Document ID: EPA-HQ-OAR-2002-0076-0428
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2005-06-14T04:00Z

MEMORANDUM
From:
BART
Project
Team,
EPA
Office
of
Air
Quality
Planning
and
Standards
To:
Docket
for
BART
Guidelines,
OAR
2002­
0076
Subject:
Fine
Particles:
Overview
of
Atmospheric
Chemistry,
Sources
of
Emissions,
and
Ambient
Monitoring
Data
Date:
April
1,
2005
A.
Introduction
Particulate
matter
is
a
chemically
and
physically
diverse
mixture
of
discrete
solid
particles
and
liquid
droplets.
It
exists
in
the
air
in
a
range
of
particle
sizes,
from
submicrometer
to
more
than
30
micrometers
in
size.
The
composition
of
particles
varies
throughout
this
range
of
sizes,
depending
on
the
age
of
the
particle,
the
nature
of
the
source
of
pollutant
emissions,
and
the
source's
operating
characteristics.
This
regulation
focuses
on
reducing
ambient
concentrations
of
the
PM2.5
size
fraction
of
PM.
The
term
PM2.5
is
used
to
describe
the
fraction
of
particles
whose
nominal
aerodynamic
diameter
is
less
than
or
equal
to
2.5
micrometers.
PM2.5
in
the
ambient
air
is
defined
operationally
as
the
set
of
particles
measured
(
and
associated
concentration)
by
the
Federal
Reference
Method
sampling
device.
Since
the
cut
point
of
this
sampling
device
is
not
perfectly
sharp,
some
particles
smaller
than
2.5
micrometers
are
not
retained
and
some
particles
larger
than
2.5
micrometers
are
captured
by
sampling
devices.
This
is
important
because
there
are
two
relevant
modes
to
the
PM
size
distribution,
fine
PM
(
nominally
PM2.5)
and
coarse
PM
(
nominally
from
2.5
to
10
micrometers
aerodynamic
diameter).
These
modes
overlap
slightly,
but
they
are
generally
associated
with
distinctly
different
source
types
and
formation
processes.
Fine
particles
emitted
directly
into
the
air
in
a
stable
solid
or
liquid
chemical
form
are
referred
to
as
"
primary"
particles.
Particles
formed
near
their
source
by
condensation
processes
in
the
atmosphere
are
also
considered
to
be
primary
particles.
PM2.5
that
is
formed
by
chemical
reactions
of
gases
in
the
atmosphere
is
considered
to
be
"
secondarily"
formed
particulate
matter.
PM2.5
in
the
atmosphere
is
composed
of
a
complex
mixture
of
constituents:
sulfate;
nitrate;
ammonium;
particle­
bound
water;
black
carbon
(
also
known
as
elemental
carbon);
a
great
variety
of
organic
compounds;
and
miscellaneous
inorganic
material
(
sometimes
called
"
crustal
material,"
which
includes
geogenic
dust
and
metals).
Atmospheric
PM2.5
also
contains
a
large
number
of
elements
in
various
compounds
and
concentrations.
Some
organic
materials
such
as
pollen,
spores,
and
plant
detritus
are
also
found
in
both
the
fine
and
coarse
particle
modes
but
from
different
sources
or
mechanisms.
Crustal
materials
such
as
calcium,
aluminum,
silicon,
magnesium,
and
iron
are
found
predominately
in
coarse
mode
particles.
Nitrate
is
generally
found
in
the
fine
particle
mode,
but
it
is
also
found
in
the
coarse
mode
particles,
coming
primarily
from
the
reaction
of
gasphase
nitric
acid
with
preexisting
coarse
particles.
Primary
coarse
particles
are
usually
formed
by
mechanical
processes.
This
includes
material
emitted
from
such
sources
as
wind­
blown
dust,
road
dust,
and
particles
formed
by
abrasion,
crushing,
and
grinding.
Some
combustion­
generated
particles
such
as
fly
ash
and
soot
also
are
found
in
the
coarse
mode.
Primary
PM2.5
includes
soot
from
diesel
engines,
a
wide
variety
of
organic
compounds
condensed
from
incomplete
combustion
or
cooking
operations,
and
compounds
such
as
arsenic,
selenium,
and
zinc
that
condense
from
vapor
formed
during
combustion
or
smelting.
The
concentration
of
primary
PM2.5
in
the
air
depends
on
source
emission
rates,
transport
and
dispersion,
and
removal
rate
from
the
atmosphere.
Secondary
PM
is
formed
by
chemical
reactions
of
gas­
phase
precursors
in
the
atmosphere.
These
reactions
form
condensable
vapors
that
either
form
new
particles
or
condense
onto
other
particles
in
the
air.
Most
of
the
sulfate
and
nitrate
and
a
portion
of
the
organic
compounds
in
the
atmosphere
are
formed
by
such
chemical
reactions.
Secondary
PM
formation
depends
on
numerous
factors
including
the
concentrations
of
precursors;
the
concentrations
of
other
gaseous
reactive
species
such
as
ozone,
hydroxyl
radicals,
peroxy
radicals,
or
hydrogen
peroxide;
atmospheric
conditions
including
solar
radiation,
temperature,
and
relative
humidity
(
RH);
and
the
interactions
of
precursors
and
pre­
existing
particles
with
cloud
or
fog
droplets
or
in
the
liquid
film
on
solid
particles.
Several
atmospheric
aerosol
species,
such
as
ammonium
nitrate
and
certain
organic
compounds,
are
semivolatile
and
are
found
in
both
gas
and
particle
phases.
Given
the
complexity
of
PM
formation
processes,
new
information
from
the
scientific
community
continues
to
emerge
to
improve
our
understanding
of
the
relationship
between
sources
of
PM
precursors
and
secondary
particle
formation.
Certain
particles,
such
as
sulfates,
nitrates,
and
certain
organics,
readily
take
up
water
and
are
considered
to
be
hygroscopic.
As
a
result
of
the
equilibrium
of
water
vapor
with
liquid
water
in
hygroscopic
particles,
many
ambient
particles
contain
some
amount
of
liquid
water.
When
filter
samples
are
weighed
at
lower
relative
humidity
levels
according
to
the
PM2.5
Federal
reference
method
specifications,
the
filters
are
desiccated
and
much
of
this
water
is
removed,
but
some
particlebound
water
will
be
measured
as
a
component
of
the
particle
mass.
Particle­
bound
water
in
the
ambient
air
increases
with
higher
relative
humidities.
This
phenomenon
is
important
because
it
affects
the
size
of
certain
particles,
and
in
turn,
their
1
US
EPA
(
1996).
Air
Quality
Criteria
Document
for
Particulate
Matter
­
Volumes
I,
II
and
III,
EPA/
600/
P­
95001aF,
EPA/
600/
P­
95/
001bF,
and
EPA/
600/
P­
95/
001cF.
Also:
USEPA,
2003.
Air
Quality
Criteria
for
Particulate
Matter
(
Fourth
External
Review
Draft).
EPA/
600/
P­
99/
002aD
and
bD.
U.
S.
Environmental
Protection
Agency,
Office
of
Research
and
Development,
National
Center
For
Environmental
Assessment,
Research
Triangle
Park
Office,
Research
Triangle
Park,
NC.
June
2003.
Available
electronically
at
http://
cfpub.
epa.
gov/
ncea/
cfm/
partmatt.
cfm.
properties
of
light
scattering
and
aerodynamics.
Differences
in
relative
humidity
can
result
in
different
measured
particle
size
distributions,
mass
concentrations,
and
resulting
visibility
impairment
levels.
Regional
emission
reduction
strategies
to
reduce
PM2.5,
particularly
hygroscopic
particles
such
as
sulfates
and
nitrates,
should
also
provide
significant
visibility
improvements,
both
in
urban
areas
and
in
federal
class
I
areas
(
national
parks
and
wilderness
areas).
The
following
discussion
elaborates
on
the
relationship
between
source
types
and
the
composition
of
PM2.5.
More
information
and
references
on
the
composition
of
PM
may
be
found
in
the
EPA
1996
PM
Air
Quality
Criteria
Document
and
draft
updates
we
released
during
the
past
year.
1
B.
Concentration,
composition
and
sources
of
fine
PM
The
relative
contribution
of
PM2.5
components
varies
significantly
by
region
of
the
country.
Data
on
PM2.5
composition
primarily
in
urban
areas
is
available
from
the
EPA
Speciation
TABLE
2.
PM2.5
CHEMICAL
COMPOSITION
DATA,
SEPTEMBER
2001
­
AUGUST
2002
URBAN
SITES
RURAL
SITES
T
o
tal
T
o
tal
R
EGION
Es
t
ima
ted
C
a
rbo
n
C
rus
ta
l
UR
B
A
N
Es
t
ima
ted
C
a
rbo
n
C
rus
t
a
l
R
UR
A
L
(#
s
ite
s
)
M
e
t
ric
Sul
fa
te
Ammo
nium
N
it
rate
M
ass
M
a
t
e
ria
l
T
O
T
A
L
Sulf
a
t
e
Ammo
nium
N
it
rate
M
as
s
M
a
te
rial
T
O
T
A
L
Southeas
t
Mass
4.0
1.7
0.7
5.1
0.6
12.1
3.2
1.4
0.4
2.8
0.5
8.3
(
8
urban/
11
rural)
%
33%
14%
6%
42%
5%
100%
39%
17%
5%
34%
6%
100%

