AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
O3-initiated Heterogeneous Oxidation of Linoleic Acid and its Dependence on Ambient Temperature and Relative Humidity
Guang Zeng, Yunhong Zhang, YONG LIU, University of Colorado Denver
Abstract Number: 54 Working Group: Aerosol Chemistry
Abstract Despite recent extensive studies towards heterogeneous oxidation of unsaturated organics (mostly oleic acid) initiated by ozone, little is known about effects of ambient temperature and relative humidity on chemical transformation of organic aerosols. In this work, we used ATR-IR flow reactor and linoleic acid (LA) as a proxy for atmospheric unsaturated organics to investigate its heterogeneous oxidation by O3 over a wide range of temperatures (257-313 K) and relative humidities (0-80% RH). Pseudo-first order rate constants and overall reactive uptake coefficients were acquired from absorbance changes in peaks located near 1743 cm-1; 1710 cm-1; 1172 cm-1 and 1110 cm-1, which can be assigned to C=O in ester; C=O in acid; C-C and C-O stretching modes, respectively. Experimental results showed that heterogeneous loss of O3 onto LA has an uptake probability close to 5×10-4 at 293K. It was found that uptake coefficients increased with increasing temperature, and within the temperature range studied here the O3 reactive uptake was enhanced by one order of magnitude. It was also noted such temperature enhancement effect on the reaction kinetics was much more effective at lower temperatures. For example, the Pseudo-first order rate constants increased nearly by 7-fold when temperature was changed from 257 K to 273 K. Such behavior can be explained by change in physical state of LA at lower temperatures which was confirmed by shifts of both CH2 and C=O stretching modes. A decrease in absorbance ratio of (A1743/A1710) with decreasing temperature was also observed, an indicative of a smaller amount of α-acyloxyalkyl hydroperoxide-related products formed during oxidation of LA with O3 at lower temperatures. In addition, RH study results showed that water vapor promoted heterogeneous reactive uptake of O3 on LA surface. Furthermore, water uptake studies showed that O3 initiated heterogeneous oxidation enhanced hydrophilicity of organic surface.