Laboratory experiments show that the optical absorptivity of model organic matter is not an intrinsic property, but a strong function of relative humidity, temperature, and insolation. Suites of representative polyfunctional C_(x)H_(y)O_(z) oligomers in water develop intense visible absorptions upon addition of inert electrolytes. The resulting mixtures reach mass absorption cross sections σ(532 nm) ~ 0.1 m^(2)/gC in a few hours, absorb up to 9 times more solar radiation than the starting material, can be half-bleached by noon sunlight in ~ 1 h, and can be repeatedly recycled without carbon loss. Visible absorptions red-shift and evolve increasingly faster in subsequent thermal aging cycles. Thermochromism and its strong direct dependences on ionic strength and temperature are ascribed to the dehydration of >CH−C(OH)C═C< unsaturations by a polar E1 mechanism, and bleaching to photoinduced retrohydration. These transformations are deemed to underlie the daily cycles of aerosol absorption observed in the field, and may introduce a key feedback in the earth’s radiative balance

Colussi, A. J.

Guzmán, Marcelo I.

Hoffmann, M. R.

Rincón, Angela G.

English

Caltech Authors - Main

 1
Supporting Information 
Thermochromism of Model Organic Aerosol Matter  
Angela G. Rincón,  Marcelo I. Guzmán, M. R. Hoffmann and A. J. Colussi* 
W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 
91125 
 
 
FIGURE S1. Absorbances of the photolyzed solution after 19h at 25°C in the dark (AT) 
minus absorbances before photolysis (black solid line), or after photolysis for 2h (blue 
line) (APHOT) of 70mM PA solutions in the absence, and presence of 1.5M Na2SO4, 2M 
NH4HSO4 (ABS), or 2M NH4ClO4  
 
 2
Time /h
0 1 2 3 4
A
bs
or
ba
nc
e
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
 
 
 
FIGURE S2A. Absorbances Aλ vs. time in a 120 mM PA solution previously photolyzed 
for 4.5 h, after diluting it by half with 4 M ABS, and then heated at 60°C. λ = 290nm (red 
trace); 300nm (blue trace). Curves closely follow exponential growth kinetics: Aλ(t) = 
Aλ(∞) [1 – exp(-kλ t)]; See text. 
 
 
 
FIGURE S2B. Absorbances Aλ vs. time in a 60 mM PA, 2M ABS solution previously 
photolyzed for 4.5 h, maintained in the dark at 60°C. From the top: λ = 350, 360, 380, 
400, 450 and 500 nm. Inset, from the top: λ = 310, 320, 330, 340 and 346 nm. See text. 
 
 3
 
FIGURE S3. Normalized absorbances Aλ(t)/Aλ(20h) vs. time of a previously photolyzed 
(for 4.5 h) 60 mM PA, 2M ABS solution, maintained in the dark at 60 °C. From the top: 
λ = 280, 320, 300, 400, 420, 450, 560 and 650 nm. 
 
 
FIGURE S4. Ratios of initial rates ABS[∂Aλ(t)/∂t]0/[∂Aλ(t)/∂t]0 in experiments involving 
62 mM PA solutions previously photolyzed for 4.5 h, in the absence {[∂Aλ(t)/∂t]0}or 
presence{ABS[∂Aλ(t)/∂t]0}of 2M ABS. Red circles and line: in stage (C). Blue circles and 
line: after the second thermal treatment, stage (D). See main text. 
 
 
.  
 4
σ, 
φo
r σ
x 
φ/a
.u
.
λ / nm
300 400 500 600 700 800
0.01
0.1
1
σ, 
φo
r σ
x 
φ/a
.u
.
 
FIGURE S5. Solar energy fluxes φ(λ) at the Earth’s surface at zero zenith angle (bold 
orange curve). Absorption cross sections σ(λ) and σ(λ)×φ(λ) products for a 60 mM PA, 
2M ABS solution, after stage (D) (re-photolysis) [σD(λ), blue triangles], [σD(λ)×φ(λ), 
bold blue curve], followed by thermal treatment for 21h at 60 °C [σC(λ), red triangles], 
[σC(λ)×φ(λ), bold red curve]. 60mM PA, 2M ABS solution after stage (C) at 25C for 12h; 
[σ( λ), green circles]. 
 
 
FIGURE S6. ∫
700
300
λλφλσ d)()(  during the photobleaching of a 80m MPA, 2M ABS 
solution after stage (C). See text.  
 
 5
Chemicals and Procedures  
 Ammonium bisulfate (ABS, Fluka >99%), sodium sulfate (EMD Chemicals, 99%), 
ammonium perchlorate (Mallinckrodt, 99%) salts were used as received. 1 M sulfuric 
acid (VWR) was used to adjust the pH of the solutions as indicated. All solutions were 
made in deionized, ultrapure water (resistivity 18.2 MΩ cm) from a Millipore purification 
system. Salts were added to PA solutions prior to, or immediately after photolysis, as 
indicated. 
 Negative ion mass spectra of directly infused sample solutions (previously diluted 
with chromatographic grade methanol, Chromasolv, purity > 99.9%) were acquired using 
a ± 0.1 Da resolution  electrospray ionization mass spectrometer (Agilent 1100). Sample 
solutions were also analyzed via HPLC-MS in a system provided with tandem UV-
detection a (Hewlett-Packard 1100 series) using a Nova-Pak C18 column (300 m × 3.9 
mm, Waters). The eluents were 92.5% of 0.1% acetic acid in water solution (A) and 7.5% 
methanol (B). B programmed to increase linearly from 7.5% to 90.0% between 8 and 
45min.  
 
