Forest–atmosphere exchange of reactive nitrogen in a remote region – Part II: Modeling annual budgets

[摘要] To monitor the effect of current nitrogen emissions andmitigation strategies, total (wet  +  dry) atmospheric nitrogen deposition toforests is commonly estimated using chemical transport models or canopybudget models in combination with throughfall measurements. Since fluxmeasurements of reactive nitrogen (N r ) compounds are scarce, drydeposition process descriptions as well as the calculated flux estimates andannual budgets are subject to considerable uncertainties. In this study, wecompared four different approaches to quantify annual dry deposition budgetsof total reactive nitrogen ( Σ N r ) at a mixed forest sitesituated in the Bavarian Forest National Park, Germany. Dry depositionbudgets were quantified based on (I) 2.5 years of eddy covariance fluxmeasurements with the Total Reactive Atmospheric Nitrogen Converter (TRANC);(II) an in situ application of the bidirectional inferential flux modelDEPAC (Deposition of Acidifying Compounds), here called DEPAC-1D; (III) asimulation with the chemical transport model LOTOS-EUROS (Long-Term OzoneSimulation – European Operational Smog) v2.0, using DEPAC as dry depositionmodule; and (IV) a canopy budget technique (CBT). Averaged annual Σ N r dry deposition estimates determined fromTRANC 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 drydeposition, using concentrations and meteorological drivers measured at thesite, was 5.8  ±  0.1 kg N ha −1  a −1 . In comparison to TRANCfluxes, DEPAC-1D estimates were systematically higher during summer and inclose agreement in winter. Modeled Σ N r deposition velocities( v d ) of DEPAC-1D were found to increase with lower temperatures and higherrelative humidity and in the presence of wet leaf surfaces, particularlyfrom May to September. This observation was contrary toTRANC-observed fluxes. LOTOS-EUROS-modeled annual dry deposition was6.5  ±  0.3 kg N ha −1  a −1 for the site-specific weighting ofland-use classes within the site's grid cell. LOTOS-EUROS showed substantialdiscrepancies to measured Σ N r deposition during spring andautumn, which was related to an overestimation of ammonia (NH 3 )concentrations by a factor of 2 to 3 compared to measured values as aconsequence of a mismatch between gridded input NH 3 emissions and thesite's actual (rather low) pollution climate. According to LOTOS-EUROSpredictions, ammonia contributed most to modeled input Σ N r concentrations, whereas measurements showed NO x as the prevailingcompound in Σ N r concentrations. Annual deposition estimatesfrom measurements and modeling were in the range of minimum and maximumestimates 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-onlydeposition, we estimated an annual total nitrogen deposition input between11.5 and 14.8 kg N ha −1  a −1 , which is within the critical loadranges proposed for deciduous and coniferous forests.

[学科分类] 大气科学