Although　weather　conditions　significantly　affect　air　pollutant　concentrations,　few　quantitative　studies　have　been　conducted　on　the　effects　of　long-term　and　seasonal　changes　in　meteorology　on　air　quality.　Hence,　in　this　study,　the　trends　in　Shandong　Province,　China,　for　six　criteria　pollutants　(viz.,　sulfur　dioxide　[SO2],　carbon　monoxide　[CO],　particulate　matter　[PM]　with　an　aerodynamic　diameter　of　＜　10　mu　m　[PM10],　PM　with　an　aerodynamic　diameter　of　＜　2.5　mu　m　[PM2.5],　nitrogen　dioxide　[NO2],　and　ozone　[O-3])　were　analyzed　for　the　period　of　2013-2019,　when　overall　emissions　of　air　pollutants　decreased,　and　the　Weather　Research　and　Forecasting　model　coupled　with　Chemistry　(WRF/Chem)　was　applied　to　evaluate　the　role　of　inter-annual　and　seasonal　meteorological　changes.　Five　of　the　six　criteria　pollutants　exhibited　a　sharp　drop　in　concentration　until　2017　and　a　gradual　decline　afterward,　with　the　maximum　and　minimum　annual　values　occurring　during　winter　and　summer,　respectively.　In　contrast,　the　level　of　O-3　rose　between　2013　and　2019　and　displayed　the　opposite　seasonal　trend.　Also,　the　diurnal　concentrations　of　the　first　five　criteria　pollutants　showed　a　typical　bimodal　distribution,　whereas　those　of　the　O-3　showed　a　typical　unimodal　distribution.　Furthermore,　a　trimodal　distribution　was　observed　for　the　ratios　between　the　diurnal　PM2.5　and　PM10　concentrations.　Using　2013　as　the　baseline,　the　inter-annual　meteorological　changes　accounted　for　only　3.4-18.6%　of　the　decrease　in　the　five　criteria　pollutants-with　little　effect　on　the　O-3-between　2015　and　2019,　indicating　that　emission　control　measures　drove　the　long-term　improvement　in　air　quality　during　these　years.　However,　seasonal　meteorological　factors,　which　favored　diffusion　during　summer　and　winter　but　accumulation　during　spring　and　autumn,　played　a　larger　role　in　the　short　term　for　all　six　species,　especially　during　winter,　when　they　reduced　concentrations　(excluding　those　of　SO2　in　2019　and　O-3　altogether)　by　6.5-31.0%.