In　order　to　evaluate　the　secondary　aerosol　formation　potential　at　a　suburban　site　of　Beijing,　in　situ　perturbation　experiments　in　a　potential　aerosol　mass　(PAM)　reactor　were　carried　out　in　the　winter　of　2014.　The　variations　of　secondary　aerosol　formation　as　a　function　of　time,　OH　exposure,　and　the　concentrations　of　gas　phase　pollutants　and　particles　were　reported　in　this　study.　Two　periods　with　distinct　secondary　aerosol　formation　potentials,　marked　as　Period　I　and　Period　II,　were　identified　during　the　observation.　In　Period　I,　the　secondary　aerosol　formation　potential　was　high,　and　correlated　well　to　the　air　pollutants,　i.　e.,　SO2,　NO2,　and　CO.　The　maximal　secondary　aerosol　formation　was　observed　with　an　aging　time　equivalent　to　about　3　days　of　atmospheric　oxidation.　In　period　II,　the　secondary　aerosol　formation　potential　was　low,　with　no　obvious　correlation　with　the　air　pollutants.　Meanwhile,　the　aerosol　mass　decreased,　instead　of　showing　a　peak,　with　increasing　aging　time.　Backward　trajectory　analysis　during　the　two　periods　confirmed　that　the　air　mass　in　Period　I　was　mainly　from　local　sources,　while　it　was　attributed　mostly　to　long　distance　transport　in　Period　II.　The　air　lost　its　reactivity　during　the　long　transport　and　the　particles　became　highly　aged,　resulting　in　a　low　secondary　aerosol　formation　potential.　Our　experimental　results　indicated　that　the　in　situ　measurement　of　the　secondary　aerosol　formation　potential　could　provide　important　information　for　evaluating　the　contributions　of　local　emission　and　long　distance　transport　to　the　aerosol　pollution.　(C)　2015　The　Research　Center　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.