Midw
est
Mass
4.1
2.1
2.6
5.2
0.7
14.7
4.2
1.8
1.1
2.8
0.7
10.6
(
8
urban/
7
rural)
%
28%
14%
18%
35%
5%
100%
40%
17%
10%
26%
7%
100%

Eas
t
Coas
t
Mass
4.5
2.3
2.0
6.2
0.6
15.6
4.1
1.6
0.7
3.1
0.5
10.0
(
10
urban/
8
rural)
%
29%
15%
13%
40%
4%
100%
41%
16%
7%
31%
5%
100%

California
Mass
1.9
2.7
7.0
9.5
1.0
22.1
0.8
0.4
0.4
2.1
0.6
4.3
(
5
urban,
14
rural)
%
9%
12%
32%
43%
5%
100%
19%
9%
9%
49%
14%
100%

Desert­
West
Mass
1.3
0.8
1.0
6.1
1.5
10.7
0.7
0.3
0.2
1.2
1.3
3.7
(
5
urban/
29
rural)
%
12%
7%
9%
57%
14%
100%
19%
8%
5%
32%
35%
100%

Nor
thw
e
s
t
Mass
1.2
0.7
0.8
4.4
0.4
7.5
0.4
0.3
0.2
1.7
0.4
3.0
(
2
urban/
17
rural)
%
16%
9%
11%
59%
5%
100%
13%
10%
7%
57%
13%
100%

East
Texas
/
South
Mass
3.3
1.6
1.1
4.2
0.9
11.1
3.0
1.4
0.8
2.4
0.8
8.4
(
3
urban/
3
rural)
%
30%
14%
10%
38%
8%
100%
36%
17%
10%
29%
10%
100%

Far
North
Eas
t
Mass
2.7
1.3
1.1
4.3
0.4
9.8
2.1
0.9
0.4
2.3
0.3
6.0
(
2
urban/
11
rural)
%
28%
13%
11%
44%
4%
100%
35%
15%
7%
38%
5%
100%

Nor
th
Plains
Mass
1.8
1.2
1.9
3.0
0.6
8.5
0.7
0.4
0.2
1.5
0.5
3.3
(
2
urban/
17
rural)
%
21%
14%
22%
35%
7%
100%
21%
12%
6%
45%
15%
100%

Source:
EPA
Spe
ciation
Trends
Netw
ork,
IM
PROVE
visibility
m
onitor
ing
ne
tw
ork
Note
s
:

1.
A
ll
units
are
in
micrograms/
cubic
meter.
A
ll
mass
numbers
represent
median
annual
average
values
for
the
time
period
September
2001­
Au
2.
A
ll
sites
included
in
analyses
had
complete
data
for
this
time
period
as
def
ined
by
50%
or
more
observations
per
quarter
for
all
major
chem
3.
A
ll
Ammonium
concentrations
are
estimated
f
rom
a
`
f
ully­
neutralized'
assumption
of
ammonium
sulf
ate
and
ammonium
nitrate.

