Ozone weekend effect in cities: Deep insights for urban air pollution control.

Studying weekend-weekday variation in ground-level ozone (O) allows one to better understand O formation conditions, with a potential for developing effective strategies for O control. Reducing inappropriately the O precursors emissions can either produce no reduction or increase surface O concentrations. This paper analyzes the weekend-weekday differences of O at 300 rural and 808 urban background stations worldwide from 2005 to 2014, in order to investigate the O weekend effect over time and assess the effectiveness of the precursors emissions control policies for reducing O levels. Data were analyzed with the non-parametric Mann-Kendall test and Theil-Sen estimator. Rural sites typically did not experience a weekend-weekday effect. In all urban stations, the mean O concentration on the weekend was 12% higher than on weekdays. Between 2005 and 2014, the annual mean of daily O concentrations increased at 74% of urban sites worldwide (+ 0.41 ppb year) and decreased in the United Kingdom (- 0.18 ppb year). Over this time period, emissions of O precursors declined significantly. However, a greater decline in nitrogen oxides (NO) emissions caused an increase in Volatile Organic Compounds (VOCs) to NO ratios leading to O formation. In France, South Korea and the United Kingdom, most urban stations showed a significant upward trend (+ 1.15% per year) for O weekend effect. Conversely, in Canada, Germany, Japan, Italy and the United States, the O weekend effect showed a significant downward trend (- 0.26% per year). Further or inappropriate control of anthropogenic emissions in Canada, Southern Europe, Japan, South Korea and the United States might result in increased daily O levels in urban areas.