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. Ozone (O3) pollution is of great concern in theYangtze River Delta (YRD) region of China, and the regional O3pollution is closely associated with dominant weather systems. With a focuson the warm seasons (April–September) from 2014 to 2018, we quantitativelyanalyze the characteristics of O3 variations over the YRD, the impactsof large-scale and synoptic-scale circulations on the O3 variations andthe associated meteorological controlling factors, based on observedground-level O3 and meteorological data. Our analysis suggests anincreasing trend of the regional mean O3 concentration in the YRD at1.8 ppb per year over 2014–2018. Spatially, the empirical orthogonalfunction analysis suggests the dominant mode accounting for 65.7 %variation in O3, implying that an increase in O3 is the dominanttendency in the entire YRD region. Meteorology is estimated to increase theregional mean O3 concentration by 3.1 ppb at most from 2014 to 2018.In particular, relative humidity (RH) plays the most important role inmodulating the inter-annual O3 variation, followed by solar radiation(SR) and low cloud cover (LCC). As atmospheric circulations can affect localmeteorological factors and O3 levels, we identify five dominantsynoptic weather patterns (SWPs) in the warm seasons in the YRD using thet-mode principal component analysis classification. The typical weathersystems of SWPs include the western Pacific Subtropical High (WPSH) underSWP1, a continental high and the Aleutian low under SWP2, an extratropicalcyclone under SWP3, a southern low pressure and WPSH under SWP4 and thenorth China anticyclone under SWP5. The variations of the five SWPs are allfavorable to the increase in O3 concentrations over 2014–2018.However, crucial meteorological factors leading to increases in O3concentrations are different under different SWPs. These factors areidentified as significant decreases in RH and increases in SR under SWP1,4 and 5, significant decreases in RH, increases in SR and air temperature(T2) under SWP2 and significant decreases in RH under SWP3. Under SWP1, 4and 5, significant decreases in RH and increases in SR are predominantlycaused by the WPSH weakening under SWP1, the southern low pressure weakeningunder SWP4 and the north China anticyclone weakening under SWP5. UnderSWP2, significant decreases in RH, increases in SR and T2 are mainlyproduced by the Aleutian low extending southward and a continental highweakening. Under SWP3, significant decreases in RH are mainly induced by anextratropical cyclone strengthening. These changes in atmosphericcirculations prevent the water vapor in the southern and northern sea frombeing transported to the YRD and result in RH significantly decreasing undereach SWP. In addition, strengthened descending motions (behind thestrengthening trough and in front of the strengthening ridge) lead todecreases in LCC and significant increases in SR under SWP1, 2, 4 and 5. Thesignificant increases in T2 would be due to weakening cold flow introducedby a weakening continental high. Most importantly, the changes in the SWPintensity can make large variations in meteorological factors and contributemore to the O3 inter-annual variation than the changes in the SWPfrequency. Finally, we reconstruct an empirical orthogonal function (EOF) mode 1 time series that is highlycorrelated with the original O3 time series, and the reconstructed timeseries performs well in defining the change in SWP intensity according tothe unique feature under each of the SWPs.

Ozone variability induced by synoptic weather patterns in warm seasons of 2014–2018 over the Yangtze River Delta region, China