Abstract. The Yangtze River Delta (YRD) is one of the most densely populated regions in China with severe air quality issues that have not been fully understood. Thus, in this study, based on 1-year (2013) continuous measurement at a National Reference Climatological Station (NRCS, 30.22° N, 120.17° E; 41.7 m a.s.l.) in the center of Hangzhou in the YRD, we investigated the seasonal characteristics, interspecies relationships, and the local emissions and the regional potential source contributions of trace gases (including O₃, NO_x, NO_y, SO₂, and CO) and particulate matter (PM_2.5 and PM₁₀). Results revealed that severe two-tier air pollution (photochemical and haze pollution) occurred in this region, with frequent exceedances in O₃ (38 days) and PM_2.5 (62 days). O₃ and PM_2.5 both exhibited distinct seasonal variations with reversed patterns: O₃ reaching a maximum in warm seasons (May and July) but PM_2.5 reaching a maximum in cold seasons (November to January). The overall results from interspecies correlation indicated a strong local photochemistry favoring the O₃ production under a volatile organic compound (VOC)-limited regime, whereas it moved towards an optimum O₃ production zone during warm seasons, accompanied by the formation of secondary fine particulates under high O₃. The emission maps of PM_2.5, CO, NO_x, and SO₂ demonstrated that local emissions were significant for these species on a seasonal scale. The contributions from the regional transport among inland cities (Zhejiang, Jiangsu, Anhui, and Jiangxi Province) on a seasonal scale were further confirmed to be crucial to air pollution at the NRCS site by using backward trajectory simulations. Air masses transported from the offshore areas of the Yellow Sea, East Sea, and South Sea were also found to be highly relevant to the elevated O₃ at the NRCS site through the analysis of potential source contribution function (PSCF). Case studies of photochemical pollution (O₃) and haze (PM_2.5) episodes both suggested the combined importance of local atmospheric photochemistry and synoptic conditions during the accumulation (related with anticyclones) and dilution process (related with cyclones). Apart from supplementing a general picture of the air pollution state in the city of Hangzhou in the YRD region, this study specifically elucidates the role of local emission and regional transport, and it interprets the physical and photochemical processes during haze and photochemical pollution episodes. Moreover, this work suggests that cross-regional control measures are crucial to improve air quality in the YRD region, and it further emphasizes the importance of local thermally induced circulation for air quality.