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Sulfate is one important inorganic aerosol in the atmosphere. It's subject to removal by two main mechanisms.

  1. Wet deposition which include rain/wash-out and cloud deposition.
  2. Dry deposition which include impaction,diffusion,gravitational settling and etc.

This master thesis says:

Sulfate aerosol, in particular, is heavily impacted by dry deposition, which accounts for 40% of the total sulfur deposition on average (Baumgardner et al., 2002; Butler & Likens, 1995) and up to 55% of total sulfur deposition in forested regions (Linderg & Lovett, 1992).
Aerosol dry deposition depends on wind speed, solar radiation, and the type and condition of ground cover (Sheih et al., 1979).

But I'm confused about the status of forested region, which have the high relative humidity, frequent rain, and more surface the aerosol can condense on (cloud deposition process: In high elevation ecosystem, the cloud droplets can be interception by vegetation). In my opinion, it should enhance the wet deposition of sulfate

Does someone know why the dry deposition perform better in forested regions?

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    $\begingroup$ Forests provide biogenic emissions that provide a chemical pathway that allows for more sulfate aerosol to be produced. So, the total percentage of deposition that is "dry" increases, since you've converted more gaseous SO2 to an aerosol. $\endgroup$ – farrenthorpe Feb 21 '16 at 22:51
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    $\begingroup$ Gravitational settling of small particles is difficult if you have strong winds and turbulence all the way down to the ground. Forests provide a way to have a relatively still boundary layer in which gravity can act. It also provides a ground covered in vegetation, leaves, etc, that further shelters small particles that have settled. $\endgroup$ – Wolfgang Bangerth Feb 21 '16 at 23:41

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