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Abstract
While dry and rain deposition of nitrate (NO3−) and ammonium (NH4+) are regularly assessed, fog deposition is often overlooked. This work assesses summer fog events contribution to nitrogen deposition and availability for forest ecosystems. Rain and fog samples were collected at Mt Åreskutan, Sweden, during CAEsAR (Cloud and Aerosol Characterization Experiment), in 2014. NH4+ + NO3− represent (31 ± 25) % of total rain ion amount, and (31 ± 42) % in fog. Based on ion concentrations and the nitrate stable isotope signatures δ(15N) and δ(18O), it was possible to detect the plume generated by the Västmanland forest fire; NOx emissions from oil rigs and Kola Peninsula; and the plume of Bardarbunga volcano, Iceland. Scavenging of ions by fog was more efficient than by rain. Rain NH4+ and NO3− deposition was (26 ± 36) μmol m−2 d−1 and (23 ± 27) μmol m−2 d−1, respectively. Fog NH4+ and NO3− contributed (77 ± 80) % to total wet deposition of these species. Upscaling rain deposition fluxes to 1 year gave an inorganic nitrogen deposition of (18 ± 16) mmol m−2 a−1 ((252 ± 224) mg m−2 a−1 N equivalents), whereas fog deposition was estimated as (59 ± 47) mmol m−2 a−1 ((826 ± 658) mg m−2 a−1 N equivalents). Annual fog deposition was four times higher than previously reported for the area which only considered rain deposition. However, great uncertainty on the calculation of fog deposition need to be bear in mind. These findings suggest that fog should be considered in deposition estimates of inorganic nitrogen and major ions. If fog deposition is not accounted for, ion wet deposition may be greatly underestimated. Further sampling of wet and dry deposition is important for understanding the influence of nitrogen deposition on forest and vegetation development, as well as soil major ion loads.
Acknowledgements
The authors would like to thank the Department of Environmental Science and Analytical Chemistry at Stockholm University and Skistar Åre for providing access to the research facilities at Mt Åreskutan. Special thanks go to the people who helped collecting rain and fog samples at Mt Åreskutan, and to S. Wexler from the Science Analytical Facility at University of East Anglia for her help with the nitrate stable isotopes analyses, and V. Víquez from the School of Physics, University of Costa Rica, for helping revising the final manuscript. Thanks to Ski Star for all the help and special thanks to Lars ‘Lumpan’ Lundberg for his support.
Disclosure statement
No potential conflict of interest was reported by the authors
Data availability
Precipitation data for Medstugan, Digernäset, and Vallbo stations was provided by the Swedish Meteorological and Hydrological Institute (SMHI, http://opendata-download-metobs.smhi.se). Back-trajectories obtained using the HYSPLIT model can be retrieved using the online platform of the model from the NOAA Air Resources Laboratory (https://ready.arl.noaa.gov/HYSPLIT_traj.php). The data collected in this study are available upon request to the corresponding author.
Supplementary material
Supplemental data for this article can be accessed here
Table S1. Mean daily precipitation estimated as the mean of daily precipitation at the three SMHI stations near Åre: Medstugan, Digernäset and Vallbo, and mean, median, standard deviation (1σ), maximum, and minimum precipitation values for the three stations (considering the study period, i.e., 77-days).
Table S2. Days in which fog was present at the sampling site and mean liquid water content γ(H2O), for each sample interval.