4.
The
regions
are
def
ined
as
f
ollow
s:

South
East:
A
L,
SC,
NC,
LA,
MS,
TN,
FL,
GA
Mid
West:
IL,
IN,
Eastern
IA,
Southern
MI,
South­
Eastern
WI,
MO,
OH,
KY
East
Coast/
North
East:
DC,
MD,
NJ,
NY,
Nor
thern
VA,
PA,
Northern
WV
CA:
All
of
CA
Desert
West:
Eastern
AZ,
CO,
NV,
UT,
Western
TX,
NM
North
West:
WA,
OR,
ID
East
Texas,
South:
Dallas,
Houston,
AK,
Southern
OK
Far
North
East:
ME,
V
T,
NH,
Upstate
NY
North
Plains:
MN,
Dakotas,
Upper
MI,
MT,
WY
Trends
Network
beginning
in
2001.
PM2.5
composition
data
for
primarily
rural
areas
(
e.
g.
national
parks
and
wilderness
areas)
is
available
from
the
IMPROVE
visibility
monitoring
network
beginning
in
1988.
Speciation
data
from
September
2001
to
August
2002
are
summarized
for
urban
and
rural
areas
in
nine
regions
in
table
2.
This
discussion
focuses
on
the
eastern
U.
S.
and
California
since
most
nonattainment
areas
will
be
located
in
those
regions.
In
general,
urban
areas
have
higher
annual
average
PM2.5
concentrations
than
nearby
rural
areas.
In
the
eastern
U.
S.
urban
areas,
ammonium
sulfate
and
total
carbon
(
comprised
of
black
carbon
and
organic
carbon)
are
the
dominant
species,
each
accounting
for
30­
40
percent
of
total
reconstructed
mass
in
most
locations.
(
Reconstructed
mass
is
the
PM
mass
calculated
by
adding
together
the
mass
from
each
of
the
main
components
of
PM
2
V.
Rao,
N.
Frank,
A.
Rush,
F.
Dimmick,
"
Chemical
Speciation
of
PM2.5
in
Urban
and
Rural
Areas,"
In
the
Proceedings
of
the
Air
&
Waste
Management
Association
Symposium
on
Air
Quality
Measurement
Methods
and
Technology,
San
Francisco,
November
13­
15,
2002.
as
obtained
from
chemical
composition
monitoring.)
Nitrate
plus
associated
ammonium
ion
is
a
more
significant
component
of
PM
mass
in
northern
regions,
such
as
the
midwest
and
east
coast,
but
is
a
less
significant
fraction
in
the
southeast.
In
California,
the
main
species
contributing
to
urban
PM2.5
mass
are
ammonium
nitrate
(
35­
40
percent)
and
total
carbon
(
43
percent),
while
sulfate
and
associated
ammonium
accounts
for
approximately
10­
15
percent.
Table
3
compares
chemical
composition
data
for
13
pairs
of
urban
and
nearby
non­
urban
sites
in
order
to
identify
the
primary
components
that
make
up
the
"
urban
increment."
To
conduct
this
analysis,
for
each
species
the
PM2.5
mass
in
the
rural
location
is
subtracted
from
the
species
mass
for
the
urban
location.
The
amount
by
which
the
urban
site
exceeds
the
nearby
rural
site
is
the
"
urban
increment."
2
Chemical
Species
West
(
3
site
pairs)
East
(
10
site
pairs)

