Estimation of In Situ pH on PM 2.5 in Hong Kong Using Online Ionic ...
Estimation of In Situ pH on PM 2.5 in Hong Kong Using Online Ionic ...
Estimation of In Situ pH on PM 2.5 in Hong Kong Using Online Ionic ...
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A Study <str<strong>on</strong>g>of</str<strong>on</strong>g> Acidity <strong>on</strong> <strong>PM</strong> <strong>2.5</strong> <strong>in</strong> H<strong>on</strong>g<br />
K<strong>on</strong>g Us<strong>in</strong>g Onl<strong>in</strong>e I<strong>on</strong>ic Chemical<br />
Compositi<strong>on</strong> Measurements by PILS<br />
<strong>PM</strong> <strong>2.5</strong><br />
HKUST<br />
Jian XUE, Alexis K. H. Lau<br />
and Jianzhen YU<br />
2010-05-11<br />
1
Outl<strong>in</strong>e<br />
1<br />
2<br />
3<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g>troducti<strong>on</strong><br />
Validati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> PILS Measurements<br />
Sampl<strong>in</strong>g<br />
4<br />
Results<br />
5<br />
Summary<br />
2<br />
2
1. <str<strong>on</strong>g>In</str<strong>on</strong>g>troducti<strong>on</strong>:<br />
Def<strong>in</strong>iti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Particle Acidity<br />
Str<strong>on</strong>g<br />
Acidity<br />
[H + ] s<br />
: total amount <str<strong>on</strong>g>of</str<strong>on</strong>g> H+ nmol m -3 <str<strong>on</strong>g>of</str<strong>on</strong>g> air,<br />
measured either by titrati<strong>on</strong> or by <str<strong>on</strong>g>pH</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> the<br />
aerosol extract <strong>in</strong> an aqueous soluti<strong>on</strong>.<br />
.<br />
Free<br />
Acidity<br />
The actual amount <str<strong>on</strong>g>of</str<strong>on</strong>g> [H + ] <strong>in</strong> the aqueous<br />
phase <str<strong>on</strong>g>of</str<strong>on</strong>g> particles <strong>in</strong> nmol m -3 <str<strong>on</strong>g>of</str<strong>on</strong>g> air;<br />
Estimated by thermodynamic models (e.g.,<br />
AIM-III ) *<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g>-situ<br />
<str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
<str<strong>on</strong>g>pH</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> the particle aqueous phase; Calculated<br />
from parameters estimated by AIM-III<br />
3<br />
*: available at: www.aim.env.uea.ac.uk/aim/aim.php/
Motivati<strong>on</strong> for study<strong>in</strong>g <str<strong>on</strong>g>In</str<strong>on</strong>g>-<str<strong>on</strong>g>Situ</str<strong>on</strong>g> <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
Acid-catalyzed effects <strong>on</strong> SOA formati<strong>on</strong> 2<br />
[H + ] s<br />
Human health, cancer related 1<br />
C<strong>on</strong>versi<strong>on</strong><br />
Acid depositi<strong>on</strong><br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> <strong>in</strong>soluble ir<strong>on</strong> to soluble Fe 3<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g>-situ<br />
<str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
4<br />
1:Sathiakumar, Critical Reviews <strong>in</strong> Toxicology, 1997;<br />
2:Jang, Science, 2002; 3:Luo, JGR, 2005
<str<strong>on</strong>g>Estimati<strong>on</strong></str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> <strong>in</strong>-<str<strong>on</strong>g>Situ</str<strong>on</strong>g> <str<strong>on</strong>g>pH</str<strong>on</strong>g> us<strong>in</strong>g<br />
