Quick, Dirty and Cheap

mistaureolinMechanics

Oct 27, 2013 (4 years and 13 days ago)

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Quick, Dirty and Cheap
Inorganic Aerosol
Thermodynamics for Regional
Air Quality Modeling

K. Max Zhang, Cornell University

Anthony S. Wexler, UC
-
Davis

What We Need


Gas
-
particle partitioning is driven by the
differences between partial pressure and the
equilibrium vapor pressure at the particle
surface.


The equilibrium vapor pressures of volatile
species (HNO
3
, HCl, NH
3
, NH
4
NO
3

and NH
4
Cl)
given aerosol composition, temperature and
relative humidity;


Water content associated with the particulate
composition.

What We Don’t (Really) Need


The detailed phase state information, while
useful for other purposes such as interpreting
laboratory results, may not be necessary for
gas particle partitioning

Limitations


We don't know the RH
very well.


We don't know if we
are in equilibrium since
we usually don't know
if we are on the rising
or falling leg of the
curve.

Tang et al., 1997

The UCD Aerosol Module


Developed by Sun, Zhang and Wexler.


Employs a simplified thermodynamics model of
the H
+
-
NH
4
+
-
Na
+
-
SO
4
2
-
-
NO
3
-
-
Cl
-

system.


The aerosol thermodynamics is calculated
without Gibbs free energy minimization.


Activity Coefficients


Mutual Deliquesce Relative Humidity


Phase States and Existence of ammonium salts


Water Content

Activity Coefficients


Low Ion Strength
-

Pitzer’s method


High Ion Strength
-

Kusik
-
Meissener/Bromley
method


Intermediate Ion Strength
-

Weighted average
between the two methods

Mutual Deliquesce Relative Humidity


Ran the AIM model to identify
MDRH’s for all possible ionic
composition and identified
“characteristic salts” (which
governs the MDRH)


Compiled results into a look
-
up
table.


Approximate combination of
solids that may exist under
different relative compositions.


Search the look
-
up table for
MDRH.

Phase States


The

aerosol

phase

is

assumed

to

be

in

pure

solid

state

when

the

RH

is

below

the

MDRH
.


If

the

RH

is

higher

than

the

DRH

of

any

possible

salts,

the

particle

phase

is

assumed

to

pure

aqueous


MDRH

is

equal

to

zero

in

“free

acid”

region,

parameterized

by

[H
+
]

/

[SO
4
2
-
]

>

1
.


Existence of Solid Ammonium Salts
(
Vapor Pressure Cap)


Constant

Activity Coefficient

Vapor
pressure

molality

Water Content


The water content associated with aerosol salts
are calculated using the ZSR method.


The single solute molality data compiled from
numerous experiments by Clegg et al. for the
AIM model, and interpolate for the appropriate
relative humidity and temperature.


Water content in a mixed phase state is taken as
a weighted mean of “dry state” and “wet state”,
similar to ISSOROPIA.

AIM vs the UCD Aerosol Module

Acknowledgement


USEPA


California Air Resources Board


VISTAS


Dr. Robin Dennis, Dr. Ajith Kaduwela and Dr.
Gail Tonnesen