Forest–atmosphere exchange of reactive nitrogen in a remote region – Part II: Modeling annual budgets
Authors: Wintjen Pascal , Schrader Frederik , Schaap Martijn , Beudert Burkhard , Kranenburg Richard , Brümmer Christian 
Published: November 2022

Abstract. To monitor the effect of current nitrogen emissions and
mitigation strategies, total (wet + dry) atmospheric nitrogen deposition to
forests is commonly estimated using chemical transport models or canopy
budget models in combination with throughfall measurements. Since flux
measurements of reactive nitrogen (Nr) compounds are scarce, dry
deposition process descriptions as well as the calculated flux estimates and
annual budgets are subject to considerable uncertainties. In this study, we
compared four different approaches to quantify annual dry deposition budgets
of total reactive nitrogen (ΣNr) at a mixed forest site
situated in the Bavarian Forest National Park, Germany. Dry deposition
budgets were quantified based on (I) 2.5 years of eddy covariance flux
measurements with the Total Reactive Atmospheric Nitrogen Converter (TRANC);
(II) an in situ application of the bidirectional inferential flux model
DEPAC (Deposition of Acidifying Compounds), here called DEPAC-1D; (III) a
simulation with the chemical transport model LOTOS-EUROS (Long-Term Ozone
Simulation – European Operational Smog) v2.0, using DEPAC as dry deposition
module; and (IV) a canopy budget technique (CBT). Averaged annual ΣNr dry deposition estimates determined from
TRANC measurements were 4.7 ± 0.2 and 4.3 ± 0.4 kg N ha−1 a−1, depending on the gap-filling approach. DEPAC-1D-modeled dry
deposition, using concentrations and meteorological drivers measured at the
site, was 5.8 ± 0.1 kg N ha−1 a−1. In comparison to TRANC
fluxes, DEPAC-1D estimates were systematically higher during summer and in
close agreement in winter. Modeled ΣNr deposition velocities
(vd) of DEPAC-1D were found to increase with lower temperatures and higher
relative humidity and in the presence of wet leaf surfaces, particularly
from May to September. This observation was contrary to
TRANC-observed fluxes. LOTOS-EUROS-modeled annual dry deposition was
6.5 ± 0.3 kg N ha−1 a−1 for the site-specific weighting of
land-use classes within the site's grid cell. LOTOS-EUROS showed substantial
discrepancies to measured ΣNr deposition during spring and
autumn, which was related to an overestimation of ammonia (NH3)
concentrations by a factor of 2 to 3 compared to measured values as a
consequence of a mismatch between gridded input NH3 emissions and the
site's actual (rather low) pollution climate. According to LOTOS-EUROS
predictions, ammonia contributed most to modeled input ΣNr
concentrations, whereas measurements showed NOx as the prevailing
compound in ΣNr concentrations. Annual deposition estimates
from measurements and modeling were in the range of minimum and maximum
estimates determined from CBT being at 3.8 ± 0.5 and 6.7 ± 0.3 kg N ha−1 a−1, respectively. By adding locally measured wet-only
deposition, we estimated an annual total nitrogen deposition input between
11.5 and 14.8 kg N ha−1 a−1, which is within the critical load
ranges proposed for deciduous and coniferous forests.