O
O O
OH
coumalic acid  
       Scheme S1 


Thermochromism of Model Organic Aerosol Matter

 1
Supporting Information 
Thermochromism of Model Organic Aerosol Matter  
Angela G. Rincón,  Marcelo I. Guzmán, M. R. Hoffmann and A. J. Colussi* 
W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 
91125 
 
 
FIGURE S1. Absorbances of the photolyzed solution after 19h at 25°C in the dark (AT) 
minus absorbances before photolysis (black solid line), or after photolysis for 2h (blue 
line) (APHOT) of 70mM PA solutions in the absence, and presence of 1.5M Na2SO4, 2M 
NH4HSO4 (ABS), or 2M NH4ClO4  
 
 2
Time /h
0 1 2 3 4
A
bs
or
ba
nc
e
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
 
 
 
FIGURE S2A. Absorbances Aλ vs. time in a 120 mM PA solution previously photolyzed 
for 4.5 h, after diluting it by half with 4 M ABS, and then heated at 60°C. λ = 290nm (red 
trace); 300nm (blue trace). Curves closely follow exponential growth kinetics: Aλ(t) = 
Aλ(∞) [1 – exp(-kλ t)]; See text. 
 
 
 
FIGURE S2B. Absorbances Aλ vs. time in a 60 mM PA, 2M ABS solution previously 
photolyzed for 4.5 h, maintained in the dark at 60°C. From the top: λ = 350, 360, 380, 
400, 450 and 500 nm. Inset, from the top: λ = 310, 320, 330, 340 and 346 nm. See text. 
 
 3
 
FIGURE S3. Normalized absorbances Aλ(t)/Aλ(20h) vs. time of a previously photolyzed 
(for 4.5 h) 60 mM PA, 2M ABS solution, maintained in the dark at 60 °C. From the top: 
λ = 280, 320, 300, 400, 420, 450, 560 and 650 nm. 
 
 
FIGURE S4. Ratios of initial rates ABS[∂Aλ(t)/∂t]0/[∂Aλ(t)/∂t]0 in experiments involving 
62 mM PA solutions previously photolyzed for 4.5 h, in the absence {[∂Aλ(t)/∂t]0}or 
presence{ABS[∂Aλ(t)/∂t]0}of 2M ABS. Red circles and line: in stage (C). Blue circles and 
line: after the second thermal treatment, stage (D). See main text. 
 
 
.  
 4
σ,
 φ
or
 σ
x 
φ/
a.
u.
λ / nm
300 400 500 600 700 800
0.01
0.1
1
σ,
 φ
or
 σ
x 
φ/
a.
u.
 
FIGURE S5. Solar energy fluxes φ(λ) at the Earth’s surface at zero zenith angle (bold 
orange curve). Absorption cross sections σ(λ) and σ(λ)×φ(λ) products for a 60 mM PA, 
2M ABS solution, after stage (D) (re-photolysis) [σD(λ), blue triangles], [σD(λ)×φ(λ), 
bold blue curve], followed by thermal treatment for 21h at 60 °C [σC(λ), red triangles], 
[σC(λ)×φ(λ), bold red curve]. 60mM PA, 2M ABS solution after stage (C) at 25C for 12h; 
[σ( λ), green circles]. 
 
 
FIGURE S6. ∫
700
300
λλφλσ d)()(  during the photobleaching of a 80m MPA, 2M ABS 
solution after stage (C). See text.  
 
 5
Chemicals and Procedures  
 Ammonium bisulfate (ABS, Fluka >99%), sodium sulfate (EMD Chemicals, 99%), 
ammonium perchlorate (Mallinckrodt, 99%) salts were used as received. 1 M sulfuric 
acid (VWR) was used to adjust the pH of the solutions as indicated. All solutions were 
made in deionized, ultrapure water (resistivity 18.2 MΩ cm) from a Millipore purification 
system. Salts were added to PA solutions prior to, or immediately after photolysis, as 
indicated. 
 Negative ion mass spectra of directly infused sample solutions (previously diluted 
with chromatographic grade methanol, Chromasolv, purity > 99.9%) were acquired using 
a ± 0.1 Da resolution  electrospray ionization mass spectrometer (Agilent 1100). Sample 
solutions were also analyzed via HPLC-MS in a system provided with tandem UV-
detection a (Hewlett-Packard 1100 series) using a Nova-Pak C18 column (300 m × 3.9 
mm, Waters). The eluents were 92.5% of 0.1% acetic acid in water solution (A) and 7.5% 
methanol (B). B programmed to increase linearly from 7.5% to 90.0% between 8 and 
45min.  
 
O
O O
OH
coumalic acid  
       Scheme S1 


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http://authors.library.caltech.edu/18450/1/Rincon2010p9964J_Phys_Chem_Lett.pdf

Thermochromism of Model Organic Aerosol Matter

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