Min
Max
Avg
Min
Max
Avg
Sulfate
0.2
0.7
0.5
­
0.5
1.1
0.3
Est.
Ammonium
0.2
2.2
1.2
0.1
0.8
0.4
Nitrate
0.6
6.9
3.7
0.4
1.4
0.8
Total
Carbon
4.8
9.8
6.6
2.1
5.3
3.1
Crustal
0.1
0.6
0.4
­
0.1
0.8
0.3
TOTAL
EXCESS
5.8
20.1
12.4
2.0
9.4
4.8
Table
3.
Urban
Increment
Analysis
for
13
urban/
rural
pairs.
Carbonaceous
mass
is
the
largest
contributor
to
urban
increments
in
all
regions
of
the
country.
In
east
coast
and
midwestern
urban
areas,
carbon
can
account
for
as
much
as
70­
90
percent
of
the
total
urban
increment.
The
highest
local
increment
of
carbon
as
calculated
from
available
data
appears
to
be
about
10
µ
g/
m3
in
Fresno,
CA.
Nonroad
diesel,
onroad
diesel,
gasoline
highway
vehicles,
and
fire
related
activities
are
regarded
to
be
important
major
contributors
to
this
urban
excess
of
carbon.
The
relative
amounts
of
primary
versus
secondary
3
USEPA,
National
Air
Pollutant
Emissions
Trends,
Report
Number
EPA­
454/
R­
00­
002,
Research
Triangle
Park,
NC,
March
2000.
organic
compounds
in
the
ambient
air
vary
with
location
and
time
of
year.
While
it
is
difficult
to
generalize,
it
is
clear
that
both
primary
and
secondary
organic
compounds
are
significant
contributors
to
ambient
PM2.5
mass
in
many
parts
of
the
country.
The
urban
increment
for
sulfate,
on
the
other
hand,
appears
to
be
fairly
low
in
most
locations.
Rural
and
urban
sulfate
levels
are
often
very
similar,
indicating
that
sulfate
is
a
regional
pollutant
that
can
be
transported
long
distances.
This
is
consistent
with
the
fact
that
power
plants
are
the
principal
sources
of
SO2,
the
precursor
to
sulfate,
and
in
general,
these
plants
are
located
outside
urban
core
areas.
In
some
eastern
cities,
the
small
estimated
urban
excess
(
up
to
0.5
µ
g/
m3)
may
be
attributed
to
a
range
of
source
types,
including
power
plants
located
within
the
metro
area,
the
combustion
of
sulfur­
laden
fuel
oil
used
for
commercial
or
institutional
heating,
and
fuel
combustion
by
diesel
and
gasoline
motor
vehicles.
Excess
nitrate
concentrations
are
observed
predominantly
in
northern,
midwestern,
and
western
locations,
comprising
a
larger
local
contribution
than
sulfate
or
crustal
material.
Nitrate
is
particularly
high
in
the
winter
time
partly
because
it
is
less
volatile
at
colder
temperatures
and
partly
because
SO2
is
less
prone
to
react
preferentially
with
ammonium
in
the
winter
as
opposed
to
the
summer.
Local
sources
of
NOx
leading
to
excess
urban
nitrate
likely
include
mobile
sources
and
other
types
of
fuel
combustion.
Some
locations
also
show
a
small
urban
excess
of
crustal
material
(
e.
g.
inorganic
material
including
metals,
dust,
sea
salt,
and
other
trace
elements).
The
estimation
procedure
used
in
the
IMPROVE
protocol
includes
the
measurement
of
iron
and
other
trace
elements.
Therefore,
this
difference
also
reflects
oxidized
particulate
metals,
some
of
which
may
be
attributed
to
road
dust
or
industrial
sources
in
urban
areas.
We
have
developed
a
National
Emissions
Inventory
(
NEI)
inventory
for
use
in
analyzing
trends
in
emissions,
conducting
various
regulatory
analyses
for
PM,
and
for
use
in
regional
scale
modeling.
3
The
NEI
covers
all
50
States
plus
some
of
the
U.
S.
territories,
and
includes
point,
area,
onroad
and
nonroad
mobile
sources,
biogenic,
and
geogenic
emissions.
Large
stationary
sources
are
located
individually
in
the
inventory
while
county
tallies
are
used
for
smaller
stationary
sources,
and
area
and
mobile
source
category
groups.
Spatial,
temporal
and
compositional
profiles
are
used
to
allocate
these
emissions
to
time­
resolved
grids
for
chemical
transport
modeling.
The
inventory
includes
emissions
of
SO2,
NOx,
VOC,
NH3,
PM10,
and
PM2.5.
A
brief
discussion
of
each
particle
type,
their
principal
sources
(
based
on
the
NEI),
formation
mechanisms,
and
spatial
and
temporal
patterns
follows.
Primary
PM
(
Crustal
and
Carbonaceous).
This
section
addresses
inorganic
and
organic
forms
of
primary
PM.
The
main
anthropogenic
sources
of
inorganic
(
or
crustal)
particles
are:
entrainment
by
vehicular
traffic
on
unpaved
or
paved
roads;
mechanical
disturbance
of
soil
by
highway,
commercial,
and
residential
construction;
and
agricultural
field
operations
(
tilling,
planting
and
harvesting).
However,
much
of
these
emissions
are
coarse
PM
rather
than
fine
PM.
Industrial
processes
such
as
quarries,
minerals
processing,
and
agricultural
crop
processing
can
also
emit
crustal
materials,
but
their
influence
is
most
important
close
to
the
source
and
they
are
not
generally
significant
contributors
to
regional
scale
PM
problems.
Even
so,
during
certain
high
wind
events,
fine
crustal
PM
has
been
shown
to
be
transported
over
very
long
distances.
Satellite
data
and
other
studies
have
shown
that
dust
has
been
transported
into
the
US
as
a
result
of
Asian
or
African
dust
storms.
Emission
estimates
of
mechanically
suspended
crustal
PM
from
sources
within
the
US
are
often
quite
high.
However,
this
PM
is
often
released
very
close
to
the
ground,
and
with
the
exception
of
windblown
dust
events,
thermal
or
turbulent
forces
sufficient
to
lift
and
transport
them
very
far
from
their
source
are
not
usually
present.
Thus,
as
shown
in
table
1,
crustal
material
is
only
a
minor
part
of
PM2.5
annual
average
concentrations.
Primary
carbonaceous
particles
are
largely
the
result
of
incomplete
combustion
of
fossil
or
biomass
fuels.
This
incomplete
combustion
usually
results
in
emissions
of
both
black
carbon
and
organic
carbon
particles.
High
molecular
weight
organic
molecules
(
i.
e.,
molecules
with
25
or
more
carbon
atoms)
are
either
emitted
as
solid
or
liquid
particles,
or
as
gases
that
rapidly
condense
into
particle
form.
These
heavy
organic
molecules
sometimes
are
referred
to
as
volatile
organic
compounds,
but
because
their
characteristics
are
most
like
direct
PM
emissions,
they
will
be
considered
to
be
primary
emissions
for
the
purposes
of
this
regulation.
Primary
organic
carbon
also
can
be
formed
by
condensation
of
semi­
volatile
compounds
on
the
surface
of
other
particles.
The
main
combustion
sources
emitting
carbonaceous
PM2.5
are
mobile
sources
(
both
onroad
and
nonroad),
managed
burning,
wildland
fires,
open
burning
of
waste,
residential
wood
combustion,
certain
industrial
processes,
and
coal
and
oilburning
boilers
(
utility,
commercial
and
industrial).
Certain
organic
particles
also
come
from
natural
sources
such
as
decomposition
or
crushing
of
plant
detritus.
Most
combustion
processes
emit
more
organic
particles
than
black
carbon
particles.
A
notable
exception
to
this
are
diesel
engines,
which
typically
emit
more
black
carbon
particles
than
organic
carbon,
although
newer
engines
tend
to
emit
an
increasing
fraction
of
organic
carbon.
Because
photochemistry
is
typically
reduced
in
4USEPA,
2003.
Air
Quality
Criteria
for
Particulate
Matter
(
Fourth
External
Review
Draft).
EPA/
600/
P­
99/
002aD
and
bD.
U.
S.
Environmental
Protection
Agency,
Office
of
Research
and
Development,
National
Center
For
Environmental
Assessment,
Research
Triangle
Park
Office,
Research
Triangle
Park,
NC.
June
2003.
Available
electronically
at
http://
cfpub.
epa.
gov/
ncea/
cfm/
partmatt.
cfm.

5
North
American
Research
Strategy
for
Tropospheric
Ozone
and
Particulate
Matter,
Particulate
Matter
Science
for
Policy
Makers
 
