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Abstract This study investigated isotopic signatures of carbon (C), lead (Pb) and strontium (Sr) in PM 2.5 samples collected from two paired sites (urban vs. rural/background) in northern Taiwan during the summer of 2015 and the spring of 2016, respectively. Significant seasonality was revealed in the isotopic signatures of C and Pb, whereas no seasonal difference was observed in 87Sr/86Sr ratio. The values of δ13C, 206Pb/207Pb, and 208Pb/207Pb were more diverse and exhibited local features in summer. However, during the episodes of continental pollution outbreaks in springtime, the δ13C and Pb isotope ratios of PM 2.5 shifted and converged consistently toward the documented characteristics of particulate matters in northern China. Moreover, the results showed that the differences in the Pb and Sr isotopic characteristics between the paired urban and rural sites were statistically insignificant, whereas marginally lower δ13C values were observed at the urban site. It was inferred accordingly that a substantial amount of gaseous hydrocarbons emitted in local urban areas could have been converted to secondary organic aerosols with lower δ13C. On the contrary, the consistency in the spatial and temporal variations of mass concentration and isotope ratios of Pb at the two sites suggested that East-Asian continental pollution outbreak was the major source of Pb-containing particles in the northern Taiwan during springtime. It is noteworthy that there was neither seasonal nor spatial differences in the 87Sr/86Sr ratio observed in this study, which suggested that the Sr-containing particles collected in this study could be originating from a common mixture of sources, including not only natural dust but also the anthropogenic emissions, coal-combustion for instance. This investigation upon the C-Pb-Sr isotopic features evidenced the substantial impacts of the continental pollution outbreaks on the aerosol composition and air quality in the downwind areas of the East-Asian winter monsoons. Graphical abstract Unlabelled Image Highlights • Seasonal and temporal variations of C-Pb-Sr isotopes of PM 2.5 were investigated. • C and Pb in PM 2.5 exhibited significant seasonal isotope signatures. • Isotopic evidences for the impacts of Asian outflows on air quality were revealed. • Consistent Sr isotope ratios were observed from local and distant sources. [ABSTRACT FROM AUTHOR]

Taichung City has become the second largest city in Taiwan, where PM 2.5 and O 3 pollution events occur frequently. The causes of these events urgently need a comprehensive and in-depth study. Therefore, a series of well-planned observation experiments were conducted in the city and its surrounding areas. The first experiment supported the current simulation study of the major causes of air quality deterioration in Central-Western Taiwan. First, we utilized the Weather Research and Forecasting (WRF)/Community Multiscale Air Quality (CMAQ) modeling system to simulate the intensive observation period (IOP) from 31 October 2021 to 22 November 2021. The performance of WRF-CMAQ is well justified by comparison with the observations. Next, we selected the PM 2.5 (particles with aerodynamic diameters equal to or <2.5 μm) pollution event occurring on 3–6 November 2021 for detailed analysis. The results reveal that the meteorological conditions of this event were the prevailing easterly/southeasterly winds blocked by the Central Mountain Range (CMR). Central Taiwan is located on the leeward side of the CMR and embedded in a calm wind area, which is especially favorable for the accumulation of PM 2.5 during the nighttime. At noon, the PM 2.5 concentration increased from the surface to the planetary boundary layer height (PBLH), even though the PBLH was overestimated. In the afternoon, the clean sea breeze would dilute the PM 2.5 in the Taichung Basin and the urban PM 2.5 plume would be transported to the mountains. The current simulation suggests that the aloft return PM 2.5 layer would subside along with the sinking PBLH and deteriorate the PM 2.5 concentration at night. At the downtown site of the IOP (UAPRS), from the trend of radicals and nitrogen oxides, ozone (O 3) is closely involved in the formation of nitrate (NO 3 −) during both daytime and nighttime. If Taichung City is Volatile Organic Compounds (VOC)-limited according to our previous study, then reducing VOC emissions could decrease O 3 and thus the NO 3 − concentration in Taichung City. [Display omitted] • The modeling of an intensive observation campaign is comprehensively evaluated. • The modeling performed well and reproduced meteorological and chemical fields. • Aloft return PM 2.5 layer subsidence could enhance pollution at night. • NO 3 − is a major species during nighttime PM 2.5 pollution events. • Reducing VOCemissions could decrease NO 3 − in the Taichung Basin. [ABSTRACT FROM AUTHOR]