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.