aerosol chemical compositi<strong>on</strong> data<br />
activity<br />
activity<br />
coefficient<br />
coefficient<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> H + , model output<br />
n H+<br />
a H+<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> H + , model output γ H+ ]<br />
<str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
IS<br />
= −loga<br />
−log[<br />
γ × n × 1000× ρ /<br />
H<br />
+<br />
H<br />
H<br />
+<br />
+<br />
=<br />
m<br />
free<br />
free<br />
H + + <strong>in</strong><br />
<strong>in</strong><br />
mol<br />
mol<br />
m -3, -3,<br />
model<br />
model<br />
output<br />
output<br />
activity<br />
activity<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g><br />
<str<strong>on</strong>g>of</str<strong>on</strong>g><br />
H + +<br />
<strong>in</strong><br />
<strong>in</strong><br />
mol<br />
mol<br />
L<br />
L -1 -1<br />
<strong>in</strong><br />
<strong>in</strong><br />
aqueous<br />
aqueous<br />
particle<br />
particle<br />
m<br />
mass<br />
mass<br />
c<strong>on</strong>centrati<strong>on</strong><br />
c<strong>on</strong>centrati<strong>on</strong><br />
<str<strong>on</strong>g>of</str<strong>on</strong>g><br />
<str<strong>on</strong>g>of</str<strong>on</strong>g><br />
particle<br />
particle<br />
aqueous<br />
aqueous<br />
phase<br />
phase<br />
c<strong>on</strong>stituents<br />
c<strong>on</strong>stituents<br />
<strong>in</strong><br />
<strong>in</strong><br />
g<br />
g<br />
m -3 -3<br />
ρ<br />
density<br />
density<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g><br />
<str<strong>on</strong>g>of</str<strong>on</strong>g><br />
particle<br />
particle<br />
aqueous<br />
aqueous<br />
phase,<br />
phase,<br />
by<br />
by<br />
equati<strong>on</strong>s<br />
equati<strong>on</strong>s<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g><br />
<str<strong>on</strong>g>of</str<strong>on</strong>g><br />
Tang<br />
Tang<br />
and<br />
and<br />
Munkelwiz<br />
Munkelwiz<br />
(1994)<br />
(1994)<br />
5
PILS: particle <strong>in</strong>to liquid system<br />
URG annular denuders:<br />
First <strong>on</strong>e is coated with Na2CO3 to remove<br />
acid gas(HNO3, URG <strong>2.5</strong>cut HNO2, cycl<strong>on</strong>e: SO2 etc). cut<br />
particle larger than <strong>2.5</strong>um <str<strong>on</strong>g>of</str<strong>on</strong>g>f by<br />
Sec<strong>on</strong>d <strong>on</strong>e is coated with citric acid for<br />
gravity<br />
removal <str<strong>on</strong>g>of</str<strong>on</strong>g> NH3<br />
6
2. Validati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> PILS measurements:<br />
Measurement set-up: PILS<br />
PILS<br />
Time resoluti<strong>on</strong>: 30 m<strong>in</strong>s<br />
VS<br />
Filter<br />
12-24 hour<br />
No. <str<strong>on</strong>g>of</str<strong>on</strong>g> Samples 539 15<br />
Sampl<strong>in</strong>g Artifacts low<br />
high (s<strong>in</strong>gle filter)<br />
7
Validati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> PILS measurements<br />
SO 4<br />
2-<br />
*<br />
NH 4+<br />
*<br />
NO 3<br />
-#<br />
K + *<br />
slope<br />
R 2<br />
slop<br />
e<br />
R 2<br />
slop<br />
e<br />
R 2<br />
slope<br />
R 2<br />
slope<br />
R 2<br />
slope<br />
R 2<br />
Chang<br />
(2007)<br />
1.05<br />
1.02<br />
0.59<br />
0.89<br />
1.18<br />
1.06<br />
0.56<br />
0.58<br />
3.56<br />
3.16<br />
0.65<br />
0.80<br />
4.91<br />
4.03<br />
