Characteristics and degradation of organic aerosols from cooking sources based on hourly observations of organic molecular markers in urban environments

Abstract

Molecular markers in organic aerosol (OA) provide specific source information on 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 when interpreting the tracer data, the diurnal variation in cooking emissions and the oxidation process. In this study, we used online thermal desorption aerosol gas chromatography and 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 (sFAs), unsaturated fatty acids (uFAs) and oxidative decomposition products (ODPs) of unsaturated fatty acids were measured every 2 h to investigate the temporal variations and the oxidative decomposition characteristics of uFAs in urban environments. The average concentration of total fatty acids (TFAs, sum of sFAs and uFAs) was measured to be 105.70±230.28 ng m−3. The average concentration of TFAs in the polluted period (PM2.5≥35 µg m−3) was 147.06 ng m−3, which was 4.2 times higher than that in the clean period (PM2.5<35 µg m−3) and higher than the enhancement of PM2.5 (2.2 times) and organic carbon (OC) (2.0 times) concentrations when comparing the polluted period to the clean period. The mean concentration of cooking aerosol in the polluted period (4.0 µg m−3) was about 5.3 times higher than that in the clean period (0.75 µg m−3), which was similar to the trend of fatty acids. Fatty acids showed a clear diurnal variation. Linoleic acid / stearic acid and oleic acid / stearic acid ratios were significantly higher at dinnertime and closer to the cooking 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 uFAs emitted from local culinary sources was higher. The effective rate constants (kO) for the oxidative degradation of oleic acid were estimated to be 0.08–0.57 h−1, which were lower than kL (the estimated effective rate constants of linoleic acid, 0.16–0.80 h−1). Both kO and kL showed a significant positive correlation with O3, indicating that O3 was the main nighttime oxidant for uFAs in the city of Changzhou. Using fatty acids as tracers, cooking was estimated to contribute an average of 4.6 % to PM2.5 concentrations, increasing to 7.8 % at 20:00 UTC+8 h. Cooking was an important source of OC, contributing 8.1 %, higher than the contribution of PM2.5. This study investigates the variation in 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.