The　purpose　of　this　study　is　to　investigate　the　impact　of　using　an　ensemble　Kalman　filter　(EnKF)　on　air　quality　simulations　in　the　California-Mexico　border　region　on　two　days　(May　30　and　June　04,　2010)　during　Cal-Mex　2010,　The　uncertainties　in　ozone　(O-3)　and　aerosol　simulations　in　the　border　area　due　to　the　meteorological　initial　uncertainties　were　examined　through　ensemble　simulations.　The　ensemble　spread　of　surface　O-3　averaged　over　the　coastal　region　was　less　than　10　ppb.　The　spreads　in　the　nitrate　and　ammonium　aerosols　are　substantial　on　both　days,　mostly　caused　by　the　large　uncertainties　in　the　surface　temperature　and　humidity　simulations.　In　general,　the　forecast　initialized　with　the　EnKF　analysis　(EnKF)　improved　the　simulation　of　meteorological　fields　to　some　degree　in　the　border　region　compared　to　the　reference　forecast　initialized　with　NCEP　analysis　data　(FCST)　and　the　simulation　with　observation　nudging　(FDDA),　which　in　turn　leading　to　reasonable　air　quality　simulations.　The　simulated　surface　O-3　distributions　by　EnKF　were　consistently　better　than　FCST　and　FDDA　on　both　days.　EnKF　usually　produced　more　reasonable　simulations　of　nitrate　and　ammonium　aerosols　compared　to　the　observations,　but　still　have　difficulties　in　improving　the　simulations　of　organic　and　sulfate　aerosols.　However,　discrepancies　between　the　EnKF　simulations　and　the　measurements　were　still　considerably　large,　particularly　for　sulfate　and　organic　aerosols,　indicating　that　there　are　still　ample　rooms　for　improvement　in　the　present　data　assimilation　and/or　the　modeling　systems.　(C)　2014　Elsevier　B.V.　All　rights　reserved.