0.07<br />
0.46<br />
Cl -# 0.98<br />
1.00<br />
0.05<br />
Na +# 0.77<br />
1.18<br />
1.05<br />
0.56<br />
0.90<br />
Ors<strong>in</strong>i<br />
(2003)<br />
1.03<br />
0.96<br />
0.93<br />
0.88<br />
0.32<br />
0.22<br />
Hogrefe<br />
(2004)<br />
0.71-<br />
2.37<br />
0.42<br />
-0.96<br />
1.05<br />
0.94<br />
This<br />
study<br />
1.04<br />
0.97<br />
1.06<br />
0.92<br />
1.16<br />
0.96<br />
0.82<br />
0.97<br />
0.93<br />
1.04<br />
*:No. <str<strong>on</strong>g>of</str<strong>on</strong>g> comparis<strong>on</strong>= 29; # : No. <str<strong>on</strong>g>of</str<strong>on</strong>g> comparis<strong>on</strong>= 8.<br />
8
3. Sampl<strong>in</strong>g:<br />
Three observati<strong>on</strong> periods<br />
Ⅰ(21-25 Oct. 2009) Ⅱ(6-13 Nov. 2008) Ⅲ(29 Jun.- 3 Jul. 2009)<br />
High polluti<strong>on</strong><br />
period<br />
Meteorological<br />
c<strong>on</strong>diti<strong>on</strong><br />
Ⅰ<br />
T(K)<br />
Avg.±std.<br />
298.15±1.79<br />
RH<br />
Avg.±std.<br />
0.77±0.06<br />
Cold fr<strong>on</strong>t<br />
Ⅱ<br />
292.46±2.21<br />
0.48±0.09<br />
Summer low<br />
polluti<strong>on</strong> period<br />
Ⅲ<br />
301.63±1.12<br />
0.75±0.06<br />
9
4. Results: <strong>PM</strong><strong>2.5</strong> str<strong>on</strong>g acidity<br />
2−<br />
−<br />
+<br />
−<br />
+<br />
[ H ] s<br />
= 2 [ SO4<br />
] + [ NO3<br />
] + [ Cl ] −[<br />
NH<br />
4<br />
] −[<br />
Na ] −[<br />
K<br />
+<br />
+<br />
]<br />
[H + ]s (nmol m -3 )<br />
300<br />
200<br />
100<br />
period I<br />
period II<br />
period III<br />
y = 0.40 x + 12.97<br />
R 2 = 0.62<br />
0<br />
0 100 200 300 400<br />
sulfate (nmol m -3 )<br />
[H+] s<br />
is str<strong>on</strong>gly correlated with sulfate, an expected result<br />
c<strong>on</strong>sider<strong>in</strong>g SO 4<br />
2-<br />
c<strong>on</strong>tributes 84 ± 7% <str<strong>on</strong>g>of</str<strong>on</strong>g> the total ani<strong>on</strong><br />
equivalent c<strong>on</strong>centrati<strong>on</strong>.<br />
10
4. Results:<br />
<strong>PM</strong> <strong>2.5</strong> <strong>in</strong>-situ <str<strong>on</strong>g>pH</str<strong>on</strong>g> <strong>in</strong> HK<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g>-situ <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
Ⅰ<br />
Ⅱ<br />
Ⅲ<br />
Sample No.<br />
127<br />
180<br />
213<br />
Mean<br />
0.59<br />
-0.45<br />
-0.08<br />
Std. variati<strong>on</strong><br />
0.43<br />
0.59<br />
0.81<br />
No. <str<strong>on</strong>g>of</str<strong>on</strong>g> cases<br />
No. <str<strong>on</strong>g>of</str<strong>on</strong>g> cases<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0<br />
-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<br />
<strong>in</strong> <str<strong>on</strong>g>Situ</str<strong>on</strong>g> <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
Period I<br />
-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<br />
<strong>in</strong> <str<strong>on</strong>g>Situ</str<strong>on</strong>g> <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
Period II<br />
no LWC <strong>in</strong> 23<br />
cases<br />
120<br />
Period III<br />
No. <str<strong>on</strong>g>of</str<strong>on</strong>g> cases<br />
80<br />
40<br />
0<br />
-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<br />
119<br />
<strong>in</strong> <str<strong>on</strong>g>Situ</str<strong>on</strong>g> <str<strong>on</strong>g>pH</str<strong>on</strong>g>
