Direct observations of N₂O₅ reactivity on ambient aerosol particles

N 2 O 5 reactivity has been measured directly for the first time on ambient aerosol particles using an entrained aerosol flow reactor coupled to a custom‐built chemical ionization mass spectrometer at two urban locations during summer. The observed N 2 O 5 reactivity is a strong function of both relative humidity (RH) and particle chemical composition. We show that particulate organic mass loadings, together with ambient relative humidity, play a leading role in determining the reaction rate of N 2 O 5 with particles. Our observed reactivity values are both more variable and, at times, as much as a factor of ten lower than currently implemented large‐scale model parameterizations would predict. Such discrepancies have likely consequences for predictions of NO x availability and ozone production, and the sensitivity of these quantities to aerosol particle loadings.