A
NARSTO
Assessment,
Parts
1
and
2.
NARSTO
Management
Office
(
Envair),
Pasco,
Washington.
February
2003.
http://
www.
cgenv.
com/
NARSTO/
the
cooler
winter
months
for
much
of
the
country,
studies
indicate
that
the
carbon
fraction
of
PM
mass
in
the
winter
months
is
likely
dominated
by
direct
PM
emissions
as
opposed
to
secondarily
formed
organic
aerosol.
Particles
from
the
earth's
crust
may
contain
a
combination
of
metallic
oxides
and
biogenic
derived
organic
matter.
The
combustion
of
surface
debris
will
likely
entrain
some
soil.
Additionally,
emissions
from
many
processes
and
from
the
combustion
of
fossil
fuels
contain
elements
that
are
chemically
similar
to
soil.
Thus,
a
portion
of
the
emissions
from
combustion
activities
may
be
classified
as
crustal
in
a
compositional
analysis
of
ambient
PM2.5.
Secondary
PM.
Although
some
sulfate
and
nitrate
salts
(
i.
e.
calcium
sulfate,
calcium
nitrate)
and
acids
(
i.
e.
sulfuric
acid,
nitric
acid)
are
directly
emitted
by
sources
under
certain
circumstances,
sulfates
and
nitrates
are
predominately
formed
as
a
result
of
chemical
reactions
with
ammonia
and
other
compounds
in
the
atmosphere.
(
See
next
sections
for
more
detail.)
During
combustion,
very
small
combustion
nucleation
particles
(
ultrafine
particles,
less
than
0.1
µ
m)
are
produced.
These
small
particles
act
as
nucleation
sites
where
gases,
water
vapor,
and
other
nucleation
particles
can
condense
or
coagulate
and
therefore
cause
particle
growth
in
both
particle
size
and
particle
mass.
Ammonium
sulfate,
ammonium
nitrate,
and
secondarily
formed
organic
aerosols,
as
well
as
agglomerating
fine
particles,
all
may
use
these
ultrafine
particles
in
their
formation
and
growth
in
the
atmosphere.
The
secondary
organic
aerosol
(
SOA)
component
of
PM2.5
is
a
complex
mixture
of
perhaps
thousands
of
organic
compounds.
A
brief
discussion
of
the
sources
of
SO2,
NOx,
NH3,
and
organic
gases
(
including
VOC
and
semi­
volatile
compounds),
and
the
formation
of
sulfate,
nitrate
and
secondary
organic
aerosol
follows.
More
detailed
discussions
of
the
formation
and
characteristics
of
secondary
particles
can
be
found
in
the
US
EPA
Criteria
Document,
4
and
in
the
NARSTO
Fine
Particle
Assessment,
5
on
which
much
of
the
following
discussion
is
based.
6
Sievering,
H.,
Boatman,
J.,
Gorman,
E.,
Kim,
Y.,
Anderson,
L.,
Ennis,
G.,
Luria,
M.,
Pandis,
S.
N.,
1992.
Removal
of
sulfur
from
the
marine
boundary
layer
by
ozone
oxidation
in
sea­
salt.
Nature
360,
571­
573.

7
McHenry,
J.
N.,
Dennis,
R.
L.,
1994.
The
relative
importance
of
oxidation
pathways
and
clouds
to
atmospheric
ambient
sulfate
production
as
predicted
by
the
Regional
Acid
Deposition
Model.
Journal
of
Applied
Meteorology
33,
890­
905.
Also:
Langner,
J.,
Rodhe,
H.,
1991.
A
global
three
dimensional
model
for
the
tropospheric
sulfur
cycle.
Journal
of
Atmospheric
Chemistry
13,
225­
263.
Sulfate.
SO2
is
emitted
mostly
from
the
combustion
of
fossil
fuels
in
boilers
operated
by
electric
utilities
and
other
industry.
Less
than
20
percent
of
SO2
emissions
nationwide
are
from
other
sources,
mainly
from
other
industrial
processes
including
oil
refining
and
pulp
and
paper
production.
The
formation
of
sulfuric
acid
from
the
oxidation
of
SO2
is
an
important
process
for
most
areas
in
North
America.
There
are
three
different
pathways
for
this
transformation.
First,
gaseous
SO2
can
be
oxidized
by
the
hydroxyl
radical
(
OH)
to
create
sulfuric
acid.
This
gaseous
SO2
oxidation
reaction
occurs
slowly
and
only
in
the
daytime.
The
hydroxl
radical
is
an
important
product
of
the
atmospheric
chemistry
process
that
forms
ozone
through
the
oxidation
of
NOx
to
form
nitric
acid.
It
is
also
involved
in
the
formation
of
secondary
organics.
Second,
SO2
can
dissolve
in
cloud
water
(
or
fog
or
rain
water),
and
there
it
can
be
oxidized
to
sulfuric
acid
by
a
variety
of
oxidants,
or
through
catalysis
by
transition
metals
such
as
manganese
or
iron.
If
ammonia
is
present
and
taken
up
by
the
water
droplet,
then
ammonium
sulfate
will
form
as
a
precipitant
in
the
water
droplet.
After
the
cloud
changes
and
the
droplet
evaporates,
the
sulfuric
acid
or
ammonium
sulfate
remains
in
the
atmosphere
as
a
particle.
This
aqueous­
phase
production
process
involving
oxidants
can
be
very
fast;
in
some
cases
all
the
available
SO2
can
be
oxidized
in
less
than
an
hour.
Third,
SO2
can
be
oxidized
in
reactions
in
the
particlebound
water
in
the
aerosol
particles
themselves.
This
process
takes
place
continuously,
but
only
produces
appreciable
sulfate
in
alkaline
(
dust,
sea­
salt)
coarse
particles.
6
Oxidation
of
SO2
has
been
also
observed
on
the
surfaces
of
black
carbon
and
metal
oxide
particles.
During
the
last
twenty
years,
much
progress
has
been
made
in
understanding
the
first
two
major
pathways,
but
some
important
questions
still
remain
about
the
smaller
third
pathway.
Models
indicate
that
more
than
half
of
the
sulfuric
acid
in
the
eastern
United
States
and
in
the
overall
atmosphere
is
produced
in
clouds.
7
8
Wayne,
R.
P.,
1991.
The
nitrate
radical:
physics,
chemistry
and
the
atmosphere.
Atmospheric
Environment
25A,
1­
203.

