Impact of urbanization on the air pollution “holiday effect” in Taiwan

► Holiday effects are found for almost all six pollutants over six divisions in Taiwan. ► The strength of the holiday effect is mainly caused by the degree of urbanization. ► The degree of division urbanization is related to population and vehicle number. ► The degree of division urbanization is related to anthropogenic sources. ► Normalized holiday effects by population are mostly weakening in the urban divisions.