Ship　emissions　are　an　important　factor　affecting　air　quality　in　coastal　areas.　The　adverse　effects　of　ship　emissions　on　coastal　air　pollutants　are　more　pronounced　in　the　presence　of　onshore　wind.　However,　existing　studies　are　based　on　the　summer　half　of　the　year　and　specific　stations,　so　the　frequency　of　onshore　winds　and　their　effect　on　the　transport　of　ship　emissions　are　not　fully　understood　for　the　whole　year　or　for　different　seasons.　Therefore,　in　this　study,　the　Yangtze　River　Delta　(YRD)　region　was　selected　as　the　target　study　area,　and　the　WRF/Chem　(The　Weather　Research　and　Forecast　model　coupled　with　Chemistry)　model　was　used　to　investigate　the　impact　of　ship　emissions　on　PM2.5　in　coastal　areas　under　the　influence　of　onshore　wind　in　2018.　There　are　three　main　findings.　(1)　The　contribution　of　ship　emissions　to　PM2.5　under　the　influence　of　onshore　wind　was　more　significant　than　either　the　annual　average　contribution　or　the　contribution　under　the　influence　of　non-onshore　wind,　especially　in　coastal　areas.　The　contributions　of　ship　emissions　to　PM2.5　during　onshore　wind　hours　reached　9.3　μg　m–3　and　7.4　μg　m–3　in　Shanghai　and　Ningbo,　respectively,　which　were　1.8　and　1.3　times　the　annual　average,　respectively.　(2)　During　onshore　wind　periods,　the　contribution　of　ship　emissions　to　PM2.5　increases　sig-nificantly　not　only　in　coastal　areas,　but　also　inland.　In　the　central　region　200　km　from　the　coast,　ship　emissions　contributed　4.5　μg　m–3　to　PM2.5　during　onshore　wind　hours,　while　the　annual　average　was　only　2.5　μg　m–3.　(3)　Under　the　influence　of　onshore　wind,　the　effect　of　ship　emissions　on　PM2.5　showed　obvious　seasonal　differences.　Under　the　influence　of　onshore　wind,　the　contribution　of　ship　emissions　to　PM2.5　was　larger　in　winter　than　in　other　seasons.　In　summer,　the　contribution　of　ship　emissions　to　PM2.5　differed　the　most　between　onshore　and　non-onshore　wind　periods.　In　spring,　the　contribution　of　ship　emissions　to　PM2.5　was　greater　even　though　the　frequency　of　onshore　wind　was　the　lowest.　The　contribution　of　ship　emissions　to　PM2.5　can　often　be　underestimated　when　only　annual　or　monthly　averages　are　considered.　This　is　detrimental　to　the　accurate　prevention　and　control　of　ship　emissions　by　relevant　authorities　during　periods　of　high　pollution.　The　results　of　this　study　will　help　those　authorities　to　understand　the　transport　characteristics　of　ship　emissions　in　the　presence　of　onshore　airflow　and　provide　insights　for　coastal　air　pollution　control　and　management.　©　2023,　Editorial　office　of　Journal　of　Resources　and　Ecology.　All　rights　reserved.