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 etha nol in gasoline, we considered the possibility of the formation of secondary organic aerosol (S OA) 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 NO X

.

Ethanol contains three different hydrogen t ypes: three methyl hydrogens, two methylene hydrogens, and a single hydroxyl hydrogen. Abstracti on reactions of the methylene and hydroxyl hydrogens with OH, generated from hydrogen pe roxide 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 C 4 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 [C 3 H 2 OH] i = 20.0 ppmC (based on injection volume)
[H 2 O 2 ] = 4.65 ppm

IR437[C 3 H 2 OH] i = 16.5 ppmC (based on GC reading)
[H 2 O 2 ] = 10.15 ppm
(NH 4 ) 2 SO 4 seed aerosol added ~ 0.01  g m -3

Results: Both experiments were carried out using the same procedures. H 2 O 2 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) we re 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 H 2 O 2 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 aeros ol 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 1.8Particle Volume (nL/m3) Ethanol Particle Volume