Here we present measurements of the size-resolved concentration and isotopic composition of atmospheric nitrate (NO3−) collected during a cruise in coastal California. Significant differences in air mass origin and atmospheric chemistry were observed in the two main regions of this cruise (South and Central Coast) with corresponding differences in NO3− concentration and isotope ratios. Measurements of the 17O-excess (Δ17O) of NO3− suggest that nocturnal chemistry played an important role in terms of total NO3− production (~ 50%) in the coastal Los Angeles region (South Coast), where NO3− concentrations were elevated due to the influence of sea breeze / land breeze recirculation and Δ17O(NO3−) averaged (25.3 ± 1.6)‰. Conversely, Δ17O(NO3−) averaged (22.3 ± 1.8)‰ in the Central Coast region, suggesting that the daytime OH + NO2 reaction was responsible for 60–85% of NO3− production in the marine air sampled in this area. A strong diurnal signal was observed for both the Δ17O and δ15N of NO3−. In the case of Δ17O, this trend is interpreted quantitatively in terms of the relative proportions of daytime and nighttime production and the atmospheric lifetime of NO3−. For δ15N, which had an average value of (0.0 ± 3.2)‰, the observed diurnality suggests a combined effect of isotopic exchange between gas-phase precursors and variability in reactive nitrogen sources. These findings represent a significant advance in our understanding of the isotope dynamics of nitrate and its precursor molecules, with potentially important implications for air quality modeling.