Tracking Aerosol Lifetimes using Radioactive Tracers
Atmospheric aerosol particles have a large impact on air quality and climate. While much is known about how particles move around and affect clouds in the atmosphere, little is known about how fast they are removed from the atmosphere because direct observations are complex and elusive. In their quest for insight into the full aerosol lifecycle, scientists are looking for ways to better constrain aerosol particle removal simulated by global models.
Researchers including scientists at the Department of Energy’s Pacific Northwest National Laboratory (PNNL), used radioactive isotopes released from the Fukushima Dai-Ichi nuclear power plant accident of March 2011 as proxy tracers for the fate of sulfate aerosol and a transport of passive tracers. They found that the ensemble of 19 global models do not transport enough aerosol to the Arctic, indicating too fast a removal in the models and errors in the simulated atmospheric transport. This work indicates areas where models can improve in simulating aerosol lifetimes.
During the Fukushima Dai-Ichi nuclear power plant accident of March 2011, the radioactive isotopes cesium-137 (137Cs) and xenon-133 (133Xe) were released in large quantities. Cesium attached to particles in the ambient air, approximately according to their available aerosol surface area. Analysis showed that 137Cs could be used as a proxy tracer for the fate of accumulation-mode sulfate aerosol. In contrast, the noble gas 133Xe behaves almost like a passive transport tracer. Global surface measurements of the two radioactive isotopes taken over several months after the Fukushima Dai-Ichi release allowed the team to derive the lifetime of the carrier aerosol particles. The research compared this data to the lifetimes simulated by 19 different atmospheric transport models constrained by observed meteorology and initialized with identical emissions of 137Cs that were assigned to an aerosol tracer with each model's default properties of accumulation-mode sulfate; 133Xe emissions were assigned to a passive tracer. Using this comparison, the research found to what extent the modeled sulfate tracer can reproduce the measurements, especially the observed loss of aerosol mass over time. They sampled simulated 137Cs and 133Xe concentrations at the same location and times as station measurements to allow a direct comparison between measured and modeled aerosol lifetime. Deviations between measured and modeled aerosol lifetimes were largest for the northernmost stations and at later time periods, suggesting that models do not transport enough aerosol toward the Arctic. Among the 19 ensemble model simulations, PNNL scientists configured the Community Atmosphere Model (CAM5) with different meteorological nudging methods, vertical resolution, and/or treatments of aerosol removal to further understand how these configurations and parameterized processes affect the aerosol lifetime. They found that a new advanced treatment of aerosol removal gave a nearly three-fold increase in aerosol lifetime, in very close agreement with the observed. CAM5 results were also sensitive to the model vertical resolution and nudging method.