Table 1.

The type of functions to characterize the relationships between predictors and pollutant concentrations, and the definition of each factor including in GAM analysis. Categorial functions represent the variables are dummy vectors with values of 1 during the time interval otherwise the values are 0. Variables in brackets indicate the list of meteorological candidates. More details can be seen in the main text.

Predictors	Functions	Factors
Interannual	Linear	Time series of days since January 1, 2015.
Monthly	Smooth	January to June.
Weekly	Category	Weekdays, Weekends, Holidays in the heating season, Tomb Sweeping Day, Holidays in the non-heating season.
Lockdown	Category	Pre-lockdown, every month from February to June in 2020.
Diurnal	Category	Three factors (Pre-lockdown, lockdown, Post-lockdown) for each hour of the day.
Meteorology	Smooth	Boundary layer height, winds, (relative humidity, dew point temperature, air temperature, surface pressure and precipitation).