Measurement report: Long-term variations in surface NOx and SO2 mixing ratios from 2006 to 2016 at a background site in the Yangtze River Delta region, China
Authors: Yin Qingqing , Ma Qianli , Lin Weili , Xu Xiaobin , Yao Jie 
Published: January 2022

Abstract. China has been experiencing rapid changes in emissions of
air pollutants in recent decades. Increased emissions of primary
particulates and reactive gases caused severe haze in several polluted
regions including the Yangtze River Delta (YRD). Measures implemented in
recent years for improving air quality have reduced the emissions of
NOx, SO2, etc. The emission changes in these gases are reflected
by tropospheric columns from satellite observations and surface measurements
of surface concentrations from urban sites. However, little is known about
the long-term variations in regional background NOx and SO2. In
this study, we present NOx and SO2 measurements from the Lin'an
station (LAN; 30∘18' N, 119∘44′ E; 138.6 m a.s.l.), one of the Global Atmosphere Watch (GAW) stations in China. We
characterize the seasonal and diurnal variations and study the long-term
trends of NOx and SO2 mixing ratios observed at LAN from 2006 to
2016. We also interpret the observed variations and trends in terms of
changes in meteorological conditions as well as emission of these gases. The
overall average mixing ratios of NOx (NO2) and SO2 during
2006–2016 were 13.6 ± 1.2 ppb (12.5 ± 4.6) and 7.0 ± 4.2 ppb, respectively. The averaged seasonal variations showed maximum values of
NOx and SO2 in December (23.5 ± 4.4 ppb) and January (11.9 ± 6.2 ppb), respectively, and minimum values of 7.1 ± 0.8
and 2.8 ± 2.3 ppb (both in July), respectively. The average diurnal
variation characteristics of NOx and SO2 differed considerably
from each other, though the daily average mixing ratios of both gases were
significantly correlated (R2=0.29, P<0.001). The annual
average mixing ratio of NOx increased during 2006–2011 and then
decreased significantly at 0.78 ppb/yr (−5.16 %/yr, P<0.01). The
annual 95th and 5th percentiles of hourly NOx mixing ratios
showed upward trends until 2012 and 2014, respectively, before a clear
decline. The annual average mixing ratio of SO2 decreased significantly
at 0.99 ppb/yr (−8.27 %/yr, P<0.01) from 2006–2016. The annual
95th and 5th percentiles of hourly SO2 mixing ratios all
exhibited significant (P<0.001) downward trends at 3.18 and
0.19 ppb/yr, respectively. Changes in the total NOx and SO2
emissions as well as the industrial emissions in the YRD region were
significantly correlated with the changes in annual NOx and SO2 mixing ratios. The significant decreases in NOx from 2011 to 2016 and
SO2 from 2006 to 2016 highlight the effectiveness of relevant control
measures on the reduction in NOx and SO2 emissions in the YRD
region. A decrease in annual SO2 / NOx ratio was found, suggesting
a better efficacy in the emission reduction in SO2 than NOx. We
found gradual changes in average diurnal patterns of NOx and SO2,
which could be attributed to increasing contributions of vehicle emissions
to NOx and weakening impacts of large sources on the SO2
concentration. This study reaffirms China's success in controlling both
NOx and SO2 in the YRD but indicates at the same time a necessity
to strengthen the NOx emission control.