Period I : High Polluti<strong>on</strong><br />
C<strong>on</strong>c (µg m -3 )<br />
40<br />
30<br />
20<br />
10<br />
sulfate (L)<br />
amm<strong>on</strong>ium (L)<br />
nitrate (L)<br />
LWC (R)<br />
Period I<br />
120<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
0<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g> situ <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
2<br />
1<br />
0<br />
-1<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g> situr <str<strong>on</strong>g>pH</str<strong>on</strong>g> ((L)<br />
R+/- (R)<br />
RH (R)<br />
Period I<br />
1.6<br />
1.2<br />
0.8<br />
-2<br />
2008-10-21 2008-10-22 2008-10-23 2008-10-24 2008-10-25 2008-10-26<br />
0.4<br />
12
Period II: Cold fr<strong>on</strong>t<br />
C<strong>on</strong>c (µg m -3 )<br />
20<br />
10<br />
sulfate (L)<br />
amm<strong>on</strong>ium (L)<br />
nitrate (L)<br />
LWC (R)<br />
Period II<br />
15<br />
10<br />
5<br />
0<br />
0<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g> situ <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
2<br />
1<br />
0<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g> situr <str<strong>on</strong>g>pH</str<strong>on</strong>g> (L)<br />
R+/- (R)<br />
RH (R)<br />
Period II<br />
1.5<br />
1.1<br />
-1<br />
0.7<br />
-2<br />
2008-11-8 2008-11-9 2008-11-10 2008-11-11 2008-11-12 2008-11-13 2008-11-14<br />
0.3<br />
13
Period III: Summer low polluti<strong>on</strong> period<br />
C<strong>on</strong>c (µg m -3 )<br />
5<br />
sulfate (L)<br />
amm<strong>on</strong>ium (L)<br />
nitrate (L)<br />
LWC (R)<br />
Period III<br />
20<br />
10<br />
0<br />
0<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g> situ <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
3<br />
2<br />
1<br />
0<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g> situr <str<strong>on</strong>g>pH</str<strong>on</strong>g> (L)<br />
R+/- (R)<br />
RH (R)<br />
Period III<br />
1.2<br />
0.8<br />
-1<br />
-2<br />
2009-6-29 2009-6-30 2009-7-1 2009-7-2 2009-7-3 2009-7-4<br />
0.4<br />
14
Pr<strong>in</strong>cipal Comp<strong>on</strong>ent Aanalysis: Identify<strong>in</strong>g major<br />
factors <strong>in</strong>fluenc<strong>in</strong>g <str<strong>on</strong>g>pH</str<strong>on</strong>g> IS<br />
Factor Load<strong>in</strong>g<br />
1 2 3 4<br />
P Cl- -.952 -.070 -.098 .070<br />
P Na+ -.901 -.096 -.005 .262<br />
P NH4+ .868 .442 -.157 -.132<br />
R +/- .483 .841 -.079 .048<br />
R N/S .432 .819 .317 .022<br />
P SO4= .391 -.804 -.433 -.069<br />
<str<strong>on</strong>g>pH</str<strong>on</strong>g> IS .155 .750 -.113 .603<br />
P NO3- .030 .123 .988 -.056<br />
RH -.379 .089 -.028 .906<br />
R +/-<br />
: (equivalent c<strong>on</strong>centrati<strong>on</strong> ratio <str<strong>on</strong>g>of</str<strong>on</strong>g> total cati<strong>on</strong>s and ani<strong>on</strong>s).<br />
R N/S<br />
: (ratio <str<strong>on</strong>g>of</str<strong>on</strong>g> NH 4+<br />
and SO 4<br />
2-<br />
mole c<strong>on</strong>centrati<strong>on</strong>)<br />
15
L<strong>in</strong>ear Regressi<strong>on</strong>: <strong>in</strong>-situ <str<strong>on</strong>g>pH</str<strong>on</strong>g> <strong>in</strong> HK is welldescribed<br />
us<strong>in</strong>g R +/- and RH<br />
<str<strong>on</strong>g>pH</str<strong>on</strong>g> IS<br />
y = 0.95 x + 0.00<br />
= 4.94R<br />
+ /-<br />
+ 3.11RH<br />
−<br />
5.70<br />
