Modeling dry flux of ammonia and nitric acid between the atmosphere and Lake Balaton

N-deposition from atmosphere contributes to the eutrophication of Lake Balaton (Hungary). To estimate the share of N-input compared to the effect of other sources, measurements have started in the 70's of the last century. However, in previous calculations the flux of N-gases (NH3 and HNO3) was estimated using deposition velocity parameters determined for terrestrial ecosystems. These simplifications were accompanied by an overestimation of the role of these compounds. According to our results for years 2001-2004, ammonia has a mean net emission flux from the lake (32.7 t N year(-1)), while nitric acid deposition takes -21.8 t N year(-1) that are one order of magnitude lower than sum of other deposition forms (esp. wet N-deposition). The pH range in lake water (pH = 8.3-8.9) allows the bi-directional flux for ammonia. Ammonia exchange can act as a buffering system, i.e., in case of a high N-load to the lake from other sources (rivers, waste water, run-off, etc.) N-accumulations can be buffered through nitrogen emission in form of NH3 as a consequence of the elevated compensation-point concentration. From this reason, eutrophication of Lake Balaton is phosphorus limited. Comparing the measured ammonia flux with the fluxes calculated by compensation-point models based on single Henry's law theory and by Hales-Drewes theory, it can be concluded that in our case latter theory describes better the exchange processes, suggesting that effect of carbon dioxide on the solubility of ammonia can not be excluded. However, in contrast with Hales and Drewes who suggested a decreased solubility of ammonia in presence of CO2, we find an opposite effect, i.e., CO2 favors the solubility of ammonia in the slight alkaline pH-range representative to the lake.