9
Seinfeld,
J.
H.,
Pandis,
S.
N.,
1998.
Atmospheric
Chemistry
and
Physics:
From
Air
Pollution
to
Climate
Change.
J.
Wiley,
New
York.
The
sulfuric
acid
formed
from
the
above
pathways
reacts
readily
with
ammonia
to
form
ammonium
sulfate,
(
NH4)
2SO4.
If
there
is
not
enough
ammonia
present
to
fully
neutralize
the
produced
sulfuric
acid
(
one
molecule
of
sulfuric
acid
requires
two
molecules
of
ammonia),
part
of
it
exists
as
ammonium
bisulfate,
NH4HSO4
(
one
molecule
of
sulfuric
acid
and
one
molecule
of
ammonia)
and
the
particles
are
more
acidic
than
ammonium
sulfate.
In
extreme
cases
(
in
the
absence
of
sufficient
ammonia
for
neutralization),
sulfate
can
exist
in
particles
as
sulfuric
acid,
H2SO4.
Sulfuric
acid
often
exists
in
the
plumes
of
stacks
where
SO2,
SO3,
and
water
vapor
are
in
much
higher
concentrations
than
in
the
ambient
atmosphere,
but
these
concentrations
become
quite
small
as
the
plume
is
cooled
and
diluted
by
mixing.
Nitrate.
The
main
sources
of
NOx
are
combustion
of
fossil
fuel
in
boilers
and
onroad
mobile
sources.
Together
they
account
for
almost
80
percent
of
NOx
emissions,
with
stationary
and
mobile
source
fuel
combustion
each
accounting
for
about
half
of
these
emissions.
Nitrates
are
formed
from
the
oxidation
of
oxides
of
nitrogen
into
nitric
acid
either
during
the
daytime
(
reaction
with
OH)
or
during
the
night
(
reactions
with
ozone
and
water).
8
Nitric
acid
continuously
transfers
between
the
gas
and
the
condensed
phases
through
condensation
and
evaporation
processes
in
the
atmosphere.
However,
unless
it
reacts
with
other
species
(
such
as
ammonia,
sea
salt,
or
dust)
to
form
a
neutralized
salt,
it
will
volatize
and
not
be
measured
using
standard
PM2.5
measurement
techniques.
9
The
formation
of
aerosol
ammonium
nitrate
is
favored
by
the
availability
of
ammonia,
low
temperatures,
and
high
relative
humidity.
Because
ammonium
nitrate
is
not
stable
in
higher
temperatures,
nitrate
levels
are
typically
lower
in
the
summer
months
and
higher
in
the
winter
months.
The
resulting
ammonium
nitrate
is
usually
in
the
sub­
micrometer
particle
size
range.
Reactions
with
sea­
salt
and
dust
lead
to
the
formation
of
nitrates
in
coarse
particles.
Nitric
acid
may
be
dissolved
in
ambient
aerosol
particles.
Secondary
Organic
Aerosol
(
SOA).
The
organic
component
of
ambient
particles
is
a
complex
mixture
of
hundreds
or
even
thousands
of
organic
compounds.
These
organic
compounds
are
either
emitted
directly
from
sources
(
i.
e.
primary
organic
aerosol)
or
can
be
formed
by
reactions
in
the
ambient
air
(
i.
e.
10
As
discussed
earlier,
high
molecular
weight
organic
molecules
(
i.
e.,
molecules
with
25
or
more
carbon
atoms)
are
either
emitted
directly
as
particles
or
as
liquids
that
rapidly
condense
onto
existing
particles.
Because
these
condensable
emissions
act
primarily
as
direct
PM
emissions,
they
are
to
be
regulated
as
direct
PM2.5
emissions,
not
as
VOC
precursors,
for
the
purposes
of
this
regulation.

11
Jang,
M.;
Czoschke,
N.;
Lee,
S;
Kamens,
R.
Heterogenous
Atmospheric
Aerosol
Production
by
Acid­
Catalyzed
Particle­
Phase
Reactions,
Science,
vol.
298,
p.
814­
817,
October
25,
2002.
secondary
organic
aerosol,
or
SOA).
Volatile
organic
compounds10
are
key
precursors
in
both
the
SOA
and
ozone
formation
processes.
The
lightest
organic
molecules
(
i.
e.,
molecules
with
six
or
fewer
carbon
atoms)
occur
in
the
atmosphere
mainly
as
vapors
and
typically
do
not
directly
form
organic
particles
at
ambient
temperatures
due
to
the
high
vapor
pressure
of
their
products.
However,
they
participate
in
atmospheric
chemistry
processes
resulting
in
the
formation
of
ozone
and
certain
free
radical
compounds
(
such
as
the
hydroxyl
radical
[
OH])
which
in
turn
participate
in
the
oxidation
of
semivolatile
organic
compounds
to
form
secondary
organic
aerosols,
sulfates
and
nitrates.
These
VOCs
include
all
alkanes
with
up
to
six
carbon
atoms
(
from
methane
to
hexane
isomers),
all
alkenes
with
up
to
six
carbon
atoms
(
from
ethene
to
hexene
isomers),
benzene
and
many
low­
molecular
weight
carbonyls,
chlorinated
compounds,
and
oxygenated
solvents.
The
relative
importance
of
organic
compounds
in
the
formation
of
organic
particles
varies
from
area
to
area,
depending
upon
local
emissions
sources,
atmospheric
chemistry,
and
season
of
the
year.
Intermediate
weight
organic
molecules
(
i.
e.,
compounds
with
7
to
24
carbon
atoms)
often
exhibit
a
range
of
volatilities
and
can
exist
in
both
the
gas
and
aerosol
phase.
For
this
reason
they
are
also
referred
to
as
semivolatile
compounds.
Semivolatile
compounds
react
in
the
atmosphere
to
form
secondary
organic
aerosols.
These
chemical
reactions
are
accelerated
in
warmer
temperatures,
and
studies
show
that
SOA
typically
comprises
a
higher
percentage
of
carbonaceous
PM
in
the
summer
as
opposed
to
the
winter.
The
production
of
SOA
from
the
atmospheric
oxidation
of
a
specific
VOC
depends
on
four
factors:
its
atmospheric
abundance,
its
chemical
reactivity,
the
availability
of
oxidants
(
O3,
OH,
HNO3),
and
the
volatility
of
its
products.
In
addition,
recent
work
by
Jang
and
others
suggests
that
the
presence
of
acidic
aerosols
may
lead
to
an
increased
rate
of
SOA
formation.
11
Aromatic
compounds
such
as
toluene,
xylene,
and
trimethyl
benzene
are
considered
to
be
the
most
significant
anthropogenic
SOA
precursors
and
have
been
estimated
to
be
responsible
for
50
12
Grosjean,
D.,
Seinfeld,
J.
H.,
1989.
Parameterization
of
the
formation
potential
of
secondary
organic
aerosols.
Atmospheric
Environment
23,
1733­
1747.