Calculated <str<strong>on</strong>g>pH</str<strong>on</strong>g> IS us<strong>in</strong>g<br />
regressi<strong>on</strong> l<strong>in</strong>e<br />
(a) L<strong>in</strong>ear regressi<strong>on</strong> variables: R +/- and RH<br />
5<br />
4<br />
3<br />
R 2 = 0.93<br />
2<br />
1<br />
0<br />
-1<br />
-2<br />
-3<br />
-3 -2 -1 0 1 2 3 4 5<br />
AIM-derived <str<strong>on</strong>g>pH</str<strong>on</strong>g> IS<br />
16
L<strong>in</strong>ear Regressi<strong>on</strong>:<br />
us<strong>in</strong>g NH 4+ /SO 4<br />
2-<br />
and RH to describe <strong>in</strong>-situ <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
<str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
IS<br />
= 1.34 R<br />
/<br />
+ 3.25 RH − 54.24<br />
N<br />
S<br />
Calculated <str<strong>on</strong>g>pH</str<strong>on</strong>g> IS us<strong>in</strong>g regressi<strong>on</strong><br />
l<strong>in</strong>e<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
-1<br />
-2<br />
-3<br />
(b) L<strong>in</strong>ear regressi<strong>on</strong> variables: R N/S and RH<br />
high nitrate<br />
episode<br />
y = 0.75 x - 0.01<br />
R 2 = 0.75<br />
-3 -2 -1 0 1 2 3 4 5<br />
R +/- is a more<br />
suitable<br />
parameter than<br />
R N/S to<br />
describe <str<strong>on</strong>g>pH</str<strong>on</strong>g> IS .<br />
17<br />
AIM-derived <str<strong>on</strong>g>pH</str<strong>on</strong>g> IS
Test<strong>in</strong>g the predict<strong>in</strong>g capability <str<strong>on</strong>g>of</str<strong>on</strong>g> the<br />
l<strong>in</strong>ear regressi<strong>on</strong> equati<strong>on</strong><br />
Approach: First, use randomly selected 90% <str<strong>on</strong>g>of</str<strong>on</strong>g> the cases to<br />
obta<strong>in</strong> a regressi<strong>on</strong> equati<strong>on</strong>; Then the regressi<strong>on</strong> equati<strong>on</strong><br />
is used to predict the <str<strong>on</strong>g>pH</str<strong>on</strong>g> IS<br />
for the rema<strong>in</strong><strong>in</strong>g 10% cases.<br />
<str<strong>on</strong>g>pH</str<strong>on</strong>g> IS<br />
= .91R<br />
+<br />
+ 3.13 RH 5.70 <str<strong>on</strong>g>pH</str<strong>on</strong>g> IS<br />
= .96R<br />
+<br />
+ 3.11RH<br />
5. 72 = .97R<br />
+<br />
+ 3.10 RH 5. 72<br />
4<br />
/-<br />
−<br />
4<br />
/-<br />
−<br />
<str<strong>on</strong>g>pH</str<strong>on</strong>g> IS<br />
4<br />
/-<br />
−<br />
by<br />
2<br />
1<br />
y=0.99x-0.01<br />
y = - 0.01<br />
R 2 =0.94<br />
R 2 = 0.94<br />
0<br />
-1<br />
-2<br />
-3<br />
-3 -2 -1 0 1 2<br />
2<br />
1<br />
y=1.02x-0.02<br />
y = - 0.02<br />
R 2 =0.92 R 2 = 0.92<br />
0<br />
-1<br />
-2<br />
-3<br />
-2 -1 0 1 2<br />
By model output<br />
2<br />
1<br />
y=1.05x+0.02<br />
y = + 0.02<br />
R 2 =0.91 R 2 = 0.91<br />
0<br />
-1<br />
-2<br />
-2 -1 0 1 2<br />
18
Comparis<strong>on</strong> with <strong>in</strong> situ <str<strong>on</strong>g>pH</str<strong>on</strong>g> measurements<br />
<strong>in</strong> the literatures<br />
Locati<strong>on</strong><br />
<str<strong>on</strong>g>In</str<strong>on</strong>g>-situ <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
method<br />
Literatures<br />
H<strong>on</strong>g K<strong>on</strong>g<br />
Mean:-0.03±0.77,<br />
range <str<strong>on</strong>g>of</str<strong>on</strong>g> -1.87 to 3.12<br />
PILS<br />
This study<br />
H<strong>on</strong>g K<strong>on</strong>g<br />
Mean:-0.4<br />
