Abstract: Abstract : The composite characteristics of PM_2.5-O₃ in Shandong Province were relatively prominent and also showed obvious spatial differences. To study characteristics of co-pollution of surface O₃ and PM_2.5 in Shandong province, spatial distributions of the PM_2.5 and O₃ concentrations were analyzed and the relationship between O₃ and PM_2.5, meteorological parameters and their precursors were explored based on the monitoring data of national and provincial ambient air quality monitoring sites and meteorological sites from 2016 to 2020. The results indicated that: (1) During 2016-2020, the air quality in Shandong Province improved gradually with excellent and good days increasing by 7.1% and heavily polluted days decreasing by 3.5%. Except for the 90th percentile of the maximum daily 8-hour average of the O₃ concentrations (O_{3 MDA8} ^90th), which increased by 9.6%, the concentrations of SO₂, PM₁₀, PM_2.5, CO and NO₂ decreased by 61.3%, 29.8%, 28.6%, 26.3% and 11.4%, respectively. The annual average values of PM_2.5 in all cities of Shandong Province declined with decreasing ratios ranging from 18.4% to 34.9%. The O_{3 MDA8} ^90th increased in all the cities except for Dezhou City. The largest increasing ratio of O_{3 MDA8} ^90th was 30.8% in Binzhou City. The highest average PM_2.5 concentrations was observed in January while the highest average O₃ concentration was observed in June. The PM_2.5 concentrations in January decreased and then increased from 2016 to 2020, while the O₃ concentrations in June increased from 2016 to 2020. (2) The PM_2.5 and O₃ concentrations in inland cities were higher than those in coastal cities with higher PM_2.5 concentration observed in western inland areas and higher O₃ concentrations in central and northern inland areas. Synergistic pollution characteristics of PM_2.5 and O₃ were obvious in the central and western Shandong Province. There were 224 days characterized by the synergistic pollution of PM_2.5 and O₃ during the whole study period, which mainly occurred from February to November. The days with the synergistic pollution of PM_2.5 and O₃ decreased year by year. (3) The correlationship between concentrations of PM_2.5 or O₃ and meteorological factors, as well as threshold of meteorological factors during PM_2.5, O₃ or PM_2.5-O₃ composite pollutant days, were also carried out. The results showed that the daily average PM_2.5 and O₃_8 h concentrations had  negative correlations with their precursors and meteorological parameters. Moreover, spatial differences existed in the correlation between PM_2.5 and O₃, their precursors and meteorological parameters. The statistical results show that PM_2.5-O₃ complex pollution favorably occurred when the atmospheric temperature, relative humidity, wind speed and atmospheric pressure ranged from 14.9-24.1 °C, from 55.5%-75.1%, 0.6-2.9 m/s and 992.8-1018.8 hPa, respectively. In most cities of Shandong Province, the average values of atmospheric temperature, relative humidity and atmospheric pressure during the synergistic pollution of PM_2.5 and O₃ were between those during the PM_2.5 pollution and those during the O₃ pollution. But the average wind speed during the synergistic pollution of PM_2.5 and O₃ was lower than those during the PM_2.5 pollution and during the O₃ pollution. In summary, the study showed that on a whole the PM_2.5 concentration declined but the O₃ concentration enhanced, and synergistic changes of PM_2.5 and O₃ became more obvious during the ‘13th Five-Year Plan’ period. Meteorological factors played a role in the formation and accumulation of PM_2.5 and O₃.
Keywords : PM_2.5 and O₃; pollution characteristics; temporal and spatial distribution; meteorological factors; Shandong Province