Department of Earth System Science

We introduce and apply to dust aerosols an efficient method to track tracer age (time since emission) as a function of space and time in large-scale geophysical models. Our mass-age tracking (MAT) method follows the full tracer lifecycles directly and does not depend on proxy, ensemble, or Green's function techniques. MAT sends a mass-age tracer through the same algorithms that the host models use to predict tracer mass processes and then estimates age as the ratio of mass-age to mass. We apply MAT to size-resolved dust aerosol tracers to study the age of dust that remains in the atmosphere and the age of dust at deposition. The results include the first global distribution maps of aerosol age. Dust age varies with location, time, and particle size and is strongly sensitive to climate, wind and precipitation in particular. The global average age of dust at deposition agrees with residence time at ∼2.7 days, while dust in the atmosphere is, on average, twice as old. As expected, older dust prevails far from sources, at higher altitudes and in smaller sizes. Dust age exhibits a seasonal cycle, stronger for larger dust particles, that peaks in April–June, the period of maximum Asian and North African emissions. The oldest dust at deposition falls in the Antarctic and South Pacific Convergence Zone about 1 month after emission. The mass-weighted ages provided by MAT are useful for investigating and parameterizing the evolution of aerosol physical and chemical properties.