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

Abstract. Through measurements of NO₂, O₃ 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 (τ_ss) of NO₃ and N₂O₅. During some nights, high NO₃ (∼ 200 pptv) and N₂O₅ (∼ 1 ppbv) mixing ratios were associated with values of τ_ss that exceeded 1 h for NO₃ and 3 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.