concentration was reduced by these two factors by about 10 % and 15 %, respectively. Therefore, precipitation and forest canopy contact time should be taken into account when determining atmospheric 222Rn concentrations through measurements of its short-lived progeny with one-filter detectors.
During a campaign at K-puszta station in the Carpathian Basin (Hungary) we used the nocturnal inversion trapping of 222Rn, originating from soil, to verify the stability of the nocturnal boundary layer. The nocturnal 222Rn concentrations were monitored at two different heights (0.1 m and 6.5 m above the ground) together with the 222Rn flux density from the soil. 14 nights were selected that were characterized by strong cooling, light winds and clear skies. Typical nocturnal accumulation of 222Rn was investigated in a very shallow layer from sunset to sunrise in the following morning. The conservation of on average 72% (s.d. = 20%) of 222Rn emitted from soil in a shallow boundary layer supported the hypothesis of a two-stratum structure, where the shallow very stable boundary layer is decoupled and isolated from the atmosphere above it. In the large continental basin at K-Puzsta the stable structure was interrupted only intermittently by strong turbulent motions.
Finally the flux of total bacteria up to the high altitude Alpine environment was estimated by parallel observation of 222Rn concentrations and estimated 222Rn fluxes. Four campaigns were conducted over a 4-month period from late June to early October 2010 at the High Altitude Research Station Jungfraujoch, Switzerland (3580 m asl).. During stable weather conditions, the concentration of total bacteria was fairly constant and no significant flux of total bacteria was observed between the planetary boundary layer and the high altitude. The average background concentration of total bacterial cells at Jungfraujoch was about 3.4 ∙ 104 cells m-3 (s.d. = 0.8 ∙ 104 cells m-3). Following windy conditions, both flux and concentration of total bacterial celnt ls increased. This confirms that wind speed is an importafactor for the bacterial cells to become airborne. First large-scale estimates of total bacterial flux during fair weather conditions provided values between about 1 to 4 times 102 cells m-2s-1.
Our results show that 222Rn serves well as atmospheric tracer to investigate and quantify the atmospheric mixing and transport processes and to estimate the regional flux density of atmospheric trace compounds and bacteria. These results can be also used to improve regional or global atmospheric transport and climate models.