Radionuclides reveal age and source of aerosols collected over central North Atlantic

Abstract:

Aerosol filter samples were collected daily during summer 2013, at the Pico Mountain Observatory (PMO, 38.47°N, 28.40°W, 2,225 m a.s.l.), Azores Islands. PMO monitors free troposphere air and aerosols transported from neighboring continents; North America has the most frequent influence due to predominantly westerly winds in mid-latitude regions, while aerosols from Europe and Africa are sampled occasionally. The residence time during long-range transport in the atmosphere has a critical impact on aerosol chemical and physical properties, and it can be estimated by measuring activities of radionuclides attached to aerosols. ²¹⁰Pb (t_1/2 = 22.1 years) and ²¹⁰Po (t_1/2 = 138 days) are daughter nuclides in the decay chain of ²²²Rn, an inert gas species produced throughout the Earth’s crust and emitted into the atmosphere. Due to different rates of decay, the activity ratio of ²¹⁰Po to ²¹⁰Pb can be used to estimate atmospheric residence times of the carrier aerosols. ²¹⁰Po activity counting of 58 samples was conducted to investigate aerosol residence times in this study. ²¹⁰Po activity was measured twice serially for each aerosol sample to predict the initial activity of ²¹⁰Po on the sampling date and the activity of very slowly decaying ²¹⁰Pb. Aerosol ages calculated by the activity ratio of ²¹⁰Po to ²¹⁰Pb were compared with air tracer ages simulated using the FLEXible PARTicle dispersion model (FLEXPART) and studied together with aerosol particle physical properties. The activity of terrestrial radionuclides per unit of aerosol mass can also reveal source information of the aerosols. FLEXPART backward trajectories will be used to verify correlations between source regions and activity of radionuclides in aerosols. In previous research related to long-range atmospheric transport to PMO, FLEXPART has proven to be reliable in identifying upwind source regions.