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

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Abstract
Long‐term observation of the trace gases NO2, ozone, and nitrate radicals (NO3) 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 NO3 concentration was determined at 7.8 ppt. Maximum lifetimes of more than 5000 s were observed, but average NO3 lifetimes are close to 250 s. As nitrate radicals are expected to play an important role in the nonphotochemical conversion of NOx to HNO3, long‐term observations offer a key for investigation of possible sink mechanisms of NO3 molecules mediated by both homogeneous and heterogeneous reactions. Analysis of the data shows that the NO3 lifetime is inversely proportional to the NO2 concentration, thus suggesting that the removal of N2O5 is actually the dominating NO3 loss process. The mean N2O5 concentration from this long‐term measurement was calculated at 1 ppb with an N2O5 lifetime between 1000 s and 20000 s covering 90% of the data.

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