Dear all,

I would just like to confirm somethings on dry deposition.

Question 1: Is the parameter (deposition velocity) in HYSPLIT actually settling velocity in dry deposition due to gravity?

Question 2: Is deposition velocity applied to all particles at any time for dry deposition during simulations, regardless of model layers?

If so, the particles in the higher model layer will slowly settle toward the ground. Once the particles are close to the ground layer, the dry deposition occurs. HYSPLIT will calculate dry deposition (the flux: mass/m^2/s) using deposition velocity (m/s) and the air concentration (mass/m^3) above the ground layer?

I am very sorry that I have little knowledge in physics. I am not sure if I understand it correctly.

Your help would be much appreciated. Thanks.

Kind Regards,

Ming

## Deposition Velocity

### Re: Deposition Velocity

This section of the tutorial may help you.

https://www.ready.noaa.gov/documents/Tu ... inear.html

If particle diameter, density, and shape are all 0, then the computational particle is treated as a gas and

no gravitational settling is applied, but dry deposition may occur when the computational particle is in the lowest layer.

If particle diameter, density and shape are defined then gravitational settling is applied to the computational particle.

gravitational settling occurs no matter what layer the particle is in.

https://www.ready.noaa.gov/documents/Tu ... inear.html

If particle diameter, density, and shape are all 0, then the computational particle is treated as a gas and

no gravitational settling is applied, but dry deposition may occur when the computational particle is in the lowest layer.

If particle diameter, density and shape are defined then gravitational settling is applied to the computational particle.

gravitational settling occurs no matter what layer the particle is in.

### Re: Deposition Velocity

Dear alicec,

Thank you very much for your reply.

I attempt to specify a value for the deposition velocity, rather than calculating it from the particle diameter and density.

From what I understand, for particle dispersion, if a dry deposition velocity is specified, then that value is used as the particle gravitational settling velocity rather than the value computed from the particle diameter and density.

In gravitational deposition, the deposition velocity is the settling velocity due to the gravity-induced drag. Therefore, the gravitational settling velocity and the deposition velocity are the same thing.

More importantly, this value (deposition velocity) will be added to the vertical velocity in the particle trajectory advection computation. This probably means adding a settling velocity to computational particles motion throughout the atmosphere (not just in the surface/ground layer), so that computational particles will slowly settle toward the ground.

Once the computational particles are in the surface/ground layer, the dry deposition occurs. HYSPLIT will calculate dry deposition (the flux: mass/m^2/s) using the given deposition velocity (m/s) and the computational particles concentration (mass/m^3) above the ground layer.

Am I correct? Please confirm.

Another question:

According to the tutorial, there is an equation to calculate the deposition/setting velocity.

Vg = dp2 g (ρg - ρ) (18 μ)-1

μ is the viscosity of air. It should change with height in the atmosphere.

I am just wondering how HYSPLIT handles it, if the setting velocity is a constant for all computational particles in all layers. Does HYSPLIT use a constant to represent the viscosity, regardless of height?

My apologies for asking too many questions. I am just trying to understand how HYSPLIT works.

Your help would be much appreciated. Thanks.

Kind Regards,

Ming

Thank you very much for your reply.

I attempt to specify a value for the deposition velocity, rather than calculating it from the particle diameter and density.

From what I understand, for particle dispersion, if a dry deposition velocity is specified, then that value is used as the particle gravitational settling velocity rather than the value computed from the particle diameter and density.

In gravitational deposition, the deposition velocity is the settling velocity due to the gravity-induced drag. Therefore, the gravitational settling velocity and the deposition velocity are the same thing.

More importantly, this value (deposition velocity) will be added to the vertical velocity in the particle trajectory advection computation. This probably means adding a settling velocity to computational particles motion throughout the atmosphere (not just in the surface/ground layer), so that computational particles will slowly settle toward the ground.

Once the computational particles are in the surface/ground layer, the dry deposition occurs. HYSPLIT will calculate dry deposition (the flux: mass/m^2/s) using the given deposition velocity (m/s) and the computational particles concentration (mass/m^3) above the ground layer.

Am I correct? Please confirm.

Another question:

According to the tutorial, there is an equation to calculate the deposition/setting velocity.

Vg = dp2 g (ρg - ρ) (18 μ)-1

μ is the viscosity of air. It should change with height in the atmosphere.

I am just wondering how HYSPLIT handles it, if the setting velocity is a constant for all computational particles in all layers. Does HYSPLIT use a constant to represent the viscosity, regardless of height?

My apologies for asking too many questions. I am just trying to understand how HYSPLIT works.

Your help would be much appreciated. Thanks.

Kind Regards,

Ming

### Re: Deposition Velocity

If I understand what you are saying correctly, then yes, you are correct.

If you define a constant settling velocity for the particle (by specifying the deposition velocity for a 'particulate' computational particle),

then you over-ride the calculation and the settling velocity is the value that you specified everywhere. The equation that you reference is not utilized in that case.

If you define a constant settling velocity for the particle (by specifying the deposition velocity for a 'particulate' computational particle),

then you over-ride the calculation and the settling velocity is the value that you specified everywhere. The equation that you reference is not utilized in that case.

### Re: Deposition Velocity

Dear alicec,

Thank you very much for your reply. I really appreciate your help. It does make sense to me now. Thanks.

Kind Regards,

Ming

Thank you very much for your reply. I really appreciate your help. It does make sense to me now. Thanks.

Kind Regards,

Ming