Estimation of In Situ pH on PM 2.5 in Hong Kong Using Online Ionic ...

A Study <strong>of</strong> Acidity on PM 2.5 in Hong
Kong Using Online Ionic Chemical
Composition Measurements by PILS
PM 2.5
HKUST
Jian XUE, Alexis K. H. Lau
and Jianzhen YU
2010-05-11
1

Outline
1
2
3
<strong>In</strong>troduction
Validation <strong>of</strong> PILS Measurements
Sampling
4
Results
5
Summary
2
2

1. <strong>In</strong>troduction:
Definition <strong>of</strong> Particle Acidity
Strong
Acidity
[H + ] s
: total amount <strong>of</strong> H+ nmol m -3 <strong>of</strong> air,
measured either by titration or by <strong>pH</strong> <strong>of</strong> the
aerosol extract in an aqueous solution.
.
Free
Acidity
The actual amount <strong>of</strong> [H + ] in the aqueous
phase <strong>of</strong> particles in nmol m -3 <strong>of</strong> air;
Estimated by thermodynamic models (e.g.,
AIM-III ) *
<strong>In</strong>-situ
<strong>pH</strong>
<strong>pH</strong> <strong>of</strong> the particle aqueous phase; Calculated
from parameters estimated by AIM-III
3
*: available at: www.aim.env.uea.ac.uk/aim/aim.php/

Motivation for studying <strong>In</strong>-<strong>Situ</strong> <strong>pH</strong>
Acid-catalyzed effects on SOA formation 2
[H + ] s
Human health, cancer related 1
Conversion
Acid deposition
<strong>of</strong> insoluble iron to soluble Fe 3
<strong>In</strong>-situ
<strong>pH</strong>
4
1:Sathiakumar, Critical Reviews in Toxicology, 1997;
2:Jang, Science, 2002; 3:Luo, JGR, 2005

<strong>Estimation</strong> <strong>of</strong> in-<strong>Situ</strong> <strong>pH</strong> using
aerosol chemical composition data
activity
activity
coefficient
coefficient
<strong>of</strong> H + , model output
n H+
a H+
<strong>of</strong> H + , model output γ H+ ]
<strong>pH</strong>
IS
= −loga
−log[
γ × n × 1000× ρ /
H
+
H
H
+
+
=
m
free
free
H + + in
in
mol
mol
m -3, -3,
model
model
output
output
activity
activity
<strong>of</strong>
<strong>of</strong>
H + +
in
in
mol
mol
L
L -1 -1
in
in
aqueous
aqueous
particle
particle
m
mass
mass
concentration
concentration
<strong>of</strong>
<strong>of</strong>
particle
particle
aqueous
aqueous
phase
phase
constituents
constituents
in
in
g
g
m -3 -3
ρ
density
density
<strong>of</strong>
<strong>of</strong>
particle
particle
aqueous
aqueous
phase,
phase,
by
by
equations
equations
<strong>of</strong>
<strong>of</strong>
Tang
Tang
and
and
Munkelwiz
Munkelwiz
(1994)
(1994)
5

PILS: particle into liquid system
URG annular denuders:
First one is coated with Na2CO3 to remove
acid gas(HNO3, URG 2.5cut HNO2, cyclone: SO2 etc). cut
particle larger than 2.5um <strong>of</strong>f by
Second one is coated with citric acid for
gravity
removal <strong>of</strong> NH3
6

2. Validation <strong>of</strong> PILS measurements:
Measurement set-up: PILS
PILS
Time resolution: 30 mins
VS
Filter
12-24 hour
No. <strong>of</strong> Samples 539 15
Sampling Artifacts low
high (single filter)
7

Validation <strong>of</strong> PILS measurements
SO 4
2-
*
NH 4+
*
NO 3
-#
K + *
slope
R 2
slop
e
R 2
slop
e
R 2
slope
R 2
slope
R 2
slope
R 2
Chang
(2007)
1.05
1.02
0.59
0.89
1.18
1.06
0.56
0.58
3.56
3.16
0.65
0.80
4.91
4.03
0.07
0.46
Cl -# 0.98
1.00
0.05
Na +# 0.77
1.18
1.05
0.56
0.90
Orsini
(2003)
1.03
0.96
0.93
0.88
0.32
0.22
Hogrefe
(2004)
0.71-
2.37
0.42
-0.96
1.05
0.94
This
study
1.04
0.97
1.06
0.92
1.16
0.96
0.82
0.97
0.93
1.04
*:No. <strong>of</strong> comparison= 29; # : No. <strong>of</strong> comparison= 8.
8

