Chemical and meteorological influences on the lifetime of NO₃ at a semi-rural mountain site during PARADE

Through measurements of NO2, O-3 and NO3 during the PARADE campaign (PArticles and RAdicals, Diel observations of mEchanisms of oxidation) in the German Taunus mountains we derive nighttime steady-state lifetimes (tau(ss)) of NO3 and N2O5. During some nights, high NO3 (aEuro-200aEuro-pptv) and N2O5 (aEuro-1aEuro-ppbv) mixing ratios were associated with values of tau(ss) that exceeded 1aEuro-h for NO3 and 3aEuro-h for N2O5 near the ground. Such long boundary-layer lifetimes for NO3 and N2O5 are usually only encountered in very clean/unreactive air masses, whereas the PARADE measurement site is impacted by both biogenic emissions from the surrounding forest and anthropogenic emissions from the nearby urbanised/industrialised centres. Measurement of several trace gases which are reactive towards NO3 indicates that the inferred lifetimes are significantly longer than those calculated from the summed loss rate. Several potential causes for the apparently extended NO3 and N2O5 lifetimes are examined, including additional routes to formation of NO3 and the presence of a low-lying residual layer. Overall, the most likely cause of the anomalous lifetimes are related to the meteorological conditions, though additional NO3 formation due to reactions of Criegee intermediates may contribute