Abstract
Abstract. Ozone (O3) pollution is of great concern in the Yangtze River Delta (YRD) region of China, and the regional O3 pollution is closely associated with dominant weather systems. With a focus on the warm seasons (April–September) from 2014 to 2018, we quantitatively analyze the characteristics of O3 variations over the YRD, the impacts of large-scale and synoptic-scale circulations on the O3 variations and the associated meteorological controlling factors, based on observed ground-level O3 and meteorological data. Our analysis suggests an increasing trend of the regional mean O3 concentration in the YRD at 1.8 ppb per year over 2014–2018. Spatially, the empirical orthogonal function analysis suggests the dominant mode accounting for 65.7 % variation in O3, implying that an increase in O3 is the dominant tendency in the entire YRD region. Meteorology is estimated to increase the regional mean O3 concentration by 3.1 ppb at most from 2014 to 2018. In particular, relative humidity (RH) plays the most important role in modulating the inter-annual O3 variation, followed by solar radiation (SR) and low cloud cover (LCC). As atmospheric circulations can affect local meteorological factors and O3 levels, we identify five dominant synoptic weather patterns (SWPs) in the warm seasons in the YRD using the t-mode principal component analysis classification. The typical weather systems of SWPs include the western Pacific Subtropical High (WPSH) under SWP1, a continental high and the Aleutian low under SWP2, an extratropical cyclone under SWP3, a southern low pressure and WPSH under SWP4 and the north China anticyclone under SWP5. The variations of the five SWPs are all favorable to the increase in O3 concentrations over 2014–2018. However, crucial meteorological factors leading to increases in O3 concentrations are different under different SWPs. These factors are identified 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, 4 and 5, significant decreases in RH and increases in SR are predominantly caused by the WPSH weakening under SWP1, the southern low pressure weakening under SWP4 and the north China anticyclone weakening under SWP5. Under SWP2, significant decreases in RH, increases in SR and T2 are mainly produced by the Aleutian low extending southward and a continental high weakening. Under SWP3, significant decreases in RH are mainly induced by an extratropical cyclone strengthening. These changes in atmospheric circulations prevent the water vapor in the southern and northern sea from being transported to the YRD and result in RH significantly decreasing under each SWP. In addition, strengthened descending motions (behind the strengthening trough and in front of the strengthening ridge) lead to decreases in LCC and significant increases in SR under SWP1, 2, 4 and 5. The significant increases in T2 would be due to weakening cold flow introduced by a weakening continental high. Most importantly, the changes in the SWP intensity can make large variations in meteorological factors and contribute more to the O3 inter-annual variation than the changes in the SWP frequency. Finally, we reconstruct an empirical orthogonal function (EOF) mode 1 time series that is highly correlated with the original O3 time series, and the reconstructed time series performs well in defining the change in SWP intensity according to the unique feature under each of the SWPs.
Highlights
As an air pollutant, surface ozone (O3) is harmful to human health and vegetation growth, such as by damaging human lungs (Jerrett et al, 2009; Day et al, 2017) and destroying forest and agricultural crops (Yue et al, 2017)
Under SWP1, 4 and 5, significant decreases in relative humidity (RH) and increases in solar radiation (SR) are predominantly caused by the western Pacific Subtropical High (WPSH) weakening under SWP1, the southern low pressure weakening under SWP4 and the north China anticyclone weakening under SWP5
We analyzed the O3 spatiotemporal distribution characteristics, quantified the contribution of meteorological conditions to the O3 variations, explored how changes in synoptic weather patterns (SWPs) and corresponding meteorological factors lead to O3 increase in the Yangtze River Delta (YRD) over 2014–2018 and assessed the contributions of SWP frequency and intensity to the interannual O3 variation in the region
Summary
Surface ozone (O3) is harmful to human health and vegetation growth, such as by damaging human lungs (Jerrett et al, 2009; Day et al, 2017) and destroying forest and agricultural crops (Yue et al, 2017). Gao et al (2020) discussed influences of six SWPs on O3 levels in the YRD and revealed differences in O3 pollution levels due to minor changes in atmospheric circulations. Due to the recent increases in O3 level over the YRD (Gao et al, 2017; Xie et al, 2017), studies on characterizing the O3 variation in the region and understanding the mechanisms for the variation are urgently required. To this end, the temporal and spatial variations in surface O3 including 5-year trend over the YRD are quantitatively investigated, and the mechanisms of meteorological influences on the O3 variations are analyzed.
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