Abstract
Molecular markers in organic aerosol (OA) provide specific source information of PM2.5, and the contribution of cooking organic aerosols to OA is significant, especially in urban environments. However, the low time resolution of offline measurements limits the effectiveness in interpreting the tracer data, the diurnal variation of cooking emission and the oxidation process. In this study, we used on-line thermal desorption aerosol gas chromatography mass spectrometry (TAG) to measure organic molecular markers in fine particulate matter (PM2.5) at an urban site in Changzhou, China. The concentrations of saturated fatty acids (SFA), unsaturated fatty acids (uFAs), and oxidative decomposition products of unsaturated fatty acids (ODPs) were measured every two hours to investigate the temporal variations and the oxidative decomposition characteristics of uFAs in urban environment. The average concentration of total fatty acids (TFAs, sum of sFAs and uFAs) was measured to be 105.70±230.28 ng/m3. The average concentration of TFAs in polluted period (PM2.5>35 μg/m3) was 147.06 ng/m3, which was 4.2 times higher than that in clean period (PM2.5<35 μg/m3), higher than the enhancement of PM2.5 (2.2 times) and OC (2.0 times) concentrations comparing polluted period to clean period. The mean concentration of cooking aerosol in the polluted period (3.63 μg/m3) was about 3.9 times higher than that in the clean period (0.90 μg/m3), which was similar to the trend of fatty acid. During the rising period of PM2.5, TFAs concentration tends to reach the peak earlier than PM2.5. Fatty acids showed a clear diurnal variation. Linoleic acid /Palmitic acid and Oleic acid /Palmitic acid ratios were significantly higher at dinning time, and closer to the source profile. By performing backward trajectory clustering analysis, under the influence of short-distance air masses from surrounding areas, the concentrations of TFAs and PM2.5 were relatively high; while under the influence of air masses from easterly coastal areas, the oxidation degree of unsaturated fatty acids emitted from local culinary sources were higher. The effective rate constants (kO) for the oxidative degradation of oleic acid were estimated to be 0.12–0.41 h-1, which were lower than kL (the estimated effective rate constants of linoleic acid, 0.25–0.50 h-1). Both kO and kL showed a significant positive correlation with O3, indicating that O3 was the main night-time oxidants for uFAs in the Changzhou City. Using fatty acids as tracers, cooking was estimated to contribute an average of 4.8% to PM2.5 concentrations, rising to 6.1% at dinner time; while the contribution to total OC is more than double the contribution to PM2.5. This study investigates the variation of the concentrations and oxidative degradation of fatty acids and corresponding oxidation products in ambient air, which can be a guide for the refinement of aerosol source apportionment, and provide scientific support for the development of cooking source control policies.
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