Using data obtained during the 1985 Nitrogen Species Methods Comparison Study (Atmos. Environ. 22, 1517 (1988)), several measurement methods for sampling ambient ammonia are compared. Eight days of continuous measurements at Pomona College, a smog receptor site in Los Angeles, provided an extensive data base for assessment of the following methods: Fourier transform infrared spectroscopy (FTIR), three filter pack configurations, a simple and an annular denuder, and the transition flow reactor. FTIR was defined as the reference method and it reported hourly NH3 concentrations ranging from >60 to 2280 nM/cu m (>1.5 - 57 ppb) during the course of the study, the highest values coming from the influence of nearby livestock operations. Although only limited quality assurance procedures were carried out, the following conclusions can, nevertheless, be drawn: relative to the FTIR average values, (1) for 4-6 h sampling periods, the averages of the three filter packs from three research groups were 83-130% and the annular denuder average was 87%, and (2) for 10-12 h sampling periods, the simple denuder averaged 90% and the two transition flow reactors were 77-98%. Possible reasons for the reported systematic biases are presented, but these are not able to fully explain the large range of differences reported by the various methods.