Air　source　heat　pumps　(ASHPs)　have　been　widely　used　for　providing　buildings　with　heating　in　winter　and　cooling　in　summer.　In　certain　locations,　with　an　increased　level　of　air　pollutions,　fouling　of　the　outdoor　coil　in　a　space　cooling　ASHP　unit　may　appear,　which　affects　the　operating　performances　of　the　ASHP　unit.　Furthermore,　the　operating　performances　of　an　ASHP　unit　can　also　be　affected　by　ambient　air　temperature　(T-a).　In　this　paper,　a　field　study　on　the　effects　of　outdoor　coil　fouling　at　different　T-a　on　the　operating　performances　of　a　field　space　cooling　ASHP　unit　is　presented.　The　study　results　demonstrated　that　the　operating　performances　of　the　field　ASHP　unit　were　significantly　affected　by　the　changes　in　both　T-a　and　percentage　of　fouled　outdoor　coil　surface　(PFOCS).　For　example,　at　T-a　=30　degrees　C,　the　variation　in　PFOCS　from　0　to　100%　led　to　a　reduction　in　COP　for　the　field　ASHP　unit　from　2.7　to　2.0.　At　a　fixed　PFOCS　of　60%,　the　changes　in　T-a　from　26　to　33　degrees　C　resulted　in　a　reduction　in　COP　from　2.6　to　2.1.　In　order　to　more　conveniently　investigate　the　combined　impacts　of　fouling　on　the　operating　performances　of　an　ASHP　unit　at　different　Ta,　an　equivalent　ambient　air　temperature　rise　(ETR)　was　proposed.　Based　on　the　measured　and　evaluated　operating　parameters　of　the　field　ASHP　unit　at　different　Ta　and　PFOCS　values,　ETR　values　at　0　-　40%,　40　-　60%,　60　-　100%　and　0　-　100%　PFOCS　ranges　were　evaluated.　These　ETR　values　implied　that　defouling　of　a　fouled　outdoor　coil　may　be　initiated　when　PFOCS　was　at　40　-　60%.　With　the　ETR　values　for　the　full　PFOCS　range　of　0-　100%,　the　impact　of　full　fouling　on　the　operating　performances　of　an　ASHP　unit　may　be　evaluated　as　if　it　was　operated　with　a　clean　outdoor　coil　but　at　an　equivalent　ambient　air　temperature　of　T-a　+　ETR.　(C)　2018　Elsevier　B.V.　All　rights　reserved.