the semi-volatile nature of secondary organic aerosol (soa) in the mexico city metropolitan area...
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The semi-volatile nature of secondary organic aerosol (SOA) in the Mexico City Metropolitan Area
November 2, 2007
EAS Graduate Student Symposium
Christopher J. Hennigan
SOA background
Carbonaceous particulate matter
Elemental carbon
(EC, Black carbon, soot)
Organic carbon (OC)
Primary organic aerosol (POA)
Secondary organic aerosol (SOA)
SOA importance
• Can be a major fraction of PM2.5
– 10-80% of OC
• Still poorly understood– Sources– Formation mechanisms– Chemical composition– Impact on climate change (e.g., role as CCN)
MIRAGE field study
• MIRAGE – part of multi-agency field study in 2006
•T1 ground site: located at Universidad Tecnológica de Tecámac (Tecamac University), approximately 30 km northeast and downwind of Mexico City center
PM2.5 online measurements• Inorganic ions: Na+, NH4
+, Ca2+, Mg2+, Cl-, NO3-, SO4
2-, (PILS-IC)
• Water Soluble Organic Carbon (PILS-WSOC)
Gas-phase measurements included CO, NOx, HNO3, NH3, OH, VOCs
Meteorology parameters
PILS-WSOC measurement • Measures water soluble fraction of OC aerosol on-line
• PILS-WSOC vs. filter-extracted WSOC compare favorably (slopes = 0.88 – 1.35)
• WSOC and SOA by EC tracer method highly correlated (R2 = 0.70 - 0.79) observed WSOC/SOA 0.67 – 0.75
• Excellent agreement (R2 = 0.86 - 0.93) between oxygenated organic carbon (OOC) aerosol via AMS and WSOC; 88% of OOC water soluble
• Less than 10% of primary OC is water soluble (summer, fall, and winter in Tokyo)
• WSOC is an approximate measure of SOA
[Sullivan et al., 2004; Zhang et al., 2005; Miyazaki et al., 2006; Kondo et al., 2007]
March 27-29 chemical composition
•Typical diurnal patterns
•Correlation suggests similar sources and atmospheric processing
• Use nitrate behavior to investigate SOA?
7
6
5
4
3
2
1
WS
OC
(µg
C m
-3)
12:00 AM3/27/2006
12:00 PM 12:00 AM3/28/2006
12:00 PM 12:00 AM3/29/2006
12:00 PM
Time (CST)
20
15
10
5
0
Nit
rate
(µ
g m
-3)
WSOC Nitrate
5
4
3
2
WS
OC
(µ
g C
m-3
)
2015105Nitrate (µg m
-3)
R2 = 0.80
N = 162
Nitrate production
1000
800
600
400
200
0
Sol
ar r
adia
tion
(W m
-2)
12:00 AM3/27/2006
12:00 PM 12:00 AM3/28/2006
12:00 PM 12:00 AM3/29/2006
12:00 PM
Time (CST)
7
6
5
4
3
2
1
WS
OC
(µg C
m-3)
20
15
10
5
0
Nitr
ate
(µg
m-3
)
WSOC Nitrate solar radiation
Average NO3- concentration increase (7:00am – 11:00am) = 13.8 μg m-3
∫11am
7am
k * [OH] * [NO2] * dt = 15 μg m-3Average morning HNO3 (g) production
ISORROPIA thermodynamic equilibrium model prediction: NO3
-
(NO3- + HNO3)
> 0.90
NO3- from secondary photochemical production; WSOC from SOA formation
Avg. increase (7:00am – 11:00am): NO3- = 13.8 μg m-3 (300%); WSOC = 1.6 μg C m-3 (50%)
Nitrate, WSOC concentration decrease
1000
800
600
400
200
0
Sol
