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Forest-atmosphere exchange of reactive nitrogen in a remote region - Part I: Measuring temporal dynamics

Long-term dry deposition flux measurements of reactive nitrogen based on the eddy covariance or the aero- dynamic gradient method are scarce. Due to the large diversity of reactive nitrogen compounds and high technical requirements for the measuring devices, simultaneous measurements of individual reactive nitrogen compounds are not affordable. Hence, we examined the exchange patterns of to- tal reactive nitrogen (6Nr) and determined annual dry deposition budgets based on measured data at a mixed forest exposed to low air pollution levels located in the Bavarian Forest National Park (NPBW), Germany. Flux measurements of 6Nr were carried out with the Total Reactive Atmospheric Nitrogen Converter (TRANC) coupled to a chemiluminescence detector (CLD) for 2.5 years. The average 6Nr concentration was 3.1 μg N m−3. Denuder measurements with DELTA samplers and chemiluminescence measurements of nitrogen oxides (NOx ) have shown that NOx has the highest contribution to 6Nr (∼ 51.4 %), followed by ammonia (NH3) (∼ 20.0 %), ammonium (NH+4 ) (∼ 15.3 %), nitrate NO3 (∼ 7.0 %), and nitric acid (HNO3) (∼ 6.3 %). Only slight seasonal changes were found in the 6Nr concentration level, whereas a seasonal pattern was observed for the contribution of NH3 and NOx . NH3 showed highest contributions to 6Nr in spring and summer, NOx in autumn and winter. We observed deposition fluxes at the measurement site with median fluxes ranging from −15 to −5 ng N m−2 s−1 (negative fluxes indicate deposition). Median deposition velocities ranged from 0.2 to 0.5 cm s−1. In general, highest deposition velocities were recorded during high solar radiation, in particular from May to September. Our results suggest that seasonal changes in composition of 6Nr, global radiation (Rg), and other drivers correlated with Rg were most likely influencing the deposition velocity (vd). We found that from May to September higher temperatures, lower relative At humidity, and dry leaf surfaces increase vdof 6Nr.the measurement site, 6Nr concentration did not emerge as a driver for the 6Nrvd. No significant influence of temperature, humidity, friction velocity, or wind speed on 6Nr fluxes when using the meandiurnal-variation (MDV) approach for filling gaps of up to 5 days was found. Remaining gaps were replaced by a monthly average of the specific half-hourly value. From June 2016 to May 2017 and June 2017 to May 2018, we estimated dry deposition sums of 3.8 and 4.0 kg N ha−1 a−1, respectively. Adding results from the wet deposition measurements, we determined 12.2 and 10.9 kg N ha−1 a−1 as total nitrogen deposition in the 2 years of observation. This work encompasses (one of) the first long-term flux measurements of 6Nr using novel measurements techniques for estimating annual nitrogen dry deposition to a remote forest ecosystem.



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