*5.4. Weekend Effect*

The weekend effect refers to the phenomenon of different atmospheric pollutants concentration between weekends and weekdays, mainly caused by human activities. On weekdays, the main anthropogenic precursor emissions are due to commuting driving, especially during the day. However, on the weekends, the main anthropogenic precursors might come from family-related recreational activities. The difference in vehicle emissions between weekdays and weekends is mainly related to fuel combustion products, fuel consumption, and the traffic patterns of gasoline and diesel vehicles. Heavy-duty diesel vehicles are the main source of NOx and black carbon (BC), while light-duty gasoline vehicles are the main source of CO2. In general, the diesel-fueled vehicles showed a significant reduction during weekends, resulting in a significant reduction in NOx and BC emissions. If the atmospheric system was at nitrogen oxide saturation in urban areas, the reduction in NOx emission on weekends may lead to a reduction in O3 titrations, which can reduce the inhibition of O3 formation, resulting in an increased O3 concentration on weekends. The "ozone quenching hypothesis" and "NOx reduction hypothesis" in the weekend effect prove that NOx plays a complex role in ozone production and termination [16]. The concentration of atmospheric O3 increased from weekdays to weekends for a number of sites in the Northern Front Range metropolitan area (NFRMA) of Colorado, with weekend reductions in NO2 at two sites in downtown Denver between 2000 and 2015, indicating that the region was in a NOx-saturated ozone production regime [91]. Similar results were found in Shenyang city from 2013 to 2015 [29]. Koo et al. [92] investigated the weekend effect in the Midwest (north-central and northeast) of the United States in the summer of 2005, and the results showed that the reduction in O3 on weekends depends on the increase in NOx emissions leading to an increase in O3 titration. The investigation of the

weekend effect on O3 in Beijing in 2014 showed that the O3 concentration at weekends was overall higher than that on weekdays, especially in urban centers [93]. The atmospheric O3 generation might be limited by VOCs during summer, autumn and winter. However, the weekend O3 concentration was lower than on the working days in spring, suggesting that O3 production might be limited by NOx in this season. On the urban scale, the weekend effect in the central area of the city was larger than that in the suburb, but on the local scale, the weekend effect showed a downward trend, with an increase in urbanization. Weekend O3 changes depend on the intensity of the sunshine and the ratio of VOCs to NOx emissions [94].
