Characteristics and degradation of organic aerosols from cooking sources based on hourly observations of organic molecular markers in urban environments
Authors: Li Rui , Zhang Kun , Li Qing , Yang Liumei , Wang Shunyao , Liu Zhiqiang , Zhang Xiaojuan , Chen Hui , Yi Yanan , Feng Jialiang , Wang Qiongqiong , Huang Ling , Wang Wu , Wang Yangjun , Yu Jian Zhen , Li Li 
Published: March 2023

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.