Particle Deposition in AERMOD: Overvie...Particle Deposition in AERMOD: Overview 2018 Regional/State/Locals Modeling Workshop Boston, MA James Thurman U.S. EPA/OAQPS/AQAD/AQMG 6/19/2018

Particle Deposition in AERMOD: Overview

2018 Regional/State/Locals Modeling Workshop

Boston, MAJames Thurman

U.S. EPA/OAQPS/AQAD/AQMG

6/19/2018 U.S. Environmental Protection Agency 1

Background• Recent interest in AERMOD deposition

– Polyfluoroalkyl sulfonate (PFAS), perfuorooctanoic acid (PFOA), Perfluorooctanesulfonic acid (PFOS)

– Hg deposition• AERMOD incorporates dry and wet deposition for particles and

gases– Generally not used for regulatory applications but can be incorporated if

important (Section 7.2.1.3 of Appendix W)• Will focus only on particle deposition here



Dry deposition fluxFd=CdxVdep

Fd=dry deposition flux (mg/m2/s)rate at which a mass is deposited to a surfacefrom the air over an area

Cd=concentration (mg/m3) calculated at zrVdep=deposition velocity (m/s)zr=deposition roughness height (m)=zo+1zo=surface roughness (m) from meteorological file


Wet deposition fluxFw=10-3rpxWpr

Fw=wet deposition flux (mg/m2/hr)rp=column average concentration (mg/m3) of particulate in airWp=Washout coefficientr=precipitation rate (mm/hr) from meteorological file

AERMOD methods of particle dry deposition• METHOD 1

– DEFAULT • From ISCST3• Based on Pleim et al. (1984)

– Inputs by size bin• Diameter (microns)• Mass fraction (0 to 1)• Density (g/cm3)

• Method 2– Added early 2000’s (Wesely et al., 2002)– Simplified approach when particle size distribution not well known– Non-default– Inputs

• Fine mass fraction (0 to 1)• Mean particle diameter (microns) of fine mass fraction


When do I use…?• Method 1

– A significant fraction (> 10%) of total particulate mass has a diameter of 10 microns or larger or,

– The particle size distribution is known• Method 2

– Particle size distribution is not well known and,– When a small fraction (< 10%) of total particular mass has a diameter

of 10 microns or larger• See Section 3.3.4 of AERMOD User’s guide


How are input parameters used?• Method 1

– Diameter and density used to calculate numerous variables yielding deposition velocities

– Fraction used in adjustment factor in concentration and deposition calculations (see model debug file)

• Method 2– Diameter and fraction used to calculate variables yielding deposition

velocities– Fraction used to in adjustment factor in concentration and deposition

calculations (see model debug file)



Modified Method 1 deposition velocity

Vd,i= Deposition velocity for bin i (m/s)Ra= Hourly varying aerodynamic resistance (s/m) (independent of method)Rp,i= Hourly varying deposition layer resistance (s/m) for bin i*Vg,i= Gravitational settling velocity (m/s) for bin iVdphor= Deposition velocity due to phoretic effects (0.0001 m/s)

*Rp,i replaces the Rd,i variable in the original Method 1 formulation as recommended by Wesely et al. (2002)

dphorigigipaipa

id VVVRRRR

V ++++

= ,,,,

,1


Method 2 deposition velocity

Vd1= Deposition velocity for fine particle mode (m/s)Vd2= Deposition velocity for coarse particle mode (m/s)Vdep= Total deposition velocity (m/s)Ra= Hourly varying aerodynamic resistance (s/m) (independent of method)Rp= Hourly varying resistance component (s/m) Ffine= Fine mass fraction0.002= Gravitational settling velocity (m/s) for coarse mode** Gravitational settling velocity assumed to be 0 m/s for fine mode

pad RR

V+

=1

1

+++=

papad RRRR

V002.0

1002.02


Key differences between dry deposition methods

• No Vg for fine mode in Method 2• Assumed Vg=0.002 m/s for coarse mode

– Reasonable compared to Method 1 Vg for coarse particles• No Vdphor for Method 2

( )( )µ

ρρ18

10 ,24

,iCFiairi

ig

SDiamgV

−−=


Key differences between dry deposition methods (continued)

• Rp calculation

( )inertiadjip xSchmidtuG

R+

= 3/2*

,1

*

500u

Rp =

( )( )LuRp 3001

500

*−−

=

Method 1 (see Appendix for variable calculations; diameter dependent)

Method 2 stable (L > 0)

Method 2 unstable (L < 0)

Other differences between dry deposition methods

• Method 1– Concentrations and deposition calculated for each particle bin– Total concentration is sum across bins

• Method 2– One concentration using the total deposition velocity


AERMOD method for wet deposition

• Same for both dry deposition methods• Key difference

– Particle bins looped for Method 1 and only 1 particle size bin for Method 2• See Appendix for variables and equations


Particle and gas deposition and depletion in AERMOD• To calculate deposition use DEPOS, DDEP, or WDEP on

