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

Abstract

Abstract. Long-term dry deposition flux measurements of reactive nitrogen based on the eddy covariance or the aerodynamic 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 total reactive nitrogen (ΣNr) 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 ΣNr were carried out with the Total Reactive Atmospheric Nitrogen Converter (TRANC) coupled to a chemiluminescence detector (CLD) for 2.5 years. The average ΣNr 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 ΣNr (∼51.4 %), followed by ammonia (NH3) (∼20.0 %), ammonium (NH4+) (∼15.3 %), nitrate NO3- (∼7.0 %), and nitric acid (HNO3) (∼6.3 %). Only slight seasonal changes were found in the ΣNr concentration level, whereas a seasonal pattern was observed for the contribution of NH3 and NOx. NH3 showed highest contributions to ΣNr 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 ngNm-2s-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 ΣNr, 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 humidity, and dry leaf surfaces increase vd of ΣNr. At the measurement site, ΣNr concentration did not emerge as a driver for the ΣNrvd. No significant influence of temperature, humidity, friction velocity, or wind speed on ΣNr fluxes when using the mean-diurnal-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 kgNha-1a-1, respectively. Adding results from the wet deposition measurements, we determined 12.2 and 10.9 kgNha-1a-1 as total nitrogen deposition in the 2 years of observation. This work encompasses (one of) the first long-term flux measurements of ΣNr using novel measurements techniques for estimating annual nitrogen dry deposition to a remote forest ecosystem.

Reactive nitrogen fluxes over peatland and forest ecosystems using micrometeorological measurement techniques

“…Flechard et al, 2013) in space and time, which complicates the development of surface-atmosphere exchange schemes. On the basis of the FOR dataset, Wintjen et al (2022) demonstrate that mean diurnal flux courses stratified by concentration, temperature, moisture, or surface wetness reveal the strongest dependence on the actual N r concentration regardless of flux direction, i.e. situations of deposition or emission.…”

“…These gases and particles differ significantly in physical and chemical properties and in their contribution to N r . The ratio of the compounds varies throughout the annual cycle (Wintjen et al, 2022) and further depends on site characteristics and surrounding conditions. Little is known about the overall dependency of N r on the climatic parameters.…”

“…The different site-specific shares are presented alongside the flux dataset in the form of monthly integrated HONO, HNO 3 , aerosol NH 4 , and aerosol NO 3 measured by DELTA denuder and filter samplers. Further details for the WET and FOR sites can be found in the publications of Hurkuck et al (2014) and Wintjen et al (2022), respectively. In more than a third (38.3 %) of the campaign time, net emission of N r was observed at the peatland site.…”

“…Sutton et al, 2009), half-hour fluxes from eddy-covariance measurements allow for investigating the mechanisms governing the surfaceatmosphere exchange on different temporal scales. For example, high temporal resolution helps to identify the bidirectional exchange of NH 3 (Nemitz et al, 2001;Zhu et al, 2015;Schulte et al, 2021) and reveals phases of net emission vs. net deposition (McGinn et al, 2016;Zöll et al, 2016;Wintjen et al, 2022). A clearer picture can be derived about the interplay of stomatal (Geßler et al, 2002) and non-stomatal pathways (Schrader et al, 2016); the exchange between soil and the atmosphere; seasonal site-specific conditions, e.g.…”

“…Although TRANC data do not reveal single compound patterns, applying the TRANC technology reduces effort and investments for studies focussing on N r balances and characteristics (e.g. Zöll et al, 2019;Wintjen et al, 2022).…”