13
Odum,
J.
R.,
Jungkamp,
T.
P.
W.,
Griffin,
R.
J.,
Flagan,
R.
C.,
Seinfeld,
J.
H.,
1997.
The
atmospheric
aerosol­
forming
potential
of
whole
gasoline
vapor.
Science
276,
97­
99.
to
70
percent
of
total
SOA
in
some
airsheds.
12
As
organic
gases
such
as
aromatics
are
oxidized
in
the
gas
phase
by
species
such
as
the
hydroxyl
radical
(
OH),
ozone
(
O3),
and
the
nitrate
radical
(
NO3)
their
oxidation
products
accumulate.
Some
of
these
products
have
low
volatility
and
condense
on
available
particles
in
an
effort
to
establish
equilibrium
between
the
gas
and
condensed
phases.
Man­
made
sources
of
aromatics
gases
are
mobile
sources,
petrochemical
manufacturing
and
solvents.
The
experimental
work
of
Odum
and
others13
showed
that
the
secondary
organic
aerosol
formation
potential
of
gasoline
could
be
accounted
for
solely
in
terms
of
its
aromatic
fraction.
Some
of
the
biogenic
hydrocarbons
emitted
by
trees
are
also
considered
to
be
important
precursors
of
secondary
organic
particulate
matter.
Terpenes
(
 ­
and
 ­
pinene,
limonene,
carene,
etc.)
and
the
sesquiterpenes
are
expected
to
be
major
contributors
to
SOA
in
areas
with
significant
vegetation
cover,
but
isoprene
is
not.
Terpenes
are
very
prevalent
in
forested
areas,
especially
in
the
southeastern
U.
S.
The
rest
of
the
anthropogenic
hydrocarbons
(
higher
alkanes,
paraffins,
etc.)
have
been
estimated
to
contribute
5­
20
percent
to
the
SOA
concentration
depending
on
the
area.

SOA­
Forming
Organic
Gases
Non
SOA­
Forming
Organic
Gases
Anthropogenic
­
Aromatics
(
esp.
toluene,
xylenes,
trimethyl­
benzenes)
­
Higher
alkanes
(>
6
C
atoms)
­
Lower
alkanes
<
6
C
atoms,(
ethane
to
hexane
isomers)
­
Benzene
­
Lower
MW
carbonyls,
chlorinated
compounds
&
oxygenated
solvents
Biogenic
­
Terpenes
(
esp.
 ­
and
 ­
pinene,
limonene,
carene)
­
Sesquiterpenes
­
Isoprene
Table
4.
Role
of
Organic
Gases
in
Secondary
Organic
Aerosol
Formation.

The
contribution
of
the
primary
and
secondary
components
of
organic
aerosol
to
the
measured
organic
aerosol
concentrations
14
Hildemann,
L.
M.,
Cass,
G.
R.,
Mazurek,
M.
A.,
Simoneit,
B.
R.
T.,
1993.
Mathematical
modeling
of
urban
organic
aerosol
properties
measured
by
high
resolution
gaschromatography
Environmental
Science
and
Technology
27,
2045­
2055.

15
Turpin,
B.
J.,
Lim,
H.
J.,
2000.
Species
contributions
to
PM
mass
concentrations:
Revisiting
common
assumptions
for
estimating
organic
mass,
Aerosol
Science
and
Technology,
vol.
35,
no.
1,
p.
602­
610.

16
Anderson,
N.,
R.
Strader,
and
C.
Davidson
(
2003)
Airborne
reduced
nitrogen:
ammonia
emissions
from
agriculture
and
other
sources,
Environment
International,
remains
a
controversial
issue.
Most
of
the
research
performed
to
date
has
been
done
in
southern
California,
and
more
recently
in
central
California,
while
fewer
studies
have
been
completed
on
other
parts
of
North
America.
Early
studies
suggested
that
the
majority
of
the
observed
organic
particulate
matter
was
secondary
in
nature.
Later
investigators
focusing
on
the
emissions
of
primary
organic
material
proposed
that
80
percent
or
so
of
the
organic
aerosol
in
Southern
California
on
a
monthly
basis
resulted
from
direct
organic
particle
emissions.
14
More
recent
studies
suggest
that
the
primary
and
secondary
contributions
are
highly
variable
even
during
the
same
day.
Studies
of
pollution
episodes
indicated
that
the
contribution
of
SOA
to
the
organic
particulate
matter
varied
from
20
percent
to
80
percent
during
the
same
day.
15
Despite
significant
progress
that
has
been
made
in
understanding
the
origins
and
properties
of
SOA,
it
remains
the
least
understood
component
of
PM2.5.
The
reactions
forming
secondary
organics
are
complex
and
the
number
of
intermediate
and
final
compounds
formed
is
voluminous.
Some
of
the
best
efforts
to
unravel
the
chemical
composition
of
ambient
organic
aerosol
matter
have
been
able
to
quantify
the
concentrations
of
hundreds
of
organic
compounds
representing
only
10­
20
percent
of
the
total
organic
aerosol
mass.
For
this
reason,
SOA
continues
to
be
a
significant
topic
of
research
and
investigation.

C.
The
role
of
ammonia
in
sulfate,
nitrate
&
secondary
organic
aerosol
formation
Ammonia
(
NH3)
is
a
gaseous
pollutant
that
is
emitted
by
natural
and
anthropogenic
sources.
Emissions
inventories
for
ammonia
are
considered
to
be
among
the
most
uncertain
of
any
species
related
to
PM.
One
recent
estimate
shows,
however,
that
livestock
(
73
percent)
and
fertilizer
application
(
17
percent)
are
the
two
primary
sources
of
emissions.
16
(
Note
that
these
29:
277­
286.

17
Seinfeld,
J.
H.,
Pandis,
S.
N.,
1998.
Atmospheric
Chemistry
and
Physics:
From
Air
Pollution
to
Climate
Change.
J.
Wiley,
New
York.

18
NARSTO,
2003.
Particulate
Matter
Science
for
Policy
Makers­­
A
NARSTO
Assessment.
Parts
1
and
2.
NARSTO
Management
Office
(
Envair),
Pasco,
Washington.
http://
www.
cgenv.
com/
NARSTO.