MODELOPT line– DEPOS: Total deposition flux calculated (dry+wet)– DDEP: Dry deposition flux calculated– WDEP: Wet deposition flux calculated– Deposition values are not averages but total for the period, year, day, hour,

etc.– Default units of g/m2 for averaging period– Automatically invokes dry (DRYDPLT) and/or wet depletion (WETDPLT)


Particle and gas deposition and depletion in AERMOD (continued)

• Calculate deposition without depletion– NODRYDPLT and/or NOWETDPLT on MODELOPT line

• Calculate depletion without deposition calculations– DRYDPLT and/or WETDPLT but not DEPOS, DDP, or WDEP on MODELOPT

line• Either way, you need to enter particle information on SO pathway


Example: All fine particulates• Assume 1 particle size bin for Method 1 and 100% fine for

Method 2– Diameter: 1 micron– Density: 1.5 g/cm3

– Fraction: 1• Use DEPOS, DEPOS, DDEP, WDEP keywords on

MODELOPT pathway• Use DEPOS keyword with DEBUGOPT pathway

– Outputs hourly velocities and other parameters– Hard coded filename PDEP.DAT



CO STARTINGCO TITLEONE METHOD 1 CO MODELOPT CONC DEPOS DDEP WDEP FLATCO AVERTIME ANNUALCO POLLUTID OTHERCO RUNORNOT RUNCO DEBUGOPT DEPOSCO ERRORFIL ERRORS.OUTCO FINISHED

SO STARTINGSO LOCATION RUR1 POINT 0.0 0.0 0.0SO SRCPARAM RUR1 1000.0 100.000 425.000 26.500 5.600SO PARTDIAM RUR1 1.0SO MASSFRAX RUR1 1.0SO PARTDENS RUR1 1.5SO SRCGROUP RUR1 RUR1SO FINISHED

Method 1 Method 2CO STARTINGCO TITLEONE METHOD 1 CO MODELOPT CONC DEPOS DDEP WDEP FLATCO AVERTIME ANNUALCO POLLUTID OTHERCO RUNORNOT RUNCO DEBUGOPT DEPOSCO ERRORFIL ERRORS.OUT

CO FINISHED

SO STARTINGSO LOCATION RUR1 POINT 0.0 0.0 0.0SO SRCPARAM RUR1 1000.0 100.000 425.000 26.500 5.600SO METHOD_2 RUR1 1.0 1.0SO SRCGROUP RUR1 RUR1SO FINISHED


YYMMDDHH ISRC ICAT Method No. Ra Rp Vg(i) Vdep(i)OPTIONS: RegDFAULT CONC DEPOS DDEP WDEP ELEV DRYDPLT WETDPLT RURAL

11010101 1 1 METHOD_1 0.277318E+02 0.228991E+05 0.349755E-04 0.196038E-0311010102 1 1 METHOD_1 0.328232E+02 0.269521E+05 0.349755E-04 0.189478E-0311010103 1 1 METHOD_1 0.678853E+02 0.484997E+05 0.349755E-04 0.173001E-03

YYMMDDHH ISRC ICAT Method No. Ra Rp Vg(i) Vdep(i)OPTIONS: NonDFAULT CONC DEPOS DDEP WDEP FLAT DRYDPLT WETDPLT RURAL

11010101 1 - METHOD_2 0.277318E+02 0.168350E+04 0.349755E-04 0.584374E-0311010102 1 - METHOD_2 0.328232E+02 0.198413E+04 0.349755E-04 0.495798E-0311010103 1 - METHOD_2 0.678853E+02 0.357143E+04 0.349755E-04 0.274777E-03

PDEP.DAT Method 1

PDEP.DAT Method 2



Annual average concentrationNo deposition


Annual average concentrationMethod 1


Annual average concentration ratioMethod 1/No deposition

Minimum ratio: 0.999Mean ratio: 0.991Median ratio: 0.999Maximum ratio: 1.0


Annual average concentrationMethod 2


Annual average concentration ratioMethod 2/No deposition



Annual average concentration ratioMethod 1/Method 2



Annual deposition (g/m2)Method 1


Annual deposition (g/m2)Method 2


Annual dry deposition ratioMethod 1/Method 2


Deposition velocity ratiosMethod 1/Method 2Minimum ratio: 0.017Mean ratio: 0.368Median ratio: 0.251Maximum ratio: 4.208

Rp ratiosMethod 1/Method 2Minimum ratio: 9.641Mean ratio: 23.924Median ratio: 13.978Maximum ratio: 414.542

Summary• Methods 1 and 2 give different results for deposition velocities

and deposition flux• Concentrations appear to change little depending on which

method used• Need more research in how deposition incorporated in

AERMOD


Useful links

• ISCST3 user’s guide volume 2 (METHOD 1)– https://www3.epa.gov/ttn/scram/userg/regmod/isc3v2.pdf

