Abstract
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.
Highlights
[2] Atmospheric aerosols have large impacts on climate, biogeochemistry and human health and these impacts often depend on the aerosol age, i.e., time since emission/formation
[19] Dust lifetime, which as mentioned above, is known as its residence time, is the average time that dust particles are expected to stay in the atmosphere and is defined as the total global dust burden in the atmosphere divided by the dust deposition rate
Dust age is defined as the time elapsed since a dust particle entered the atmosphere and is computed by mass‐age tracking (MAT)
Summary
[2] Atmospheric aerosols have large impacts on climate, biogeochemistry and human health and these impacts often depend on the aerosol age, i.e., time since emission/formation. [11] Our motivation for developing MAT stems from our efforts to model the effects of atmospheric aerosols on ocean biogeochemistry [Han et al, 2008; Krishnamurthy et al, 2009] For this purpose, we needed an age‐tracking method to satisfy the following requirements that, collectively, are not met by any of the previous methods: firstly, it runs online in GCMs/CTMs so that instantaneous (rather than climatological) tracer age is always known and can be coupled between atmosphere and ocean; secondly, it is computationally inexpensive; thirdly, it is generic and applies to any tracer simulated; fourthly, it gives mean mass‐weighted ages (which may be empirically related to aerosol solubility); lastly, it is deterministic and reproduces the same ages for any given meteorology and tracer physics. AAbbreviations: t is residence time, adps is age at deposition, awet is age at wet deposition, adry is age at dry deposition, and aair is age of dust aloft. bGlobal mean residence time (in days) from Zender et al [2003]
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