Chemical and meteorological influences on the lifetime of NO.sub.3 at a semi-rural mountain site during PARADE

Through measurements of NO.sub.2, O.sub.3 and NO.sub.3 during the PARADE campaign (PArticles and RAdicals, Diel observations of mEchanisms of oxidation) in the German Taunus mountains we derive nighttime steady-state lifetimes (τ.sub.ss) of NO.sub.3 and N.sub.2 O.sub.5 . During some nights, high NO.sub.3 (∼ 200 pptv) and N.sub.2 O.sub.5 (∼ 1 ppbv) mixing ratios were associated with values of τ.sub.ss that exceeded 1 h for NO.sub.3 and 3 h for N.sub.2 O.sub.5 near the ground. Such long boundary-layer lifetimes for NO.sub.3 and N.sub.2 O.sub.5 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 NO.sub.3 indicates that the inferred lifetimes are significantly longer than those calculated from the summed loss rate. Several potential causes for the apparently extended NO.sub.3 and N.sub.2 O.sub.5 lifetimes are examined, including additional routes to formation of NO.sub.3 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 NO.sub.3 formation due to reactions of Criegee intermediates may contribute.