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. The characteristics of ozone variations and the impacts of synoptic andlocal meteorological factors in northern China were quantitatively analyzedduring the warm season from 2013 to 2017 based on multi-city in situ ozoneand meteorological data as well as meteorological reanalysis. Thedomain-averaged maximum daily 8 h running average O3 (MDA8 O3 )concentration was 122±11 µ g m −3 , with an increase rate of7.88  µ g m −3  yr −1 , and the three most polluted months wereclosely related to the variations in the synoptic circulation patterns,which occurred in June (149  µ g m −3 ), May (138  µ g m −3 )and July (132  µ g m −3 ). A total of 26 weather types (merged into fiveweather categories) were objectively identified using the Lamb–Jenkinsonmethod. The highly polluted weather categories included the S–W–N directions(geostrophic wind direction diverts from south to north), low-pressure-related weather types (LP) and cyclone type, which the study area controlledby a low-pressure center (C), and the corresponding domain-averaged MDA8 O3 concentrations were 122, 126 and 128  µ g m −3 , respectively.Based on the frequency and intensity changes of the synoptic circulationpatterns, 39.2 % of the interannual increase in the domain-averaged O3 from 2013 to 2017 was attributed to synoptic changes, and theintensity of the synoptic circulation patterns was the dominant factor.Using synoptic classification and local meteorological factors, thesegmented synoptic-regression approach was established to evaluate andforecast daily ozone variability on an urban scale. The results showedthat this method is practical in most cities, and the dominant factors arethe maximum temperature, southerly winds, relative humidity on the previousday and on the same day, and total cloud cover. Overall, 41 %–63 % of theday-to-day variability in the MDA8 O3 concentrations was due to localmeteorological variations in most cities over northern China, except for twocities: QHD (Qinhuangdao) at 34 % and ZZ (Zhengzhou) at 20 %. Ourquantitative exploration of the influence of both synoptic and localmeteorological factors on interannual and day-to-day ozone variability willprovide a scientific basis for evaluating emission reduction measures thathave been implemented by the national and local governments to mitigate airpollution in northern China.

Quantifying the impact of synoptic circulation patterns on ozone variability in northern China from April to October 2013–2017