Seasonal variations in surface ozone as influenced by Asian summer monsoon and biomass burning in agricultural fields of the northern Yangtze River Delta

► Long-term surface O3 was observed for the first time in the northern Yangtze River Delta. ► Pollutants from industrialized area with the prevailing wind increase O3 in spring. ► Monsoonal incursions of the maritime air masses depress O3 in summer. ► Crop residue burning makes a significant contribution to the peak O3 in June.