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. China has been experiencing rapid changes in emissions ofair pollutants in recent decades. Increased emissions of primaryparticulates and reactive gases caused severe haze in several pollutedregions including the Yangtze River Delta (YRD). Measures implemented inrecent years for improving air quality have reduced the emissions ofNOx, SO2, etc. The emission changes in these gases are reflectedby tropospheric columns from satellite observations and surface measurementsof surface concentrations from urban sites. However, little is known aboutthe long-term variations in regional background NOx and SO2. Inthis study, we present NOx and SO2 measurements from the Lin'anstation (LAN; 30∘18' N, 119∘44′ E; 138.6 m a.s.l.), one of the Global Atmosphere Watch (GAW) stations in China. Wecharacterize the seasonal and diurnal variations and study the long-termtrends of NOx and SO2 mixing ratios observed at LAN from 2006 to2016. We also interpret the observed variations and trends in terms ofchanges in meteorological conditions as well as emission of these gases. Theoverall average mixing ratios of NOx (NO2) and SO2 during2006–2016 were 13.6 ± 1.2 ppb (12.5 ± 4.6) and 7.0 ± 4.2 ppb, respectively. The averaged seasonal variations showed maximum values ofNOx and SO2 in December (23.5 ± 4.4 ppb) and January (11.9 ± 6.2 ppb), respectively, and minimum values of 7.1 ± 0.8and 2.8 ± 2.3 ppb (both in July), respectively. The average diurnalvariation characteristics of NOx and SO2 differed considerablyfrom each other, though the daily average mixing ratios of both gases weresignificantly correlated (R2=0.29, P&lt;0.001). The annualaverage mixing ratio of NOx increased during 2006–2011 and thendecreased significantly at 0.78 ppb/yr (−5.16 %/yr, P&lt;0.01). Theannual 95th and 5th percentiles of hourly NOx mixing ratiosshowed upward trends until 2012 and 2014, respectively, before a cleardecline. The annual average mixing ratio of SO2 decreased significantlyat 0.99 ppb/yr (−8.27 %/yr, P&lt;0.01) from 2006–2016. The annual95th and 5th percentiles of hourly SO2 mixing ratios allexhibited significant (P&lt;0.001) downward trends at 3.18 and0.19 ppb/yr, respectively. Changes in the total NOx and SO2emissions as well as the industrial emissions in the YRD region weresignificantly correlated with the changes in annual NOx and SO2 mixing ratios. The significant decreases in NOx from 2011 to 2016 andSO2 from 2006 to 2016 highlight the effectiveness of relevant controlmeasures on the reduction in NOx and SO2 emissions in the YRDregion. A decrease in annual SO2 / NOx ratio was found, suggestinga better efficacy in the emission reduction in SO2 than NOx. Wefound gradual changes in average diurnal patterns of NOx and SO2,which could be attributed to increasing contributions of vehicle emissionsto NOx and weakening impacts of large sources on the SO2concentration. This study reaffirms China's success in controlling bothNOx and SO2 in the YRD but indicates at the same time a necessityto strengthen the NOx emission control.

atmospheric chemistry and physics

Measurement report: Long-term variations in surface NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; and SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; mixing ratios from 2006 to 2016 at a background site in the Yangtze River Delta region, China