《Table 2.Emission scenario setting for an FC-affected event and an SP-affected event.》

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《"Source Contributions to PM_(2.5) under Unfavorable Weather Conditions in Guangzhou City, China"》

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Studies have shown that air pollution in the PRD region is an integrated result of local（sources in the city areas），regional（other city sources within the region）and superregional（all sources outside the study region）contributions（Kemball-Cook et al.，2009；Li et al.，2012）.Understanding the specific contribution is fundamental for designing effective emission control strategies.One challenge is that the source contribution varies if weather conditions are different.In order to take the impact of weather into account，historical haze episodes from 2013 to 2016 were identified and classified according to weather systems.In this study，a haze episode was defined as a period with at least three consecutive haze days（a haze day refers to daily visibility less than 10 km and daily relative humidity （RH）less than 90%；rainy days are not considered） .A similar method can be found in Wu et al.（2007）.These data were collected from 86 operational automatic weather monitoring stations throughout Guangdong，and the data quality was well controlled by the China Meteorological Administration（CMA）.Weather systems were classified by analyzing the weather charts of the selected haze episode.A similar method was used in previous studies（Chan and Chan，2000；Huang et al.，2006；Saskia Buchholza et al.，2010；Li et al.，2011，2016）.According to this method，four types of weather systems—namely，“foreside of a cold front”（FC），“sea high pressure”（SP），“equalizing pressure”（EP）and“others”（Os，such as mainland high pressure and tropical cyclones）—were diagnosed as unfavorable weather systems in Guangdong Province（see Table 1）.A general introduction to these weather systems is presented in the supplementary material（Fig.S1，see Electronic Supplementary Material），and more detailed information about the statistical data is given by Gao et al.a.On the one hand，the fouryear statistical result showed that the haze episodes exhibited a declining trend.The total number of haze episodes from2013 to 2016 was 61，35，30 and 19，respectively，which indicated that the emission control efforts were effective.The variation of annual concentrations of PM2.5also verified this positive aspect，with PM2.5concentrations of 53，49，32 and30μg m-3，respectively.On the other hand，SP was the most frequent unfavorable weather system，accounting for 49.0%from 2013 to 2016.Among all the SP episodes，the lowest visibility（daily averaged）ranged from 0.7 km to 3.8 km，with the highest concentrations of PM2.5（daily averaged）ranging from 80μg m-3to 184μg m-3，respectively.FC was the second most dominant unfavorable weather system，the occurrence of which comprised 26.9%of the total.Low visibility（0.6–5.4 km）and high PM2.5（79–222μg m-3）concentrations were also recorded.EP and Os were the last two，with occurrences of 16.6%and 7.6%，respectively.Since the total occurrence of haze episodes under SP and FC was over 75%，significantly higher than that under EP and Os（p<0.05），targeted investigations were conducted by examining an SP-affected event（19–25 November 2014）and an FC-affected event（19–25 November 2010），with the aim of explaining the characteristics of source contributions under these two typical unfavorable weather systems.