Alkyl nitrates were measured over the northeastern plains of Colorado during the Stratospheric Tropospheric Exchange: Radiation, Aerosols and Ozone (STERAO) 1996 campaign and over the North Atlantic during the North Atlantic Regional Experiment (NARE) 1997. Alkyl nitrate measurements were performed by two different laboratories enabling an intercomparison of analytical techniques. A weighted bivariate linear regression analysis of comparable samples (National Oceanic and Atmospheric Administration versus National Center for Atmospheric Research) for the four alkyl nitrates measured by both techniques (2-propyl nitrate, 1-propyl nitrate, 2-butyl nitrate, and 3-pentyl nitrate) suggests there is neither a significant calibration difference nor a systematic offset. The precision of the measurements is indicated by the scatter in the correlations. Good precision is observed for the 2-butyl nitrate measurements (square of the correlation coefficient, R2=0.90). However, relatively small values of R2 and relatively large uncertainties in the determination of the slopes and intercepts (2&sgr; confidence intervals) for 2-propyl nitrate, 1-propyl nitrate, and 3-pentyl nitrate suggest significant imprecision when compared to ambient mixing ratios for at least one of the measurement techniques. Vertical profiles during NARE 1997 show high levels of alkyl nitrates in well-defined photochemical pollution layers of continental origin. A comparison of alkyl nitrate to hydrocarbon ratios with a sequential reaction scheme model suggests our understanding of C4-C5 alkyl nitrate photochemistry is adequate to reproduce the observations. Air masses with larger ratios of alkyl nitrate to hydrocarbons were observed during NARE 1997 than STERAO 1996, consistent with the NARE 1997 measurements being in the more remote North Atlantic. These results suggest that information related to the photochemical age of an air mass can be derived from alkyl nitrate to hydrocarbon ratios. ¿ 2001 American Geophysical Union