Ozone variability induced by synoptic weather patterns in warm seasons of 2014–2018 over the Yangtze River Delta region, China
Authors: Gao Da , Xie Min , Liu Jane , Wang Tijian , Ma Chaoqun , Bai Haokun , Chen Xing , Li Mengmeng , Zhuang Bingliang , Li Shu 
Published: April 2021

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.