3. Sampling:
Three observation periods
Ⅰ(21-25 Oct. 2009) Ⅱ(6-13 Nov. 2008) Ⅲ(29 Jun.- 3 Jul. 2009)
High pollution
period
Meteorological
condition
Ⅰ
T(K)
Avg.±std.
298.15±1.79
RH
Avg.±std.
0.77±0.06
Cold front
Ⅱ
292.46±2.21
0.48±0.09
Summer low
pollution period
Ⅲ
301.63±1.12
0.75±0.06
9

4. Results: PM2.5 strong acidity
2−
−
+
−
+
[ H ] s
= 2 [ SO4
] + [ NO3
] + [ Cl ] −[
NH
4
] −[
Na ] −[
K
+
+
]
[H + ]s (nmol m -3 )
300
200
100
period I
period II
period III
y = 0.40 x + 12.97
R 2 = 0.62
0
0 100 200 300 400
sulfate (nmol m -3 )
[H+] s
is strongly correlated with sulfate, an expected result
considering SO 4
2-
contributes 84 ± 7% <strong>of</strong> the total anion
equivalent concentration.
10

4. Results:
PM 2.5 in-situ <strong>pH</strong> in HK
<strong>In</strong>-situ <strong>pH</strong>
Ⅰ
Ⅱ
Ⅲ
Sample No.
127
180
213
Mean
0.59
-0.45
-0.08
Std. variation
0.43
0.59
0.81
No. <strong>of</strong> cases
No. <strong>of</strong> cases
60
50
40
30
20
10
60
50
40
30
20
10
0
0
-2 -1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2
in <strong>Situ</strong> <strong>pH</strong>
Period I
-2 -1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2
in <strong>Situ</strong> <strong>pH</strong>
Period II
no LWC in 23
cases
120
Period III
No. <strong>of</strong> cases
80
40
0
-2 -1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2
119
in <strong>Situ</strong> <strong>pH</strong>

Period I : High Pollution
Conc (µg m -3 )
40
30
20
10
sulfate (L)
ammonium (L)
nitrate (L)
LWC (R)
Period I
120
100
80
60
40
20
0
0
<strong>In</strong> situ <strong>pH</strong>
2
1
0
-1
<strong>In</strong> situr <strong>pH</strong> ((L)
R+/- (R)
RH (R)
Period I
1.6
1.2
0.8
-2
2008-10-21 2008-10-22 2008-10-23 2008-10-24 2008-10-25 2008-10-26
0.4
12

Period II: Cold front
Conc (µg m -3 )
20
10
sulfate (L)
ammonium (L)
nitrate (L)
LWC (R)
Period II
15
10
5
0
0
<strong>In</strong> situ <strong>pH</strong>
2
1
0
<strong>In</strong> situr <strong>pH</strong> (L)
R+/- (R)
RH (R)
Period II
1.5
1.1
-1
0.7
-2
2008-11-8 2008-11-9 2008-11-10 2008-11-11 2008-11-12 2008-11-13 2008-11-14
0.3
13

Period III: Summer low pollution period
Conc (µg m -3 )
5
sulfate (L)
ammonium (L)
nitrate (L)
LWC (R)
Period III
20
10
0
0
<strong>In</strong> situ <strong>pH</strong>
3
2
1
0
<strong>In</strong> situr <strong>pH</strong> (L)
R+/- (R)
RH (R)
Period III
1.2
0.8
-1
-2
2009-6-29 2009-6-30 2009-7-1 2009-7-2 2009-7-3 2009-7-4
0.4
14

Principal Component Aanalysis: Identifying major
factors influencing <strong>pH</strong> IS
Factor Loading
1 2 3 4
P Cl- -.952 -.070 -.098 .070
P Na+ -.901 -.096 -.005 .262
P NH4+ .868 .442 -.157 -.132
R +/- .483 .841 -.079 .048
R N/S .432 .819 .317 .022
P SO4= .391 -.804 -.433 -.069
<strong>pH</strong> IS .155 .750 -.113 .603
P NO3- .030 .123 .988 -.056
RH -.379 .089 -.028 .906
R +/-
: (equivalent concentration ratio <strong>of</strong> total cations and anions).
R N/S
: (ratio <strong>of</strong> NH 4+
and SO 4
2-
mole concentration)
15

