Underestimation of potassium in forest dry deposition? – A simulation experiment in rural Germany

<p>Measurements of throughfall (TF) and wet deposition (WD) are a common method to assess nitrogen (N) and base cation (BC) deposition to forests. Using TF and WD, dry deposition (DD) is usually calculated with a canopy budget model (CBM) assuming similar BC to Na⁺ ratios in WD and DD. This assumption is especially uncertain for K⁺, since K⁺ is often bound to smaller particles compared to Na⁺. Here we asses this assumption by comparing the DD of K⁺ estimated with the CBM (DD_K^CBM) to the DD of K⁺ simulated with a process oriented DD model (&#8220;inferential model&#8221;, DD_K^INF). Simulation experiments were performed at two indicator forest stands (&#8220;virtual&#8221; broadleaved (BL) and coniferous (CF) forest) based on six years of daily PM_2.5 and PM₁₀ concentrations and weekly WD observations measured at the rural background research station Melpitz, Germany. On average, the K⁺:Na⁺ ratio in WD was 0.24 while the K⁺:Na⁺ ratio in DD^INF was 0.43 (CF) and 0.40 (BL), respectively. Accordingly, DD_K^CBM would need to be multiplied by a correction factor of 1.77 (CF) and 1.66 (BL) to match DD_K^INF, with substantial variation between years (lowest correction factor: 0.98, highest correction factor: 3.89). However, applying the correction factors in CBM calculations at nearby ICP Forests monitoring sites had only little effect on total (WD+DD) deposition rates of N and BC. The results were robust against changes in the meteorological data used for the inferential model. Uncertainty arises from periods affected by presence of particles larger than 10 &#181;m diameter, not covered by local measurements. A corresponding lower boundary estimate for the average underestimation of DD_K^CBM is a correction factor of 1.37 (CF) and 1.29 (BL). More work is required to assess to what extend the observed underestimation of DD_K^CBM is confirmed by other methods and at sites with different atmospheric conditions.</p>