This　paper　focused　on　VOCs　(referring　to　C2–C12　hydrocarbons)　in　Beijing　and　conducted　an　integrated　comparison　study　on　VOCs　concentration,　composition,　chemical　degradation　and　regional　transport　impact　between　haze　and　non-haze　days　in　July　and　December　of　2015,　through　VOCs　monitoring　and　WRF-Chem　modeling　simulation.　Our　monitoring　results　show　that　in　July　VOCs　was　20.1%　higher　in　haze　days　(24.5　±　6.6　ppbv)　than　in　non-haze　days　(20.4　±　7.8　ppbv),　but　VOCs:CO　in　haze　days　was　13.4%　lower　than　the　non-haze　level.　The　relative　change　in　VOCs:CO　from　non-haze　days　to　haze　days　responded　negatively　to　VOCs　reaction　ability　with　OH　radical　(K　OH　).　Combining　VOCs:CO　decline　during　8:00–14:00　with　K　OH　,　OH　level　was　estimated　to　increase　from　0.75　×　10　6　molec/cm　3　in　non-haze　days　to　1.17　×　10　6　molec/cm　3　in　haze　days.　WRF-Chem　simulations　calculate　the　regional　transport　contribution　to　Beijing　VOCs,　averagely　−0.5　ppbv　in　haze　days　and　6.3　ppbv　in　non-haze　days　in　July.　These　analyses　implied　a　significant　VOCs　chemical　degradation　and　a　weak　VOCs　regional　transport　in　summer　haze　days.　In　December,　VOCs　reached　91.0　±　37.3　ppbv　in　haze　days　and　21.4　±　14.1　ppbv　in　non-haze　days,　and　VOCs:CO　still　dropped　by　21.2%　lower　in　haze　days.　However,　the　VOCs:CO　decrease　was　mostly　contributed　by　alkanes,　acetylene　and　C4–C5　alkenes.　The　relative　change　in　VOCs:CO　from　non-haze　days　to　haze　days　seemingly　responded　positively　to　K　OH　values　and　VOCs　chemical　degradation　was　never　explicitly　found　in　both　haze　and　non-haze　days.　Model　simulations　obtain　a　higher　regional　transport　contribution　to　VOCs　in　haze　days　(4.4–31.0　ppbv)　than　in　non-haze　days　(0.5–6.5　ppbv).　The　winds　transport　also　explained　the　difference　in　VOCs　composition　and　VOCs:CO　ratio　between　haze　and　non-haze　days　of　winter,　for　prevalent　south　winds　in　haze　days　transporting　air　masses　of　South　Beijing　and　South　Hebei　that　were　more　polluted　and　characterized　by　higher　proportions　of　alkenes,　aromatics　and　CO.　©　2018　Elsevier　Ltd