19
Ibid,
at
S­
31
(
table
S.
4).
estimates
do
not
include
natural
emissions
from
soil,
which
can
be
significant.)
Ammonia
serves
an
important
role
in
neutralizing
acids
in
clouds,
precipitation
and
particles.
In
particular,
ammonia
neutralizes
sulfuric
acid
and
nitric
acid,
the
two
key
contributors
to
acid
deposition
(
acid
rain).
Deposited
ammonia
also
can
be
an
important
nutrient,
contributing
to
problems
of
eutrophication
in
water
bodies.
17
Ammonia
would
not
exist
in
particles,
if
not
for
the
presence
of
acidic
species
with
which
it
can
combine
to
form
a
particle.
In
the
eastern
US,
sulfate,
nitrate,
and
the
ammonium
associated
with
them
can
together
account
for
between
roughly
30
percent
and
75
percent
of
the
PM2.5
mass.
The
ammonium
itself
roughly
accounts
for
between
5
percent
and
20
percent
of
the
PM2.5.18
The
NARSTO
Fine
Particle
Assessment
indicates
that
sulfates
form
preferentially
over
nitrates
and
that
particle
nitrate
formation
is
affected
by
a
number
of
factors,
including
the
availability
of
sulfates,
NOx,
ammonia,
nitric
acid
and
VOCs.
The
report
also
notes
that
implementing
decreasing
ammonia
emissions
where
sulfate
concentrations
are
high
can
reduce
PM2.5
mass
concentrations,
but
may
also
increase
particle
and
precipitation
acidity.
19
As
noted
above,
this
acidification
of
particles
may
result
in
an
increase
in
the
formation
of
secondary
organic
compounds.
Moreover,
the
relationship
between
ammonia
and
sulfate­
nitrate
equilibrium
may
also
impact
SOA
formation,
although
this
link
is
not
well
understood.
Recent
studies
of
ammonia
sources
and
possible
emission
reduction
measures
indicate
that
ammonia
controls
are
a
maturing
science,
but
that
ongoing
research
will
greatly
improve
our
understanding
of
such
control
measures.
The
same
can
be
said
of
our
understanding
of
the
role
of
ammonia
in
aerosol
formation.
Based
on
the
above
information
and
further
insights
gained
from
the
NARSTO
Fine
Particle
Assessment,
it
is
apparent
that
the
formation
of
sulfate,
nitrate
&
SOA
compounds
is
a
complex,
nonlinear
process.
The
control
techniques
for
ammonia
and
the
analytical
tools
to
quantify
the
impact
of
reducing
ammonia
emissions
on
atmospheric
aerosol
formation
are
both
evolving
sciences.
Also,
there
are
indications
that
there
may
be
considerable
ambiguity
concerning
the
results
of
reducing
ammonia
emissions
and
in
some
cases,
there
may
be
undesired
consequences
of
ammonia
reductions.
Therefore,
based
on
our
current
understanding
of
ammonia's
role
in
these
complex
precursor
interactions
and
emission
reduction
processes,
it
seems
prudent
to
continue
research
on
ammonia
control
technologies
and
the
ammonia
­
sulfate
­
nitrate
­
SOA
equilibrium
before
one
undertakes
broad
national
programs
to
reduce
ammonia
emissions.
However,
as
States
and
EPA
develop
a
greater
understanding
over
the
coming
years
about
the
potential
air
quality
effects
of
reducing
ammonia
emissions
in
specific
nonattainment
areas,
it
may
be
appropriate
for
ammonia
reduction
strategies
to
be
included
in
future
SIPs.
At
this
time,
however,
we
believe
that
reducing
SO2
and
NOx
will
allow
us
to
move
with
greater
certainty
toward
achieving
our
nation's
air
quality
goals.
We
encourage
you
to
provide
comments
on
the
resolution
of
this
issue.
D.
Regional
patterns
of
carbon,
sulfate
and
nitrate,
and
indications
of
transport
Table
2
above
shows
that
much
of
the
eastern
US,
both
urban
and
non
urban
areas
alike,
is
subject
to
high
PM2.5
concentrations,
with
the
highest
concentrations
occurring
in
urban
areas.
Table
3
above
compares
the
urban
and
rural
concentrations
of
sulfate,
nitrate,
and
carbon
particles.
The
data
show
that
there
are
high
concentrations
of
sulfate
across
the
region
and
that
sulfate
at
urban
monitoring
sites
is
only
slightly
higher
than
at
nearby
non­
urban
sites.
In
contrast,
the
carbon
mass
at
urban
sites
is
significantly
higher
than
at
the
nearby
non­
urban
sites.
This
seems
to
indicate
that
sulfate
is
present
on
a
much
more
regional
scale
and
likely
is
associated
with
significant
pollutant
transport.
On
the
other
hand,
a
sizeable
fraction
of
the
carbonaceous
mass
seems
to
be
more
associated
with
urban
sources.
Mobile
sources
are
much
more
concentrated
in
urban
areas
and
may
explain
much
of
the
elevated
urban
carbon
concentrations.
However,
black
carbon
and
organic
aerosols
still
make
up
a
large
percentage
of
the
non­
urban
air
quality
composition,
indicating
that
there
is
a
regional
background
level
of
carbon
that
is
enhanced
in
urban
areas
by
local
sources.
The
atmospheric
lifetimes
of
particles
and
thus
the
distances
they
can
be
transported
vary
with
particle
size.
The
regional
nature
of
PM2.5
reflects
the
fact
that
fine
particles
can
be
transported
over
long
distances.
Ultra­
fine
and
fine
particles
rapidly
grow
in
size
into
a
relatively
stable
size
range,
generally
less
than
2
µ
m.
These
fine
particles
are
kept
suspended
by
normal
air
motions
and
have
very
low
deposition
rates
to
surfaces.
They
can
be
transported
thousands
of
kilometers
and
remain
in
the
atmosphere
for
a
number
of
days.
Thus,
they
are
important
when
considering
regional
PM
transport.
Coarse
particles
can
settle
rapidly
from
the
atmosphere
within
hours
and
normally
travel
only
short
distances.
However,
when
mixed
high
into
the
atmosphere,
as
in
some
dust
storms,
the
smaller­
sized
coarse­
mode
particles
may
have
longer
lives
and
travel
greater
distances.
Meteorology
also
plays
a
role
in
the
size
and
characteristics
of
particles.
High
temperatures
increase
reaction
rates,
which
may
explain
why
sulfate
concentrations
are
generally
greatest
in
the
summer.
Conversely,
lower
temperatures
result
in
a
greater
fraction
of
nitrates
being
in
the
particle
phase.
Fine
particles,
especially
particles
with
a
hygroscopic
component,
grow
as
the
relative
humidity
increases,
serve
as
cloud
condensation
nuclei,
and
grow
into
cloud
droplets.
If
the
cloud
droplets
grow
large
enough
to
form
rain,
the
particles
are
removed
in
the
rain.
Falling
rain
drops
impact
coarse
particles
and
remove
them.
Very
fine
particles
are
small
enough
to
diffuse
to
the
falling
drop,
be
captured,
and
be
removed
in
rain.
However,
falling
rain
drops
are
not
nearly
as
effective
in
removing
PM2.5
as
the
cloud
processes
mentioned
above.
Sulfuric
acid,
ammonium
nitrate,
ammonium
sulfates,
and
organic
particles
also
are
deposited
on
surfaces
by
dry
deposition.
Therefore,
reductions
in
SO2
and
NOx
emissions
will
decrease
both
acidic
deposition
and
PM
concentrations.