Long‐term observation of nitrate radicals at the Tor Station, Kap Arkona (Rügen)

Long‐term observation of the trace gases NO 2 , ozone, and nitrate radicals (NO 3 ) at a rural site in the Baltic Sea (near Kap Arkona, island Rügen) using differential optical absorption spectroscopy (DOAS) are presented here. The time series of these species cover more than 14 months of data. Additionally, meteorological data were taken from the Deutscher Wetterdienst station at Kap Arkona. The average nighttime NO 3 concentration was determined at 7.8 ppt. Maximum lifetimes of more than 5000 s were observed, but average NO 3 lifetimes are close to 250 s. As nitrate radicals are expected to play an important role in the nonphotochemical conversion of NO x to HNO 3 , long‐term observations offer a key for investigation of possible sink mechanisms of NO 3 molecules mediated by both homogeneous and heterogeneous reactions. Analysis of the data shows that the NO 3 lifetime is inversely proportional to the NO 2 concentration, thus suggesting that the removal of N 2 O 5 is actually the dominating NO 3 loss process. The mean N 2 O 5 concentration from this long‐term measurement was calculated at 1 ppb with an N 2 O 5 lifetime between 1000 s and 20000 s covering 90% of the data.