Contribution and sources of light absorption brown carbon at a suburban site in Guangzhou, China
Yi Ming Qin1, Hao Bo Tan2, Yong Jie Li3,*, Li Liu2,4, Chak K.Chan1,*
1School of Energy and Environment, City University of Hong Kong
2Institute of Tropical and Marine Meteorology, China Meteorological Administration, Guangzhou
3Department of Civil and Environmental Engineering, Faculty of Science and Technology, University of Macau
1
Why care about Brown carbon?
• Quantitative prediction of radiative forcing due to aerosol is still challenging.
• Black carbon? Brown carbon (BrC)?
• BrC is an ensemble of light absorbing (colored) organic compounds.
• Particles from smoldering combustion can contain substantial amounts of BrC.
• Appears light brown to yellowish, and not black as would be expected for black carbon.
2
Sources and chemical compositions of BrC
Both primary and secondary sources: • biomass burning, biological aerosols,
• secondary organic aerosol formed from anthropogenic or biogenic precursors.
Several classes of compounds: • nitroaromatic compounds, such as nitrophenols
• N-heterocyclic compounds,
• and quinones.
3
BrC studies in Pearl River Delta region
• Light absorption of brown carbon aerosol in the PRD region of China by J.-F. Yuan, et al. Atmos. Chem. Phys., 2016
BrC contributed 6.3% to 12.1% of the total aerosol light absorption at 405 nm;
4.1% to 10.0% at 532 nm.
• However, little is known about the relationship between the brown carbon and aerosol chemical compositions.
4
Sampling site
23◦00’ N, 113◦21’E, 141ma.s.l.
Sampling period: 7th Nov. 2014 -3rd Jan. 2015
Panyu site: About 15 km south of the downtown
Guanzhou Located at the summit of Dazhengang
at ~150 m
5
Instruments
Aethalometer (AE33):
Seven wavelengths aerosol light absorption
High-resolution Time-of-Flight Aerosol Mass Spectrometer (HR-Tof-AMS):
Organics factors and organic nitrate, CHN,CHON CHOxN groups (λ = 370, 470,520, 590, 660, 880 and 950nm)
6
Time series of aerosol light absorption at different wavelengths
The average absorption was 11.8 Mm-1 at 370nm , 8.6 Mm-1 at 470 nm, 7.3 Mm-1 at 520, 6.3 Mm-1 at 590nm, 5.6 Mm-1 at 660nm,3.8 Mm-1 at 880nm and 3.6 Mm-1 at 950nm.
70
60
50
40
30
20
10
0b
ab
s (M
m-1
)2014/11/11 2014/11/21 2014/12/1 2014/12/11 2014/12/21 2014/12/31
dat
babs370 babs470 babs520 babs590 babs660 babs880 babs950
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Absorption Angström exponent (AAE)
The basic theory used to describe light absorption properties is the well-known power-law relationship: Absλ=K*λ−AAE
where Absλ (unit: Mm−1 or M−1) refers to the absorption coefficient of aerosol at wavelength λ (unit: nm). K is a constant. AAE indicates the wavelength dependence of light absorption.
Pure black carbon: AAE ~1; AAE for brown carbon ≥2, up to 9.7 The presence of BrC will make the absorption stronger at shorter wavelengths:
BrC mainly absorb light at UV and short-visible wavelengths
and absorb negligibly at longer wavelength. 8
Average absorption spectra and AAE
higher than 1
Average absorption spectra over 370-950nm wavelength during the whole campaign.
Absλ=K*λ−AAE
the presence of brown carbon
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Attribution of aerosol light absorption to BC and BrC
• Power-law relationship Absλ=K*λ−AAE
• BrC mainly absorb light at UV and short-visible wavelengths and absorb negligibly at longer wavelength.
Attributed absorption of BC at a short wavelength.
