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Brown Carbon in the Continental Troposphere
Jiumeng Liu1,2, Eric Scheuer3, Jack Dibb3, Luke D. Ziemba4, Kenneth.L.Thornhill4,
Bruce E. Anderson4, Armin Wisthaler5, Tomas Mikoviny6, J Jai Devi7, Michael Bergin7,
Rodney J. Weber1*
1
School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta,
GA, USA.
2
Now at Atmospheric Sciences and Global Change Division, Pacific Northwest
National Laboratory, Richland, WA, USA.
3
Institute for the Study of Earth, Oceans, and Space, University of New Hampshire,
Durham, NH, USA.
4
NASA Langley Research Center, Hampton, VA, 23681, USA
5
Institut fuer Ionenphysik und Angewandte Physik, A-6020 Innsbruck, AUSTRIA
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Oak Ridge Associated Universities (ORAU), Oak Ridge, TN, USA
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School of Civil & Environmental Engineering, Georgia Institute of Technology,
Atlanta, GA, USA
*
Correspondence to: Rodney Weber (rodney.weber@eas.gatech.edu)
For submission to GRL
Index terms: 0305, 0345, 0365
Keywords: Brown carbon, aerosol light absorption, direct radiative forcing
Key points:
Brown carbon (BrC) is prevalent throughout the troposphere and increases relative to
BC with altitude
Optical closure is obtained between BrC plus BC and total absorption at 365nm
BrC contributes 20% to top of atmosphere absorbing aerosol forcing
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Light Scattering Measurements
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The light scattering coefficients (bsp) were measured by a TSI Integrating nephelometer at
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ambient RH at wavelengths of 450 nm, 550 nm, and 700 nm using the same inlet
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cut-point as other instruments (aerodynamic diameter of 4.1 µm). Similar to absorption,
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scattering coefficients measured at three wavelengths were first averaged over
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filter-sampling period, if larger than 75% of the period were covered by measurements.
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The averaged scattering coefficients were then extended to other wavelengths based on
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scattering Ångström exponent (SAE), following equations (1) and (2).
SAE  
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ln bsp (700)   ln bsp (450) 
ln( 700)  ln( 450)
  
bsp ( )  bsp (550)  

 550 
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(1)
 SAE
(2)
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Averaged into 1-km altitude intervals, the estimated SAE was 1.27±0.74, for this study.
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Scattering data were reported to have a 10% uncertainty for measurements at all three
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wavelengths, therefore the combined uncertainty in estimated scattering coefficients at
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various wavelengths, based on equations (1) and (2), was 17%.
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Uncertainty analysis
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Significant uncertainties exist in all of these calculations, including bap,PSAP(365),
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bap,BC(365) and the absorption of brown carbon, bap,
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discussed, we chose the wavelength pair of 470 nm and 660 nm to estimate the AAE of
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total aerosols. Other combinations result in a difference of ±20% on the estimated aerosol
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absorption coefficient at 365 nm. The PSAP absorption itself has an uncertainty of 0.2
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Mm-1 or 20% due to measurement and artifact correction, which results in a combined
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BrC(365)).
For bap,PSAP(365), as
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uncertainty (assuming all are independent) of 0.25 Mm-1 or 29%, whichever is larger.
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Similarly, bap,BC(365) was estimated based on PSAP absorption at 660 and an AAE of 1.
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A 15% difference in the AAE of BC would result in a 10% difference in retrieved BC
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absorption at 365 nm, which, combined with the PSAP measurement uncertainty, results
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in a 22.4% uncertainty in bap,BC(365).
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For BrC absorption in the extracts, the uncertainties were estimated at ±34% for the
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combined water and methanol extraction process. According to Liu et al. [2013], the
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uncertainty due to various assumptions associated with the Mie calculations were
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estimated at 30%, and the combined uncertainty in estimating bap,BrC(365) is 45%.
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Table 1. Nomenclature
PSAP
TOA
SSA
AOD
bap:
BC:
BrC
AAE
AAEBrC
AAEBC
AAEPSAP
SAE
Abs(λ):
Total_Abs(λ):
bap, BrC(λ):
bap,PSAP(λ):
bap,BC (λ)
bsp (λ)
Particle Soot Absorption Photometer
Top of Atmosphere
Single Scattering Albedo
Aerosol Optical Depth
light absorption coefficient for fine particles (M/m)
Black Carbon
Brown Carbon
Absorption Ångström Exponent
Absorption Ångström Exponent for brown carbon
Absorption Ångström Exponent for black carbon
Absorption Ångström Exponent based on the PSAP data
Scattering Ångström Exponent
light absorption measured in a solution at wavelength λ (M/m)
sum of H2O_Abs(λ) and MeOH_Abs(λ) for a filter extracted
sequentially using the two solvents (water then methanol).
Mie predicted fine particle brown carbon absorption from the sum of
water and methanol extracts (M/m), wavelength is specified in text.
Light absorption coefficient of fine particles at wavelength λ (M/m)
determined from the PSAP data.
Light absorption coefficient of BC at wavelength λ (M/m), estimated
from PSAP absorption at 660 nm, assuming non-BC light absorbers are
minimal at 660 nm and an AAEBC of 1
Light scattering coefficient of aerosols at wavelength λ (M/m),
estimated from nephelometer measurements
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Table 2. Flight periods identified to be largely impacted by biomass burning
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contributions.
Time (UTC)
2012/5/25, 22:00-22:26
2012/5/26, 21:20-21:40, 2012/5/27, 00:09-00:21
2012/6/6, 21:27-21:37, 2012/6/7, 00:19-00:36
2012/6/11 16:24-16:57, 17:56-18:11, 21:56-22:06
2012/6/15, 19:51-20:10
2012/6/16, 21:18-21:26, 2012/6/17 01:36-02:13
2012/6/17, whole flight
2012/6/22, whole flight
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Figure 1. (a) Map of NASA DC-8 research aircraft sampling regions during the DC3
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experiment based at Salina, KS. (b) Relative filter sample frequency as a function of
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altitude, with number of filters for each altitude bin given.
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Figure 2. Example solution light absorption spectra from the sum of the water and
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methanol filter extract, plotted on a (a) linear (b) log scale. Absorption Ångström
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exponent is calculated by linear regression fit to logAbs vs logλ over the wavelength
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range of 300-450nm, as the blue line in (b).
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