• AERMOD user’s guide– https://www3.epa.gov/ttn/scram/models/aermod/aermod_userguide.p

df• AERMOD deposition algorithms document (draft)

– https://www3.epa.gov/ttn/scram/7thconf/aermod/aer_scid.pdf• Deposition report

– https://www3.epa.gov/ttn/scram/7thconf/aermod/driscdep.zip


https://www3.epa.gov/ttn/scram/userg/regmod/isc3v2.pdf

https://www3.epa.gov/ttn/scram/models/aermod/aermod_userguide.pdf

https://www3.epa.gov/ttn/scram/7thconf/aermod/aer_scid.pdf

AppendixDeposition equations



Ra (aerodynamic resistance) calculations

+

=

Lz

zz

kuR d

o

da 5ln1

*

−

−

+

−

+

−

−

−

=

11611161

11611161ln1

*

Lz

Lz

Lz

Lz

kuR

od

od

a

Stable (L>0)

Unstable (L < 0)

Zd=zo+1 (zo=surface roughness from sfc file)k=von Karman constant (0.4)u*=surface friction velocity (from sfc file)L=Monin-Obukhov length (from sfc file)


Method 1 Rp calculations: Schmidt number calculation

( )( )ooo

anu PP

PP

TT

X −+

×= − 0132.01101505.0

772.14 Ta=ambient temperature (from sfc file)

To=reference temperature (273.16 K)P=surface pressure (from sfc file)Po=reference pressure (101.3 kPa)

Kinematic viscosity of air (varies hourly; particle independent)

Gust adjustment factor (varies hourly; particle independent)1=adjG w*=0

+= 2

*

2*24.01

uwG adj w*≠0

w*=convective velocity scale (from sfc file)u*=surface friction velocity (from sfc file)P=surface pressure (from sfc file)Po=reference pressure (101.3 kPa)

nuXut

2*

1 =

( )( )i

xDiamamfp

iCF Diameaax

Smfpi

4

/21

, 102

13

−

−++=

a1=1.257a2=0.4a3=0.55xmfp=6.5x10-6

Diami=particle diameter for bin i

Slip correction factor for particle bin I; non-hourly varying

( )( )µ

ρρ18

10 ,24

,iCFiairi

ig

SDiamgV

−−=

Gravitational settling velocity for particle bin I; non-hourly varyingρi=density of particle bin iρair=Air density (1.2x10-3 g/cm3) µ=absolute viscosity of air (1.81x10-4 g/cm/s)

gV

T igistop

,, = g=gravity (9.80616 m/s2)

Stop time; non-hourly varying

Inertial impaction term for particle bin i; hourly varying

−

= 1,3

10 tTinert

istopx

Brownian diffusivity of particle in air (m2/s) for particle bin i; hourly varying

×= −

i

CFaiB Diam

STD 14

, 1009.8

iB

nui D

XSchmidt,

=

Schmidt number for particle bin i; hourly varying

( )inertiadjip xSchmidtuG

R+

= 3/2*

,1

34

Wet deposition calculations (vary hourly)

111.075.3 pfall RateV =

11.18

232.0pRate

RDROP =

Raindrop fall speed

Raindrop radius

Ratep=precipitation rate (from sfc file)

nu

falle X

VRDROPR ×= 01.0

Reynolds number

( )

( )e

e

c R

RStk

++

++=

0.1ln112

1ln2.1

Critical Stokes number

Diffusion term for scavaging ratios

For each particle bin i=1,n (Method 2, n=1)

( )ieieie

i SchmidtRSchmidtRSchmidtR

Term 16.04.014 3/1,1 ++

=

Ratio of particle to raindrop diameter (adjusted for units)

Interception term for scavaging ratios

RDROPDiami

i 02.010 6−

=κ

( )( )eiii RTerm 211081.14 2,2 +×= − κκ

Stokes number

Reset critical Stokes number to minimum of Stokes number and critical Stokes number

Inertial impaction term for scavaging ratios

RDROPVV

TStk igfallistopi 01.0

,,

−=

Stkc=minimum(Stki,Stkc)

5.1

3/1,3 2

+−

−=

ci

cii StkStk

StkStkTerm

Scale by the ratio of water (1 g/cm3) to particle density (g/cm3)

iii TermTerm ρ

1,3,3 =

Collision efficiency

ECOLLi=minimum(1,(Term1,i+Term2,i+Term3,i))

Washout coefficient

RDROPECOLLZ

WASHOUT ipi 02.0

5.1= Zp=Height of top of the plume (m)

Particle scavenging ratio

64.325.1, ×

=RDROP

RateECOLLP p

iiscat

Pscati=0

Ratep > 0

Ratep = 0



Documents

Particle Deposition in AERMOD: Overvie...Particle Deposition in AERMOD: Overview 2018 Regional/State/Locals Modeling Workshop Boston, MA James Thurman U.S. EPA/OAQPS/AQAD/AQMG 6/19/2018