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

7

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

9

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.

12

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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