Linear Regression: in-situ <strong>pH</strong> in HK is welldescribed
using R +/- and RH
<strong>pH</strong> IS
y = 0.95 x + 0.00
= 4.94R
+ /-
+ 3.11RH
−
5.70
Calculated <strong>pH</strong> IS using
regression line
(a) Linear regression variables: R +/- and RH
5
4
3
R 2 = 0.93
2
1
0
-1
-2
-3
-3 -2 -1 0 1 2 3 4 5
AIM-derived <strong>pH</strong> IS
16

Linear Regression:
using NH 4+ /SO 4
2-
and RH to describe in-situ <strong>pH</strong>
<strong>pH</strong>
IS
= 1.34 R
/
+ 3.25 RH − 54.24
N
S
Calculated <strong>pH</strong> IS using regression
line
5
4
3
2
1
0
-1
-2
-3
(b) Linear regression variables: R N/S and RH
high nitrate
episode
y = 0.75 x - 0.01
R 2 = 0.75
-3 -2 -1 0 1 2 3 4 5
R +/- is a more
suitable
parameter than
R N/S to
describe <strong>pH</strong> IS .
17
AIM-derived <strong>pH</strong> IS

Testing the predicting capability <strong>of</strong> the
linear regression equation
Approach: First, use randomly selected 90% <strong>of</strong> the cases to
obtain a regression equation; Then the regression equation
is used to predict the <strong>pH</strong> IS
for the remaining 10% cases.
<strong>pH</strong> IS
= .91R
+
+ 3.13 RH 5.70 <strong>pH</strong> IS
= .96R
+
+ 3.11RH
5. 72 = .97R
+
+ 3.10 RH 5. 72
4
/-
−
4
/-
−
<strong>pH</strong> IS
4
/-
−
by
2
1
y=0.99x-0.01
y = - 0.01
R 2 =0.94
R 2 = 0.94
0
-1
-2
-3
-3 -2 -1 0 1 2
2
1
y=1.02x-0.02
y = - 0.02
R 2 =0.92 R 2 = 0.92
0
-1
-2
-3
-2 -1 0 1 2
By model output
2
1
y=1.05x+0.02
y = + 0.02
R 2 =0.91 R 2 = 0.91
0
-1
-2
-2 -1 0 1 2
18

Comparison with in situ <strong>pH</strong> measurements
in the literatures
Location
<strong>In</strong>-situ <strong>pH</strong>
method
Literatures
Hong Kong
Mean:-0.03±0.77,
range <strong>of</strong> -1.87 to 3.12
PILS
This study
Hong Kong
Mean:-0.4
range <strong>of</strong> -2.2 to 1.1
Filter
Yao et al, AE, 2006
Hong Kong
Mean:0.25
range <strong>of</strong> -0.62 to 2.35
Filter
Pathak et al,
AE,2004
Marine PM
Low to -0.8
range <strong>of</strong> 0 to 2
Filter
Fridlind and
Jacobson, JGR,
2000
California,US
range <strong>of</strong> -1 to3
Filter
Meng and Seinfeld,
AST,1995
Pittsburgh,US
Mean: about 2
range <strong>of</strong> -0.5 to 5.5
AMS
Zhang et al, EST,
2007
19

Diurnal pattern <strong>of</strong> in-situ <strong>pH</strong>
Often
Often
lower
lower
in
in
the
the
midday
midday
as
as
a
a
result
result
<strong>of</strong>
<strong>of</strong>
low
low
RH
RH
Implication:
Implication:
Together
Together
with
with
higher
higher
solar
solar
radiation,
radiation,
acid-catalyzed
acid-catalyzed
effects
effects
are
are
possibly
possibly
enhanced
enhanced
in
in
the
the
midday.
midday.
<strong>In</strong> situ <strong>pH</strong>
3
2
1
0
-1
-2
12:00 12:00 12:00
6-29 6-30 7-1 7-2 7-3 7-4
<strong>In</strong>-situ <strong>pH</strong> during period Ⅲ
20

Summary
1
PILS measurement provides high time
resolution data and low sampling artifacts
2
PM 2.5 in-situ <strong>pH</strong> in HK are acidic, varying
from -1.9 to 3.1.
3
Two factors, R +/-
and relative humidity, are
identified to be important in determining PM2.5 insitu
<strong>pH</strong> in HK: <strong>pH</strong> IS
=4.94R +/-
+3.11RH-5.70
4
<strong>In</strong>-situ <strong>pH</strong> is lower in the midday as result <strong>of</strong>
low RH, which may enhance the acid-catalyze
effect.
21

Acknowledge
This work was supported by the Research Grants Council <strong>of</strong>
Hong Kong (615406).
22