Absorption at a longer wavelength, at which BrC has negligible or no absorption
Theoretical AAE value for pure black carbon, “1”
Attributed absorption of BrC
10
2014.11.11 2014.11.21 2014.12.1 2014.12.11 2014.12.21 2014.12.31dat
50
40
30
20
10
0
babs
(Mm
-1)
babs370_BrC babs370_BC
78.9%
21.1% Light absorption contribution due to BrC : 21.1% at 370nm 15.2% at 470nm 9.6% at 520 nm 7.4% at 590nm and 7.2% at 660 nm
Contribution of aerosol light absorption to BC and BrC
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Organic factor analysis
Time series of Overall organics mass spectra Separate into a number of factors
Each factor corresponds to a large group of OA constituents with similar chemical composition and temporal behavior.
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Organic factor analysis
6
4
2
0
K+ (µ
g/m
3)
11/11 11/21 12/1 12/11 12/21 12/31Local time
8
4
0
80
60
40
20
0
x10
-3
80604020mass to charge (m/z)
0.12
0.08
0.04
0.00
60x10-3
30
0
Fra
ctio
n o
f O
A s
ign
als
0.20
0.10
0.00
0.12
0.08
0.04
0.00
80
60
40
20
0
300
200
100
0
NO
x (pp
b)
40
30
20
10
0
40
20
0
NO
3
- (µg
/m3)
20
10
0
20
15
10
5
0
30
20
10
0
SO
4
2- (µ
g/m
3)
3.0
2.0
1.0
0.0
C3 H
3 O (µ
g/m
3)
0.8
0.4
0.0
C2 H
4 O2 (µ
g/m
3)40
20
0
11/21/ 11/24/ 11/27/ 11/30/
dat
120
80
40
0
(pp
b)
OM:OC 1.47O:C 0.22H:C 1.91
OM:OC 1.33O:C 0.12H:C 1.83
OM:OC 1.85O:C 0.51H:C 1.92
OM:OC 2.08O:C 0.69H:C 1.68
OM:OC 2.36O:C 0.92H:C 1.37
HOA
COA
BBOA
SVOOA
LVOOA
4443
57
55
57
60
44
Mas
s co
nce
ntr
atio
n o
f O
A f
acto
rs (µ
g/m
3)
HOA vs. NOx
Rp=0.83
COA vs. C3H3O
Rp=0.66
BBOA vs. C2H4O2
Rp=0.71
LVOOA vs. SO4
2-
Rp=0.68
SVOOA vs. NO3
-
Rp=0.70
55
73
O3
BBOA vs. K+
Rp=0.58
POA: Hydrocarbon-like organic aerosol
(HOA); Cooking organic aerosol (COA); Biomass burning related organic
aerosol (BBOA)
SOA: Semi-volatile oxygenated organic
aerosol (SV-OOA); Low-volatility oxygenated
organic aerosol (LVOOA)
13
Correlation of light absorption by BrC with OA
15
10
5
0
ba
bs
37
0_
BrC
(Mm
-1)
6040200
HOA (g m-3
)
15
10
5
0
ba
bs
37
0_
BrC
(Mm
-1)
151050
COA (g m-3
)
BrC was associated with BBOA, biomass burning related aerosol.
BrC was also associated with LVOOA. Possible source: SOA from VOCs
14
Correlation of light absorption by BrC with CxHyN,CxHyON and CxHyOxN
CxHyN, CxHyON and CxHyOxN may be responsible for the observed optical properties of BrC.
15
Conclusions
I. Light absorption contribution due to BrC at Guangzhou, Panyu was obtained, with 21.1% of the total aerosol absorption at 370nm, 15.2% at 470nm, 9.6% at 520 nm, 7.4% at 590nm and 7.2% at 660 nm.
II. The BrC were associated with BBOA and LVOOA at this site. Possible source: Biomass burning and SOA from VOCs.
III. CxHyN, CxHyON and CxHyOxN may be responsible for the observed optical properties of BrC.
This work was supported by the National Key Project of the Ministry of Science and Technology of the People’s Republic of China (2016YFC0201901) and the Natural Science Foundation of China (41375156).
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