DRY DEPOSITION GROUP KICK-OFF MEETING

SUMMARY

Session 1 - Total deposition estimation cases

Per Erik Karlsson, presenting the study Total deposition of inorganic nitrogen to Norway spruce forests – Applying a surrogate surface method across a deposition gradient in Sweden (Karlsson et al., 2019, Atmos. Environ.)

Particulate dry deposition was estimated based on measurements using Teflon string samplers as surrogate surfaces, in combination with the net throughfall deposition for sodium

Factors to correct for dry deposition to BD samplers were derived from monthly measurements with bulk deposition samplers placed under a roof. Mean correction factors for estimating monthly wet deposition from bulk deposition measurements at nine different sites across Sweden, separated for months with high or low bulk deposition values, are presented in the article.

Values for N deposition derived from measurements exceeded for most sites the corresponding values derived from modelling with the MATCH and EMEP models.

Based on comparisons between total and throughfall deposition, the canopy uptake of atmospheric N deposition to Norway spruce forests in Sweden was estimated to be up to 7 kg N ha⁻¹ yr⁻¹.

Analysis of seasonal variations in canopy nitrogen uptake, as well as the impacts of different meteorological conditions, should be possible in such a large study. However, this is complicated since different confounding factors have to be considered. Annual estimations of N uptake, on the other hand, are possible.

Anne Thimonier, presenting the study Total deposition of nitrogen in Swiss forests: Comparison of assessment methods and evaluation of changes over two decades (Thimonier et al., 2019, Atmos. Environ.)

The study presented low-cost, readily available field methods that resulting in a reasonable estimation of total nitrogen input into forests, while avoiding the measurement of site-dependent parameters (such as leaf area index, surface deposition or stomata uptake)

They considered three different approaches based on throughfall methods, inferential method and emission based modelling. The three approaches generally yielded comparable estimates of total deposition, with some notable differences at some sites.

For both the model and the inferential method, uncertainties were related to the deposition velocities that were applied to air concentrations of N compounds, especially for NH3. The modelling approach allows a good estimation for larger areas; however, it only gives a rough estimate for a site-specific quantification. The inferential method was considered a solid approach, but choosing the appropriate deposition velocity is difficult and can lead to substantial uncertainties.

The throughfall method provided a minimum estimate of the total N deposition in the forest, without N quantification, since they used an approach (alternative to CBM) in which they estimated the ranges of possible values for total and dry deposition, and canopy uptake. The lower bounds of the ranges were derived from the measurements, while the most plausible estimates for upper bounds were derived from the literature (from studies actually quantifying canopy uptake). It was considered a useful complementary method that gives a minimum estimate of the site-level, total deposition.

Andreas Schmitz, presenting the study Comparison of Methods for the Estimation of Total Inorganic Nitrogen Deposition to Forests in Germany (Ahrends et al., 2020, Front. For. Glob. Change)

They considered three different approaches based on CBM methods; inferential method and emission-based modelling.

For the inferential method approach, they introduced site-specific, semi-empirical correction factors to deposition velocities from literature to diminish the uncertainty derived from site-specific variations. Moreover, empirical approach was used to estimate particulate N and HNO3 deposition.

There was a good agreement between the canopy budget and the inferential method when using semi-empirical correction factors for deposition velocity.

Large discrepancies were observed between the emissions-based approach and the two measurement-based approaches, which seemed to be related to a combination of meteorological conditions and tree species effects (highlighting the lower emission-based modelling estimates for spruce plots).

Very valuable information about the uncertainty of different methodologies was presented. Particurlarly about CBM-like methods:

• Related to measurements: the validity of the tracer approach could be compromised by canopy leaching, contamination, rainfall differences between samplers plots (open-throughfall); the concentration ratios may vary depending on the source (sampler);
• Related to N transformation or fluxes: there is few information on canopy exchange factors; even less information on N species transformation in the canopy or in the samplers.

Héctor García-Gómez, presenting the study Joining empirical and modelling approaches to estimate dry deposition of nitrogen in Mediterranean forests (García-Gómez et al., 2018, Environ. Pollution)

A site-specific, highly demanding method was presented. They considered a measurement-based methodology in combination with stomatal-uptake modelling.

Branch-washing experiments in field were used to establish empirical relationship between atmospheric concentration of gaseous N and dry deposition of gaseous and particulate N. DO3SE model was used to estimate stomatal deposition of gaseous N (mainly NO₂ and NH₃)

Dry deposition of oxidized N forms was predominant. Particularly, for HNO₃, which led to pronounced temporal and spatial. The difficulty for obtaining reliable measurements of HNO₃ is one of the greatest factors of uncertainty in this approach, together with the scarcity of reliable, site-specific values of LAI.

Further efforts are needed to include reliable HNO3 measurements in broad-scale atmospheric deposition monitoring networks (particularly in Mediterranean region and in summer), together with NO₂ measurements in peri-urban forests, since an important contribution to total deposition was estimated. Also, increasing the availability of reliable LAI data, at the smallest possible scale, is advisable for upscaling.

An ongoing research in foliar uptake and transformation of N was represented. This study will include isotopic analysis of deposition, genetic analysis of deposition and experiment of foliar addition of N¹⁵ and branch washing.