ar r
adia
tion
(W m
-2)
12:00 AM3/27/2006
12:00 PM 12:00 AM3/28/2006
12:00 PM 12:00 AM3/29/2006
12:00 PM
Time (CST)
7
6
5
4
3
2
1
WS
OC
(µg C
m-3)
20
15
10
5
0
Nitr
ate
(µg
m-3
)
WSOC Nitrate solar radiation
Possibly due to:– Boundary layer dilution
– Advection
– Atmospheric processing (thermodynamics)
– Combination of the above
Avg. decrease (11:00am – 12:45pm): NO3- = 14.9 μg m-3 (82%); WSOC = 2.68 μg C m-3 (56%)
Modeling nitrate thermodynamics
• Semi-volatile NH4NO3 well understood
– Exists in gas (HNO3, NH3) or particle (NH4NO3) phase at ambient conditions
– NH4NO3: T, RH, pH, aerosol composition (SO42-)
dependent
• ISORROPIA-II thermodynamic equilibrium model– Predicts aerosol H2O content
– Predicts equilibrium gas-particle partitioning of inorganic
species (including NH3(g)/NH4+, HNO3(g)/NO3
-)
[Nenes et al., 1998; Fountoukis et al., 2007]
ISORROPIA model output
20
15
10
5
0
Nitrate (µ
g m-3 )
00:003/27/2006
12:00 00:003/28/2006
12:00 00:003/29/2006
12:00
Central Standard Time
30
25
20
15
10
5Tem
pera
ture
(ºC
)
80
60
40
20
RH
(%)
100
80
60
40
20
Pe
rcen
t aer
osol
nitr
ate
Temperature RH Aerosol nitrate fraction Nitrate
[Fountoukis et al., 2007]
ISORROPIA predicts observed drop in NO3- just before noon
Correlation to SOA?
• Dramatic decrease in semi-volatile NO3
- concentration was due at least in part to thermodynamics (NO3
- evaporation)
• Was observed WSOC concentration decrease also due to the semi-volatile nature of SOA?
• Assess the possible impacts of BL dilution and advection by using a conservative tracer (CO in this case)
12:00 PM3/27/2006
6:00 PM
Time (CST)
20
15
10
5
Nitr
ate
(µg
m-3
)
35
30
25
20
15
10
5
NO
3 :CO
(µg m
-3 ppm-1)
Nitrate Nitrate:CO ratio
WSOC:CO and NO3-:CO ratios
5
4
3
2
WS
OC
(µ
g C
m-3
)
12:00 PM3/27/2006
6:00 PM
Time (CST)
20
15
10
5
Nitr
ate
(µg
m-3
)
35
30
25
20
15
10
5
NO
3 :CO
(µg m
-3 ppm-1)
12
10
8
6
4
2
WS
OC
:CO
(µg C
m-3 ppm
-1)
Nitrate Nitrate:CO ratio
WSOC WSOC:CO ratio
6
5
4
3
2
1
Wind speed (m
s-1 )
12:00 PM3/27/2006
6:00 PM
Time (CST)
35
30
25
20
15
10
5
NO
3:C
O r
atio
(µ
g m
-3 p
pm-1
)
12
10
8
6
4
2
WS
OC
:CO
ratio (µg C
m-3 pp
m-1)
Wind speed NO3:CO ratio WSOC:CO ratio
WSOC:CO and NO3-:CO
R2 = 0.46
Appears as if meteorology is driving the afternoon WSOC concentration, not thermodynamics (observed on all three days)
Summary
• Nitrate and WSOC (SOA) highly correlated (R2 = 0.80) in Mexico City Metropolitan Area (MCMA), indicating similar sources and atmospheric processing
• Nitrate experienced a rapid phase shift from the particle to the gas phase around noon, with observations and model results agreeing well; not so for SOA
• Fresh SOA with strong anthropogenic influence in the MCMA was less volatile than NH4NO3
– Semi-volatile intermediates more stable products– Oligomer formation?– Thermodynamic threshold not reached?
Fast (2 - 3hrs)