QUANTITATIVE ANALYSIS OF THE PRODUCTS OF THE HETEROGENEOUS REACTION BETWEEN OLEIC ACID AEROSOL AND OZONE
O. Vesna, H.W. Gäggeler (Univ. Bern & PSI), M. Kalberer, M. Sax (ETHZ), K. Stemmler, M. Birrer, M. Ammann (PSI) The ozonolysis products of oleic acid particles were studied in detail with simultaneous detection of the four primary products: nonanal, azelaic acid, nonanoic acid and oxo-nonanoic acid. Identification and quantification of the products was performed by GC-MS. The yields of all products were studied as a function of reaction time and relative humidity. INTRODUCTION The ozonolysis of particle phase organic molecules can lead to changes in the physicochemical properties of atmospheric aerosol particles [1]. The reaction of oleic acid (OA) - aerosol with ozone was chosen as a model system to improve the understanding of atmospheric chemical oxidation processes affecting organic particles. EXPERIMENTAL The studies of the OA-aerosol plus ozone reaction were carried out with aerosol particles of 77 nm diameter, at a concentration of 5x106 particles/cm3 at room temperature. OA particles generated by homogeneous nucleation of vapour phase OA were mixed with ozone and fed into a Teflon aerosol flow reactor (Fig. 1). Aerosol size distribution and number concentration were measured with a scanning mobility particle sizer (SMPS). The reaction time was varied by changing the reactor length. At the exit of the flow tube, O3 was destroyed on a potassium iodide denuder, and the aerosol was sampled on the filter. Gaseous products were collected by adsorption on polyurethane foam traps [2]. Gas-chromatography mass spectrometry (GC-MS) was used to identify and quantify the four major reaction products, i. e., nonanal (NN), azelaic acid (AA), nonanoic acid (NA), oxo-nonanoic acid (ON), and unreacted OA in both the gas and the particle phase. time. The yields are expressed as carbon yields (in %). As an example the yield of NN was calculated as % (NN) = 100[c(NN)×nC(NN)/c(OA)×nC(OA)] (1)
where c(NN) is the concentration of NN measured at the reactor exit, c(OA) is the concentration of OA at the inlet of the flow reactor, and nC is the number of carbon atoms in the molecular structure of the compounds. The error given is the standard deviation 1σ. Table 1: Relative carbon yields of the reaction products found in the condensed and the gas phase as function of relative humidity and reaction time.
NN MRT , s 60 150 250 359 RH,%
0 30 65 76
e
AA
NA
ON
UPa
OA loss
Product yields (%) as a function of reaction time
7.0 ± 0.0 11.9 ± 0.2 12.4 ± 1.2 14.1 ± 0.8 0.3 ± 0.1 0.6 ± 0.0 0.6 ± 0.1 0.6 ± 0.0 0.3 ± 0.0 0.3 ± 0.1 0.4 ± 0.0 0.4 ± 0.0 0.4 ± 0.0 0.4 ± 0.1 0.5 ± 0.0 0.5 ± 0.0 5.7 ± 1.6 9.2 ± 1.4 8.6 ± 2.8 12.3 ± 7.2 13.7 ± 1.5 22.3 ± 1.3 22.4 ± 1.5 27.8 ± 6.7
Product yields (%) as a function of relative humidity
11.9 ± 0.2 15.3 ± 1.7 15.9 ± 0.6 14.2 ±1.2 0.6 ± 0.0 0.7 ± 0.1 0.7 ± 0.0 0.6 ± 0.0 0.3 ± 0.1 0.5 ± 0.1 0.6 ± 0.2 0.5 ± 0..1 0.4 ± 0.1 0.6 ± 0.2 0.6 ± 0.0 0.6 ± 0.0 9.2 ± 1.4 7.1 ± 1.9 7.9 ± 1.9 12.2 ± 0.4 22.3 ± 1.3 24.1±0.2 25.7 ± 1.0 28.0 ± 0.8
a UP are unidentified products. b MRT is the mean residence time of a particle in the flow reactor.
Fig. 1: Schematic set-up of the aerosol flow tube and the sampling system. RESULTS The absolute yield of all products was determined. NN was quantified without derivatization. The other products were determined after derivatization with MTBSTFA (N-tertbutyldimethylsilyl-N-methyltrifluoroacetamide). The quantification of all products was performed with authentic standards by GC-MS [3]. The amount of OA removed by the reaction was determined from the difference between the mass of OA aerosol entering the reactor (measured by SMPS) and the concentration of unreacted OA collected at the reactor exit (measured by GC-MS). The SMPS- and the GC-MS-measurements were intercalibrated. In Table 1, the yields of the oxidation products are presented as a function of relative humidity and reaction
The analysis of the data first shows a surprising predominance of nonanal over oxo-nonanoic acid, which has not been realized in previous studies, mostly because nonanal was not detected in the gas and the particle phases in the same experiment. Currently, we have no clear explanation for this observation. We observed an increase of the OA loss with increasing relative humidity and reaction time, indicating that humidity plays an important role in the process of O3 uptake. The increasing yields of nonanal with increasing humidity could indicate that water also plays a role in interfering with the secondary processes of the Criegee intermediate formed after attack of O3 to the double bond and the decay of the primary ozonide. REFERENCES [1] Y. Rudich, Chem. Rev. 103, 5097 (2003). [2] M. Sax et al., J. Environ. Monit. 226N, 5 (2003). [3] J. Yu at al., Environ. Sci. Technol. 29, 1923 (1996).