Examining Short-Term Variability in Gamma Radiation due to Atmospheric Phenomena
ARM data from remote Graciosa Island enable interpretation of atmospheric impacts on gamma radiation measurements.
This study focused on the short-term variability of gamma radiation measured continuously at the Atmospheric Radiation Measurement (ARM) Eastern North Atlantic (ENA) site on Graciosa Island, the Azores, and its association with atmospheric conditions. Atmospheric contributions to gamma radiation near the surface are still uncertain. In particular, precipitation may increase near-surface gamma radiation due to the scavenging effect of precipitation bringing radon progeny from the upper air to the ground surface. Graciosa is an ideal site for this study as it is sufficiently remote to be clear of direct continental influence. It typically experiences relatively clean conditions with air masses coming from the central North Atlantic, however, it is also subject to periodic episodes of polluted air masses from Western Europe, North Africa, and North America. The comprehensive suite of very detailed and high quality atmospheric measurements at the ARM site provide a unique source of ancillary observations for the interpretation of the gamma radiation measurements.
Understanding the temporal variability of gamma radiation from natural sources has important practical applications, such as routine monitoring of nuclear facilities. In this context it is critical to be able to discriminate between increased levels of gamma radiation associated with artificial radioactivity, and the natural variability in gamma radiation associated with specific atmospheric conditions such as the occurrence of precipitation. This study identifies atmospheric factors that contribute to variability in gamma radiation near the surface, including convective precipitation and the diurnal cycle of thermal stability.
This work addresses the short-term variability of gamma radiation measured continuously at the Atmospheric Radiation Measurement (ARM) Eastern North Atlantic (ENA) facility located in the Graciosa Island (Azores, 39N; 28W). The temporal variability of gamma radiation is characterized by occasional anomalies over a slowly-varying signal. Sharp peaks lasting typically 2-4 hours are coincident with heavy precipitation and result from the scavenging effect of precipitation bringing radon progeny from the upper levels to the ground surface. However the connection between gamma variability and precipitation is not straightforward as a result of the complex interplay of factors such as the precipitation intensity, the PBL height, the cloud’s base height and thickness, or the air mass origin and atmospheric concentration of sub-micron aerosols, which influence the scavenging processes and therefore the concentration of radon progeny. Convective precipitation associated with cumuliform clouds forming under conditions of warming of the ground relative to the air does not produce enhancements in gamma radiation, since the droplet growth process is dominated by the fast accretion of liquid water, resulting in the reduction of the concentration of radionuclides by dilution. Events of convective precipitation further contribute to a reduction in gamma counts by inhibiting radon release from the soil surface and by attenuating gamma rays from all gamma-emitting elements on the ground. Anomalies occurring in the absence of precipitation are found to be associated with a diurnal cycle of maximum gamma counts before sunrise decreasing to a minimum in the evening, which are observed in conditions of thermal stability and very weak winds enabling the build-up of near surface radon progeny during the night.
Susana Barbosa Susana Barbosa
INESC Technology and Science
This work is financed by the ERDF European Regional Development Fund through the Operational Programme for Competitiveness and Internationalisation – COMPETE 2020 Programme within project (POCI-01-0145- FEDER-006961), and by National Funds through the Portuguese funding agency, FCT as part of project (UID/EEA/50014/2013). Data were obtained from the Atmospheric Radiation Measurement (ARM) Program sponsored by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Climate and Environmental Sciences Division.
Barbosa, S.M., P. Miranda, and E.B. Azevedo. “Short-Term Variability of Gamma Radiation at the ARM Eastern North Atlantic Facility (Azores),” Journal of Environmental Radioactivity, 172:218-231 (2017). DOI: 10.1016/j.jenvrad.2017.03.027.