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 (6N_r) 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 6N_r were carried out with the Total Reactive Atmospheric Nitrogen Converter (TRANC) coupled to a chemiluminescence detector (CLD) for 2.5 years. The average 6N_r concentration was 3.1 μg N m⁻³. Denuder measurements with DELTA samplers and chemiluminescence measurements of nitrogen oxides (NO_x ) have shown that NO_x has the highest contribution to 6N_r (∼ 51.4 %), followed by ammonia (NH₃) (∼ 20.0 %), ammonium (NH⁺₄ ) (∼ 15.3 %), nitrate NO⁻₃ (∼ 7.0 %), and nitric acid (HNO₃) (∼ 6.3 %). Only slight seasonal changes were found in the 6N_r concentration level, whereas a seasonal pattern was observed for the contribution of NH3 and NOx . NH₃ showed highest contributions to 6N_r in spring and summer, NO_x 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 6N_r, global radiation (R_g), and other drivers correlated with R_g were most likely influencing the deposition velocity (v_d). We found that from May to September higher temperatures, lower relative At humidity, and dry leaf surfaces increase v_dof 6N_r.the measurement site, 6N_r concentration did not emerge as a driver for the 6N_rvd. No significant influence of temperature, humidity, friction velocity, or wind speed on 6N_r 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 6N_r using novel measurements techniques for estimating annual nitrogen dry deposition to a remote forest ecosystem.