Distinguishing in-cloud and below-cloud short and distal N-sources from high-temporal resolution seasonal nitrate and ammonium deposition in Vienna, Austria

Abstract

Reactive nitrogen (Nr: nitrate and ammonium) washout in Vienna (Austria) precipitation events were investigated in 2019. A total of 958 samples from 61 rain events representing >90% of annual precipitation were collected at 5–30 min intervals for nitrate (NO3−) and ammonium (NH4+) analyses and meteorological information. The data revealed systematic seasonal concentration variations for all Nr-species and a clear influence of rush-hour traffic on the kinetics of N-scavenging processes. The monthly nitrate and ammonium deposition was 0.69 ± 0.21 kg ha−1 month−1 and 1.02 ± 0.30 kg ha−1 month−1, respectively. Around 30% of nitrate and 20% of ammonium was dry deposition, and ∼30% of each N-species was from distal sources associated with rainout processes. The half-life of below-cloud N-species were similar in the warmer seasons (1.7 ± 0.2 h and 2.3 ± 0.4 h for nitrate and ammonium). In winter, the ammonium half-life was significantly lower (1.4 h). Nr removal by wet-only in-cloud scavenging was slower than predicted by empirical models. HYSPLIT trajectory analysis revealed that Nr rainout from distal sources in spring had no prevailing direction, but higher Nr contributions were from N and W. In summer and winter, air masses from W, SW and SE were related to intense, medium, and low Nr contributions, respectively. The origin and path of these trajectories coincided with known NOx hotspots in Europe.