range <str<strong>on</strong>g>of</str<strong>on</strong>g> -2.2 to 1.1<br />
Filter<br />
Yao et al, AE, 2006<br />
H<strong>on</strong>g K<strong>on</strong>g<br />
Mean:0.25<br />
range <str<strong>on</strong>g>of</str<strong>on</strong>g> -0.62 to 2.35<br />
Filter<br />
Pathak et al,<br />
AE,2004<br />
Mar<strong>in</strong>e <strong>PM</strong><br />
Low to -0.8<br />
range <str<strong>on</strong>g>of</str<strong>on</strong>g> 0 to 2<br />
Filter<br />
Fridl<strong>in</strong>d and<br />
Jacobs<strong>on</strong>, JGR,<br />
2000<br />
California,US<br />
range <str<strong>on</strong>g>of</str<strong>on</strong>g> -1 to3<br />
Filter<br />
Meng and Se<strong>in</strong>feld,<br />
AST,1995<br />
Pittsburgh,US<br />
Mean: about 2<br />
range <str<strong>on</strong>g>of</str<strong>on</strong>g> -0.5 to 5.5<br />
AMS<br />
Zhang et al, EST,<br />
2007<br />
19
Diurnal pattern <str<strong>on</strong>g>of</str<strong>on</strong>g> <strong>in</strong>-situ <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
Often<br />
Often<br />
lower<br />
lower<br />
<strong>in</strong><br />
<strong>in</strong><br />
the<br />
the<br />
midday<br />
midday<br />
as<br />
as<br />
a<br />
a<br />
result<br />
result<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g><br />
<str<strong>on</strong>g>of</str<strong>on</strong>g><br />
low<br />
low<br />
RH<br />
RH<br />
<br />
Implicati<strong>on</strong>:<br />
Implicati<strong>on</strong>:<br />
Together<br />
Together<br />
with<br />
with<br />
higher<br />
higher<br />
solar<br />
solar<br />
radiati<strong>on</strong>,<br />
radiati<strong>on</strong>,<br />
acid-catalyzed<br />
acid-catalyzed<br />
effects<br />
effects<br />
are<br />
are<br />
possibly<br />
possibly<br />
enhanced<br />
enhanced<br />
<strong>in</strong><br />
<strong>in</strong><br />
the<br />
the<br />
midday.<br />
midday.<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g> situ <str<strong>on</strong>g>pH</str<strong>on</strong>g><br />
3<br />
2<br />
1<br />
0<br />
-1<br />
-2<br />
12:00 12:00 12:00<br />
6-29 6-30 7-1 7-2 7-3 7-4<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g>-situ <str<strong>on</strong>g>pH</str<strong>on</strong>g> dur<strong>in</strong>g period Ⅲ<br />
20
Summary<br />
1<br />
PILS measurement provides high time<br />
resoluti<strong>on</strong> data and low sampl<strong>in</strong>g artifacts<br />
2<br />
<strong>PM</strong> <strong>2.5</strong> <strong>in</strong>-situ <str<strong>on</strong>g>pH</str<strong>on</strong>g> <strong>in</strong> HK are acidic, vary<strong>in</strong>g<br />
from -1.9 to 3.1.<br />
3<br />
Two factors, R +/-<br />
and relative humidity, are<br />
identified to be important <strong>in</strong> determ<strong>in</strong><strong>in</strong>g <strong>PM</strong><strong>2.5</strong> <strong>in</strong>situ<br />
<str<strong>on</strong>g>pH</str<strong>on</strong>g> <strong>in</strong> HK: <str<strong>on</strong>g>pH</str<strong>on</strong>g> IS<br />
=4.94R +/-<br />
+3.11RH-5.70<br />
4<br />
<str<strong>on</strong>g>In</str<strong>on</strong>g>-situ <str<strong>on</strong>g>pH</str<strong>on</strong>g> is lower <strong>in</strong> the midday as result <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
low RH, which may enhance the acid-catalyze<br />
effect.<br />
21
Acknowledge<br />
This work was supported by the Research Grants Council <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
H<strong>on</strong>g K<strong>on</strong>g (615406).<br />
22