Forest-atmosphere exchange of reactive nitrogen in a low polluted area – temporal dynamics and annual budgets

Abstract. Accurate modeling of nitrogen deposition is essential for identifying exceedances of critical loads and designing effective mitigation strategies. However, there are still uncertainties in modern deposition routines due to a limited availability of long-term flux measurements of reactive nitrogen compounds for model development and validation. In this study, we investigate the performance of dry deposition inferential models with regard to annual budgets and the exchange patterns of total reactive nitrogen (ΣNr) at a low-polluted mixed forest located in the Bavarian Forest National Park (NPBW), Germany. Flux measurements of ΣNr were carried out with a Total Reactive Atmospheric Nitrogen Converter (TRANC) coupled to a chemiluminescence dectector (CLD) for 2.5 years. Average ΣNr concentration was approximately 5.2 ppb. Denuder measurements with DELTA samplers and chemiluminescence measurements of nitrogen oxides (NOx) have shown that NOx has the highest contribution to ΣNr (~52%), followed by ammonia (NH3) (~ 22 %), ammonium (NH4+) (~ 14 %), nitrate NO3− (~ 7 %), and nitric acid (HNO3) (~ 6 %). We observed mostly deposition fluxes at the measurement site with median fluxes ranging from −15 ng N m−2 s−1 to −5 ng N m−2 s−1 (negative fluxes indicate deposition). In general, highest deposition was recorded from May to September. ΣNr deposition was enhanced by higher temperatures, lower relative humidity, high ΣNr concentration, and dry leaf surfaces. Our results suggest that dry conditions seem to favour nitrogen dry deposition at natural ecosystems. For determining annual dry deposition budgets we used the bidirectional inferential scheme DEPAC (DEPosition of Acidifying Compounds) with locally measured input parameters, called DEPAC-1D, as gap-filling strategy for TRANC measurements. In a second approach, the mean-diurnal-variation method (MDV) was applied to gaps of up to five days whereas DEPAC-1D was used for remaining gaps. We compared them to results from the chemical transport model LOTOS-EUROS (LOng Term OzoneSimulation – EURopean Operational Smog) v2.0 and from the canopy budget technique conducted at the measurement site. After 2.5 years, dry deposition based on TRANC measurements resulted in (11.1 ± 3.4) kg N ha−1 with DEPAC-1D as gap-filling method and (10.9 ± 3.8) kg N ha−1 with MDV and DEPAC-1D as gap-filling methods. Both values are close to dry deposition by DEPAC-1D (13.6 kg N ha−1) considering the uncertainties of measured fluxes and possible uncertainty sources of DEPAC-1D. The difference of DEPAC-1D to TRANC can be related to parameterizations of reactive gases or the missing exchange path with soil. 16.8 kg N ha−1 deposition were calculated by LOTOS-EUROS for considering land-use class weighting. We further showed that predicted NH3 concentrations, an input parameter of LOTOS-EUROS, were the main reason for the discrepancyin dry deposition budgets between the different methods. On average, annual TRANC dry deposition was 4.5 kg N ha−1 a−1 for both gap-filling approaches, DEPAC-1D showed 5.3 kg N ha−1 a−1, and LOTOS-EUROS modeled 5.2 kg N ha−1 a−1 to 6.9 kg N ha−1 a−1 depending on the weighting of land-use classes within the site's grid cell. 7.5 kg N ha−1 a−1 was estimated with the canopy budget technique for the period from 2016 to 2018 as upper estimate and 4.6 kg N ha−1 a−1 as lower estimate. Our findings provide a better understanding of exchange dynamics occurring at low-polluted, natural ecosystems and show opportunities for further development of deposition models.