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Molecular markers in organic aerosol (OA) provide specific source information on PM 2.5 , 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 (PM 2.5) 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 (PM 2.5≥35 µ g m -3) was 147.06 ng m -3 , which was 4.2 times higher than that in the clean period (PM 2.5<35 µ g m -3) and higher than the enhancement of PM 2.5 (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 µ g m -3) was about 5.3 times higher than that in the clean period (0.75 µ g 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 PM 2.5 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 O 3 , indicating that O 3 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 PM 2.5 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 PM 2.5. 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. [ABSTRACT FROM AUTHOR]

The outbreak of COVID-19 promoted strict restrictions to human activities in China, which led to a dramatic decrease in most air pollutant concentrations (e.g., PM 2.5 , PM 10 , NO x , SO 2 and CO). However, an obvious increase in ozone (O 3) concentrations was found during the lockdown period in most urban areas of China. In this study, we conducted field measurements targeting ozone and its key precursors by utilizing a novel proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) in Changzhou, which is representative of the Yangtze River Delta (YRD) city cluster of China. We further applied the integrated methodology including machine learning, an observation-based model (OBM) and sensitivity analysis to obtain insights into the reasons causing the obvious increase in ozone. Major findings include the following: (1) by deweathered calculation, we found changes in precursor emissions contributed 1.46 ppbv to the increase in the observed O 3 during the full-lockdown period in 2020, while meteorology constrained 3.0 ppbv of O 3 in the full-lockdown period of 2019. (2) By using an OBM, we found that although a significant reduction in O 3 precursors was observed during the full-lockdown period, the photochemical formation of O 3 was stronger than that during the pre-lockdown period. (3) The NOx/VOC ratio dropped dramatically from 1.84 during the pre-lockdown to 0.79 in the full-lockdown period, which switched O 3 formation from a VOC-limited regime to the boundary of a NO x - and VOC-limited regime. Additionally, box model results suggested that the decrease in the NOx/VOC ratio during the full-lockdown period could increase the mean O 3 by 2.4 ppbv. Results of this study give insights into the relationship between O 3 and its precursors in urban area and demonstrate reasons for the obvious increase in O 3 in most urban areas of China during the COVID-19 lockdown period. This study also underlines the necessity of controlling anthropogenic oxygenated volatile organic compounds (OVOCs), alkenes and aromatics in the sustained campaign of reducing O 3 pollution in China. [ABSTRACT FROM AUTHOR]