Several methods for measurement of ambient HNO3 were compared over a 9-day period during the Carbonaceous Species Methods Comparison Study at Glendora, CA, in August 1986. Hourly averaged HNO3 concentrations were in the range 0.5-25 ppbv, with hourly maxima each day in excess of 15 ppbv. The measurement methods included absorption by a nylon filter in an open-face filter pack (FP), a transition flow reactor tube (TRF) in conjunction with a nylon filter, tunable diode laser absorption spectroscopy (TDLAS) and Fourier transform infrared (FT-IR) spectroscopy. The latter method was chosen as the common method for comparison of the daytime HNO3 measurements (nighttime concentrations were below the 4 ppbv detection limit of the FT-IR). Comparing the mean daytime HNO3 mixing ratios, the 1-h FP method yielded 13.9 ppbv (vs 12.9 for FT-IR), the 1-h TDLAS was 11.7 ppbv (vs 13.2 for FT-IR) and the 12-h TFR was 11.4 ppbv (vs 12.0 for FT-IR). For the nighttime periods, the means of the 2-h FP and the 12-h TER were 1.2 and 0.95 ppbv, respectively. The measured means were within the uncertainties of the methods.