Characterization and sources of volatile organic compounds in a provincial capital city of northern China in 2019–2024: Impact of public events

The COVID-19 outbreak and the Beijing Winter Olympics provided an opportunity to study the impact of human activities on volatile organic compounds (VOCs) in the atmosphere. Continuous measurements of 117 VOC species were conducted in urban Ji'nan, the capital city of Shandong Province, North China Plain, from 2019 to 2024. The total volatile organic compound concentrations ranged from 30.7 ppbv to 41.7 ppbv, and the seasonal characteristics of VOCs were generally high in autumn and winter and low in spring and summer, with alkanes as the component with the highest percentage. The average VOC volume fraction increased by 24.8% after the COVID-19 outbreak compared to that before the outbreak, among which the concentrations of aromatics increased most markedly (334.7%), and those of alkanes increased by 95.2%. Alkenes, alkynes, halocarbons, and oxygenated VOCs showed decreasing trends. The volume fraction of each VOC species showed a decreasing trend during the Beijing Winter Olympics air quality guarantee period compared with the pre-Beijing Winter Olympics period, with a 16.7%–36.3% reduction rate. The positive matrix factorization model identified six sources: vehicle emission, industrial mixing sources, solvent use, oil and gas volatilization, biogenic and secondary sources, and combustion sources. Influenced by the resumption of work and production by enterprises after the COVID-19 pandemic, solvent use increased by 26.7% after the pandemic, and the contribution of diesel vehicle emissions was significant. After the pandemic, the contribution of industrial mixing sources decreased by 28.6%, whereas coal combustion sources increased by 5.3% compared to the pre-COVID-19 period. During the Beijing Winter Olympics air quality guarantee period, coal combustion sources and vehicle emission sources decreased by 11.6% and 6.5%, respectively, and contributions from industrial sources and biogenic and secondary sources increased by 13.2% and 6.4%, respectively, compared with those during the pre-Beijing Winter Olympics period. During the Beijing Winter Olympics air quality guarantee period, the results of the backward airflow trajectory, and potential source area analysis, showed a strong influence of air mass transmission in the southwest direction, and VOCs emissions from industrial sources had higher values in the southwest region. Before the COVID-19 outbreak, there were obvious ship sources from the long-range transmission of the near-coastal region in the Yellow Sea. However, in post-COVID-19, long-range transport contributions from ship sources vanished due to the upgrading of marine oils after implementing the DECA 2.0.