Ozone weekend effects in the Beijing-Tianjin-Hebei metropolitan area, China

The ozone weekend effect (OWE) was first investigated in the metropolitan area of Beijing-Tianjin-Hebei (BTH), China, using in situ measurements from the Atmospheric Environment Monitoring Network from July 2009 to August 2011. The results indicate that there is an obvious weekly periodical variation in the surface ozone concentration. There is a lower ozone concentration from Wednesday to Friday (weekday) and a higher concentration from Saturday to Monday (weekend) at all the locations of the study. NOx also displays a weekly cycle, with the maximum level occurring on weekdays and the minimum level on weekends, especially later on Sunday night and early Monday morning. This pattern may be responsible for the higher concentration of ozone on weekends. Additionally, the vertical variations in O-3 and NOx from the 8 m, 47 m, 120 m and 280 m observation platforms on the 325 m Beijing meteorological tower displayed obvious weekly cycles that corresponded to the surface results. A smaller decrease in volatile organic compounds (VOCs; using CO as a proxy) and much lower NOx concentrations on the weekend may lead to higher VOC/NOx ratio, which can enhance the ozone production efficiency in VOC-limited regime areas. Additionally, a clear weekly cycle in the fine aerosol concentration was observed, with maximum values occurring on weekdays and minimum values occurring on weekends. Higher concentrations of aerosol on weekdays can reduce the UV radiation flux by scattering or absorbing, which leads to a decrease in the ozone production efficiency. A significant weekly cycle in UV radiation, consistent with the aerosol concentration, was discovered at the Beijing meteorological tower site (BJT), validating the assumption. A comprehensive understanding of the ozone weekend effect in the BTH area can provide deep insights into controlling photochemical pollution.