Characterization of atmospheric trace gases and particle matters in Hangzhou, China

Abstract. The Yangtze River Delta (YRD) is one of the most densely populated regions in China with severe air quality issues, which has not been fully understood. Thus, in this study, based on one-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, NOx, NOy, SO2 and CO) and particulate matters (PM2.5 and PM10). Results revealed severe two-tier air pollution (photochemical and haze pollution) occurred in this region, with frequent exceedances in O3 (38 days) and PM2.5 (62 days). O3 and PM2.5 both exhibited distinct seasonal variations with reversed patterns: O3 reaching a maximum in warm seasons (May and July) but PM2.5 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, accompanying with a formation of secondary fine particles under high O3. The emission maps of PM2.5, CO, NOx, and SO2 demonstrated that local emissions were both significant for these species on seasonal scale. The contributions from the regional transports among inland cities (Zhejiang, Jiangsu, Anhui, and Jiangxi Province) on seasonal scale were further confirmed to be crucial to air pollution at NRCS site by using the backward trajectories simulations. Air masses transported from Yellow Sea, East Sea, and South Sea were also found to be highly relevant to the elevated pollutants, especially for NOx and O3. Case studies of photochemical pollution (O3) and haze (PM2.5) 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). This study supplements a general picture of the air pollution state in the YRD region, specifically elucidates the role of local emission and regional transport, and 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 further emphasizes the importance of local thermally induced circulation on air quality.