UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

NATIONAL EXPOSURE RESEARCH LABORATORY 

Research Triangle Park, NC 27711 

OFFICE OF ' 

RESEARCH AND DEVELOPMENT

September 30, 2008 

MEMORANDUM

SUBJECT: Hypothetical SOA Production from Ethanol Photooxidation 

FROM: Tadeusz E. Kleindienst 

Research Physical Scientist (919-541-2308) 

TO: Patricia Rowley 

With recent interest in the widespread use of ethanol in gasoline, we considered the possibility of 

the formation of secondary organic aerosol (SOA) from the photooxidation of ethanol. In the 

past, this has been considered highly unlikely given that ethanol is a small molecule and the 

photooxidation pathways are generally well known. However, the same arguments had been 

used to suggest that isoprene did not form SOA and it was subsequently learned that isoprene 

photooxidation products do form SOA, although at low yields. While the reaction mechanism for 

isoprene is considerably more complex than for ethanol, we have considered the possibility that 

ethanol might form SOA if the photooxidation were conducted in the absence of NOX. 

Ethanol contains three different hydrogen types: three methyl hydrogens, two methylene 

hydrogens, and a single hydroxyl hydrogen. Abstraction reactions of the methylene and hydroxyl 

hydrogens with OH, generated from hydrogen peroxide photolysis, are expected to produce 

acetaldehyde. However, OH abstraction of a methyl hydrogen will likely produce a peroxy 

ethanol-type compound. One can then hypothesize that without NO being present, this 

intermediate will only self react to form a C4 dihydroxy peroxide. It is the disposition of this 

product which is uncertain and aerosol formation can only be ruled out by experiment. 

Based on this hypothesis, two experiments were conducted (by our laboratory naming scheme 

IR436 and IR437 dated 7/26 and 8/1/2007, respectively). The first experiment was a survey type 

experiment simply to see if ethanol SOA readily forms. For the second experiment, 

measurements were made for the reacted ethanol and a low ammonium sulfate seed aerosol 

concentration was added. The initial conditions for the two experiments are given as follows. 

IR436 

[C3H2OH]i = 20.0 ppmC (based on injection volume) 

[H2O2] = 4.65 ppm 

IR437 
[C3H2OH]i = 16.5 ppmC (based on GC reading) 

[H2O2] = 10.15 ppm 

(NH4)2SO4 seed aerosol added ~ 0.01 g m-3

Results: 

Both experiments were carried out using the same procedures. H2O2 was added to the chamber in 

the gas phase by passing air through an impinger into the chamber. Ethanol was added to the 

chamber by evaporating a know amount from a syringe. Once the initial conditions were set the 

lights (a combination of UVB and UV-310 bulbs) were turned on. Particle growth was followed 

using a Scanning Mobility Particle Sizer (SMPS). 

In both IR436 and IR437, particle formation was not detected over a period of approximately 

seven hours. As noted, IR436 was a survey experiment and the lack of particle formation led to 

an adjustment of the initial conditions to further improve organic particle formation. As a result, 

seed aerosol was added and the H2O2 concentration was doubled to more rapidly react ethanol in 

the chamber. In IR437, GC measurements were made to quantify the ethanol concentration in the 

chamber. In the experiment, approximately 10,000 g m-3 of ethanol was removed by reaction 

with OH. The results of this run including plots for the reacted ethanol and the SMPS particle 

volume are shown below. Again, no sign of aerosol formation was detected. Based on these 

results, the precision of the SMPS measurements (assuming a density of 1 g nL-1 to convert the 

SMPS volume units to mass units) were used together with the reacted ethanol to calculate an 

upper limit for the yield. This leads to an upper limit for the SOA yield of < 1.5 × 10-6. 

Ethanol / H2O2 Irradiation - IR437

0

2

4

6

8

10

12

14

16

18

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

Irradiation Time (h)

Ethanol (ppmC)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Particle Volume (nL/m3)

Ethanol Particle Volume